JP2007148128A - Pixel circuit - Google Patents

Abstract

A pixel circuit having a function of canceling variations in threshold voltages of drive transistors is simplified and rationalized.
A pixel capacitor C1 is connected between a gate G and a source S of a drive transistor T5, a light emitting element EL is connected between the source S and a cathode potential Vcat, and a sampling transistor T1 is connected between the gate G and a signal line SL. Are connected, a switching transistor T4 is connected between the drain D and the power supply Vcc, and another switching transistor T2 is connected between the source S and the signal line SL. The two switching transistors T2 and T4 operate before the video signal Vsig is sampled to the pixel capacitor C1, detect the threshold voltage of the drive transistor T5 and write it to the pixel capacitor C1, and thus with respect to the threshold voltage of the output current. Correct dependencies.
[Selection] Figure 14

Description

  The present invention relates to a pixel circuit that current-drives light-emitting elements constituting pixels of an active matrix display device. More specifically, the present invention relates to a technique for controlling the amount of current supplied to a light emitting element such as an organic EL by an insulated gate field effect transistor provided in each pixel circuit. More particularly, the present invention relates to a technique for correcting a threshold voltage of an insulated gate field effect transistor that drives a light emitting element.

  In an image display device such as a liquid crystal display, an image is displayed by arranging a large number of liquid crystal pixels in a matrix and controlling the transmission intensity or reflection intensity of incident light for each pixel in accordance with image information to be displayed. This also applies to an organic EL display using an organic EL element as a pixel, but unlike a liquid crystal pixel, the organic EL element is a self-luminous element. Therefore, the organic EL display has advantages such as higher image visibility than the liquid crystal display, no backlight, and high response speed. Further, the luminance level (gradation) of each light emitting element can be controlled by the value of the current flowing therethrough, and is greatly different from a voltage control type such as a liquid crystal display in that it is a so-called current control type.

In the organic EL display, similarly to the liquid crystal display, there are a simple matrix method and an active matrix method as driving methods. Although the former has a simple structure, there is a problem that it is difficult to realize a large-sized and high-definition display. Therefore, the active matrix method is actively developed at present. In this method, a current flowing through a light emitting element in each pixel circuit is controlled by an active element (generally a thin film transistor or TFT) provided in the pixel circuit, and is described in the following patent documents.
JP 2003-255856 A JP 2003-271095 A JP 2004-133240 A JP 2004-029791 A JP 2004-093682 A

  A conventional pixel circuit is arranged at a portion where a row scanning line for supplying a control signal and a column signal line for supplying a video signal intersect, and includes at least a sampling transistor, a capacitor, a drive transistor, and a light emitting element. . The sampling transistor conducts in response to the control signal supplied from the scanning line and samples the video signal supplied from the signal line. The capacitor unit holds an input voltage corresponding to the sampled video signal. The drive transistor supplies an output current during a predetermined light emission period in accordance with the input voltage held in the capacitor unit. In general, the output current depends on the carrier mobility and threshold voltage of the channel region of the drive transistor. The light emitting element emits light with luminance according to the video signal by the output current supplied from the drive transistor.

  The drive transistor receives the input voltage held in the capacitor portion at the gate, causes an output current to flow between the source and the drain, and energizes the light emitting element. In general, the light emission luminance of a light emitting element is proportional to the amount of current applied. Further, the output current supply amount of the drive transistor is controlled by the gate voltage, that is, the input voltage written in the capacitor. The conventional pixel circuit controls the amount of current supplied to the light emitting element by changing the input voltage applied to the gate of the drive transistor in accordance with the input video signal.

Here, the operating characteristic of the drive transistor is expressed by the following Equation 1.
Ids = (1/2) μ (W / L) Cox (Vgs−Vth) ² Formula 1
In the transistor characteristic formula 1, Ids represents a drain current flowing between the source and the drain, and is an output current supplied to the light emitting element in the pixel circuit. Vgs represents a gate voltage applied to the gate with reference to the source, and is the above-described input voltage in the pixel circuit. Vth is the threshold voltage of the transistor. Μ represents the mobility of the semiconductor thin film constituting the channel of the transistor. In addition, W represents the channel width, L represents the channel length, and Cox represents the gate capacitance. As is apparent from the transistor characteristic equation 1, when the thin film transistor operates in the saturation region, if the gate voltage Vgs increases beyond the threshold voltage Vth, the thin film transistor is turned on and the drain current Ids flows. In principle, as shown in the above transistor characteristic equation 1, if the gate voltage Vgs is constant, the same amount of drain current Ids is always supplied to the light emitting element. Therefore, if video signals of the same level are supplied to all the pixels constituting the screen, all the pixels should emit light with the same luminance, and the uniformity of the screen should be obtained.

  However, in reality, thin film transistors (TFTs) composed of semiconductor thin films such as polysilicon have variations in individual device characteristics. In particular, the threshold voltage Vth is not constant and varies from pixel to pixel. As apparent from the transistor characteristic equation 1 described above, if the threshold voltage Vth of each drive transistor varies, even if the gate voltage Vgs is constant, the drain current Ids varies and the luminance varies from pixel to pixel. , Damage the screen uniformity. Conventionally, a pixel circuit incorporating a function for canceling variations in threshold voltages of drive transistors has been developed, and is disclosed in, for example, Patent Document 3 described above.

  However, a pixel circuit incorporating a function (threshold voltage correction function) for canceling the variation in threshold voltage of the drive transistor has increased the number of elements, resulting in a complicated circuit. As the number of transistors constituting the pixel circuit increases, the number of scanning lines (gate lines) and power supply lines for controlling the transistors increases accordingly. The ratio of the gate lines and power supply lines to the pixel circuit increases, making it difficult to achieve high definition panels. In addition, when the number of gate lines and power supply lines is large, there is a problem that the crossover between wirings increases correspondingly and the manufacturing yield of the panel is deteriorated.

  In view of the above-described problems of the conventional technology, an object of the present invention is to simplify and rationalize a pixel circuit having a function of canceling a variation in threshold voltage of a drive transistor. In order to achieve this purpose, the following measures were taken. That is, the present invention is a pixel circuit which is arranged at a portion where a signal line and a required number of scanning lines intersect, and includes a light emitting element and a drive transistor for driving the light emitting element, and a pixel capacitance between the gate and source of the drive transistor. Is connected, the light emitting element is connected between the source of the drive transistor and a predetermined cathode potential, the sampling transistor is connected between the gate of the drive transistor and the signal line, and the drain of the drive transistor and the power supply A switching transistor is connected between them, and another switching transistor is connected between the source of the drive transistor and the signal line, and the sampling transistor conducts in the horizontal scanning period and is supplied from the signal line. A signal is sampled into the pixel capacitor, and the pixel capacitor An input voltage is applied to the gate of the drive transistor in response to the received video signal, the drive transistor supplies an output current corresponding to the input voltage to the light emitting element, and the output current is a threshold voltage of the drive transistor. The light emitting element emits light with a luminance corresponding to the video signal by an output current supplied from the drive transistor, and the two switching transistors have the video signal in the pixel capacitance. It operates before being sampled, and detects the threshold voltage of the drive transistor and writes it to the pixel capacitor, thereby correcting the dependency of the output current on the threshold voltage.

  Specifically, the signal line switches and supplies a signal voltage representing a video signal, a first fixed voltage fixed at a first level, and a second fixed voltage fixed at a second level. In this case, the signal voltage is applied to the gate of the drive transistor when sampling the video signal, the first fixed voltage is applied to the gate of the drive transistor when correcting the threshold voltage, and the second fixed voltage is corrected to the threshold voltage. It is given to the source of the drive transistor in a preparatory stage before starting. The two switching transistors operate during the horizontal scanning period, detect the threshold voltage of the drive transistor, and write it into the pixel capacitor.

  According to another aspect of the present invention, there is provided a pixel circuit including a light emitting element and a drive transistor for driving the signal line, which is disposed at a portion where a signal line and a required number of scanning lines intersect, and a pixel capacitance between the gate and the source of the drive transistor. Is connected, the light emitting element is connected between the source of the drive transistor and a predetermined cathode potential, the sampling transistor is connected between the gate of the drive transistor and the signal line, and the drain of the drive transistor and the power supply A switching transistor is connected between them, and another switching transistor is connected between the source and gate of the drive transistor, and the sampling transistor conducts during a horizontal scanning period and is supplied from the signal line. Is sampled into the pixel capacitance, and the pixel capacitance is An input voltage is applied to the gate of the drive transistor in accordance with the received video signal, the drive transistor supplies an output current corresponding to the input voltage to the light emitting element, and the output current is a threshold voltage of the drive transistor. The light emitting element emits light with a luminance corresponding to the video signal by an output current supplied from the drive transistor, and the two switching transistors have the video signal in the pixel capacitance. It operates before being sampled, and detects the threshold voltage of the drive transistor and writes it to the pixel capacitor, thereby correcting the dependency of the output current on the threshold voltage.

  Specifically, the signal line switches and supplies a signal voltage representing a video signal, a first fixed voltage fixed at a first level, and a second fixed voltage fixed at a second level. In this case, the signal voltage is applied to the gate of the drive transistor when sampling the video signal, the first fixed voltage is applied to the gate of the drive transistor when correcting the threshold voltage, and the second fixed voltage is corrected to the threshold voltage. It is given to the source of the drive transistor in a preparatory stage before starting. The two switching transistors operate during the horizontal scanning period, detect the threshold voltage of the drive transistor, and write it into the pixel capacitor.

  Furthermore, the present invention is a pixel circuit which is arranged at a portion where a signal line and a required number of scanning lines intersect, and includes a light emitting element and a drive transistor for driving the light emitting element, and a pixel capacitance between the gate and the source of the drive transistor. Is connected, the light emitting element is connected between the source of the drive transistor and a predetermined cathode potential, the sampling transistor is connected between the gate of the drive transistor and the signal line, the drain of the drive transistor and the variable power supply A switching transistor is connected between the gate of the drive transistor and a fixed power supply, and the sampling transistor is supplied from the signal line in a horizontal scanning period. A video signal is sampled into the pixel capacity, and the pixel capacity is An input voltage is applied to the gate of the drive transistor according to the ringed video signal, and the drive transistor supplies an output current according to the input voltage to the light emitting element, and the output current is a threshold of the drive transistor. The light emitting element emits light with a luminance corresponding to the video signal by an output current supplied from the drive transistor, and the two switching transistors have the video signal having the pixel capacitance. The threshold voltage of the drive transistor is detected and written to the pixel capacitor, thereby correcting the dependency of the output current on the threshold voltage.

  Specifically, the variable power supply takes a binary value of a high voltage and a low voltage so that the voltage between the gate and the source of the drive transistor exceeds the threshold voltage, and correction of the threshold voltage is started by applying the high voltage. The threshold voltage correction is terminated when the switching transistor connected between the transistor and the variable power supply is turned off. Further, the two switching transistors operate with a time width longer than one horizontal scanning period, and the threshold voltage of the drive transistor can be written into the pixel capacitor. Further, when the light emission of the light emitting element is finished, one switching transistor connected between the gate of the drive transistor and the fixed power source is turned on, and between the drain of the drive transistor and the variable power source. By turning off the other switching transistor connected to, a negative bias is applied to the drive transistor. In this case, the negative bias applied to the drive transistor suppresses fluctuations in the threshold voltage of the drive transistor. The fixed power supply is set such that its power supply voltage is smaller than the sum of the cathode voltage applied to the cathode of the light emitting element and the threshold voltage of the light emitting element.

  In addition, the present invention provides a pixel circuit including a light emitting element and a drive transistor for driving the signal line, which is arranged at a portion where a signal line and a required number of scanning lines intersect, and the pixel between the gate and the source of the drive transistor. A capacitor is connected, the light emitting element is connected between the source of the drive transistor and a predetermined cathode potential, a sampling transistor is connected between the gate of the drive transistor and a signal line, the drain of the drive transistor and the fixed power supply A first switching transistor is connected between the gate of the drive transistor and a variable power supply, and the sampling transistor is turned on during the horizontal scanning period from the signal line. Sampling the supplied video signal into the pixel capacity, An input voltage is applied to the gate of the drive transistor in accordance with the sampled video signal, the drive transistor supplies an output current corresponding to the input voltage to the light emitting element, and the output current is a threshold of the drive transistor. The light emitting element emits light with a luminance according to the video signal by an output current supplied from the drive transistor, and the first switching transistor and the second switching transistor have the video Before the signal is sampled in the pixel capacitor, a correction operation is performed to detect the threshold voltage of the drive transistor and write to the pixel capacitor, thereby correcting the dependency of the output current on the threshold voltage, and changing the variable The power supply switches its power supply voltage before the correction operation and passes through the second switching transistor. The gate voltage of the drive transistor is changed from high voltage to low voltage, with the voltage change that is coupled to a source voltage of the drive transistor, the field which is characterized in that the preparation operation prior to entering the correction operation.

  Specifically, as a result of the preparatory operation, after coupling, the voltage between the gate and the source of the drive transistor becomes larger than the threshold voltage of the drive transistor, and the source potential of the drive transistor is the operating point of the light emitting element. Is set to be less than In addition, the drive transistor has a dependency on the carrier mobility in addition to the threshold voltage of the channel region, and the first switching transistor cancels the dependency of the output current on the carrier mobility. A mobility correction operation that operates during a part of the horizontal scanning period, extracts an output current from the drive transistor while the video signal is sampled, and negatively feeds back the output current to the pixel capacitor to correct the input voltage I do. In this case, in order to accurately perform the mobility correction operation, the source potential of the drive transistor is set in advance to be lower than the operating point of the light emitting element.

  According to the present invention, the pixel circuit includes two switching transistors in addition to the drive transistor that drives the light emitting element and the sampling transistor that samples the video signal in the pixel capacitor. In the present invention, before the video signal is sampled into the pixel capacitance, the threshold voltage of the drive transistor is corrected by turning on and off these two switching transistors. That is, the threshold voltage of the drive transistor is detected and written to the pixel capacitance, thereby correcting the dependency of the output current on the threshold voltage. The pixel circuit of the present invention having a threshold voltage correction function is composed of a total of four transistors by adding two switching transistors in addition to the drive transistor and the sampling transistor. In this way, by simplifying and rationalizing the configuration of the pixel circuit, it is possible to reduce the number of gate lines and power supply lines that control each transistor. By reducing the number of wires, it is possible to achieve high definition and high yield of the panel.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. First, in order to clarify the background of the present invention, the basic configuration of an active matrix display device will be described with reference to FIG. As shown in the figure, this display device includes a pixel array 1, a horizontal selector 3, and a write scanner 4. The pixel array 1 is integrally formed on one panel. The horizontal selector 3 and the light scanner 4 may be built in the panel or may be externally attached. The pixel array 1 is composed of scanning lines WS arranged in rows, signal lines SL arranged in columns, and pixel circuits 2 arranged at the intersection of the two. The scanning lines WS are connected to the write scanner 4 and sequentially output control signals to sequentially select the pixel circuits 2 in units of rows. The horizontal selector 3 is connected to each signal line SL, and writes a video signal to the selected pixel circuit 2.

  FIG. 2 is a circuit diagram showing an example of the pixel circuit 2 shown in FIG. The pixel circuit 2 has the simplest configuration, and includes two transistors T1 and T5, one pixel capacitor C1, and one light emitting element EL. The sampling transistor T1 is an N-channel thin film transistor. The drive transistor T5 is a P-channel type thin film transistor. The pixel capacitor C1 is a thin film capacitor. The light emitting element EL is, for example, a two-terminal element (diode) having an organic EL thin film as a light emitting layer. These elements T1, T5, C1, and EL are integrally formed on an insulating substrate constituting the panel.

  The sampling transistor T1 is connected between the signal line SL and the gate of the drive transistor T5. The gate of the sampling transistor T1 is connected to the write scanner 4 through the scanning line WS. A pixel capacitor C1 is connected to the gate of the drive transistor T5. The source of the drive transistor T5 is connected to the power supply Vcc. The drain of the drive transistor T5 is connected to the anode of the light emitting element EL. The cathode of the light emitting element EL is grounded.

  In the horizontal scanning period, the sampling transistor T1 is applied with a control signal from the write scanner 4 and becomes conductive. As a result, the sampling transistor T1 samples the video signal supplied from the horizontal selector 3 to the signal line SL and writes it to the pixel capacitor C1. The drive transistor T5 supplies the drain current Ids to the light emitting element EL according to the video signal written in the pixel capacitor C1. As a result, the light emitting element EL emits light with a luminance corresponding to the video signal.

  In the method shown in FIG. 2, the output current Ids flowing through the light emitting element EL is controlled by changing the gate application voltage Vgs of the drive transistor in accordance with the video signal. In this example, the source of the P-channel type sampling transistor T5 is connected to the power source Vcc and is designed to always operate in the saturation region, so that it becomes a constant current source that operates according to the above-described equation 1. That is, the P-channel drive transistor T5 can always supply a constant output current Ids to the light emitting element EL according to the voltage Vgs between the gate and the source without depending on the potential of the drain connected to the light emitting element EL side.

  FIG. 3 is a graph showing the IV characteristics of the light emitting element EL. A light-emitting element typified by an organic EL element or the like has a tendency that an IV characteristic changes with time, and a solid line indicates an initial state while a dotted line indicates an IV characteristic after change with time. In the graph, the voltage V is an anode voltage. Corresponding to FIG. 2, the anode voltage V is the drain voltage of the drive transistor T5. On the other hand, the current I is the output current Ids supplied from the drive transistor T5. As described above, the pixel circuit 2 in FIG. 2 can always supply a constant output current Ids to the light emitting element EL without the drive transistor T5 depending on the drain voltage. Therefore, even if the IV characteristic of the light emitting element EL changes with time, it is possible to supply a constant current without being affected by this. Therefore, no change in luminance occurs in the light emitting element EL.

  FIG. 4 is a circuit diagram showing another example of the conventional pixel circuit 2. In order to facilitate understanding, parts corresponding to those of the prior art shown in FIG. The difference is that the drive transistor T5 is not an P-channel type but an N-channel type. In this case, the source side of the drive transistor T5 is connected to the anode side of the light emitting element EL. Therefore, the source potential is affected by the change over time of the IV characteristic of the light emitting element EL and fluctuates. The gate / source voltage Vgs changes with time of the light emitting element. As a result, the amount of output current Ids flowing through the light emitting element EL changes, and the light emission luminance changes. In addition, the threshold voltage Vth varies for each pixel circuit in the drive transistor T5. Therefore, as shown in Equation 1 above, the drain current Ids varies due to variations in Vgs and Vth, and the light emission luminance changes from pixel to pixel.

  For example, a reference example shown in FIG. 5 has been developed in advance as a pixel circuit that copes with deterioration over time of the light emitting element EL and variation in characteristics of the drive transistor T5. For ease of understanding, the reference example shown in FIG. 5 is provided with a reference number corresponding to the conventional example shown in FIG. As shown in the figure, this display device includes a pixel array 1, a horizontal selector 3, a write scanner 4, a drive scanner 5, a correction scanner 7, and a second correction scanner 8. The pixel array 1 includes pixel circuits 2 arranged in a matrix. In order to simplify the illustration, one pixel circuit 2 is shown. The pixel circuit 2 includes five transistors T1 to T5, one pixel capacitor C1, and one light emitting element EL, and has a relatively large number of elements. The pixel circuit 2 is driven by four scanning lines WS, DS, AZ, and AZ2, one signal line SL, and four power lines Vcc, Vss, Vofs, and Vcat. . There are nine control lines in total, which puts pressure on the area occupied by pixels. The scanning line WS is scanned by the write scanner 4, the DS is scanned by the drive scanner 5, AZ is controlled by the correction scanner 7, and AZ2 is controlled by the second correction scanner 8. An input signal (Vsig) is supplied from the horizontal selector 3 to the signal line SL. In this example, all transistors T1 to T5 are N-channel type. The source S of the drive transistor T5 as the center is connected to the anode of the light emitting element EL. The cathode of the light emitting element EL is connected to Vcat. The drain of the drive transistor T5 is connected to Vcc via the switching transistor T4. The gate of the switching transistor T4 is connected to the scanning line DS. The gate G of the drive transistor T5 is connected to the signal line SL via the sampling transistor T1. The gate of the sampling transistor T1 is connected to the scanning line WS. The gate G of the drive transistor T5 is connected to Vofs via the switching transistor T3. The gate of the switching transistor T3 is connected to the scanning line AZ2. A pixel capacitor C1 is connected between the gate G and the source S of the drive transistor T5. The source S of the drive transistor T5 is connected to Vss via the switching transistor T2. The gate of the switching transistor T2 is connected to the scanning line AZ.

  FIG. 6 is a timing chart for explaining the operation of the pixel circuit 2 shown in FIG. The change of ON / OFF of the transistors T1 to T4 along the time axis J is shown. The on / off control of T1 to T4 is performed by the corresponding scanner via the corresponding scanning line. This timing chart also shows changes in the potentials of the gate G and source S of the drive transistor T5. Since the transistor T4 is turned on before the timing J1, the output current is supplied to the light emitting element EL via the drive transistor T5 and the light emitting state is obtained.

  At timing J1, the transistor T3 is turned on, and the gate G of the drive transistor T5 is lowered to Vofs. Further, since the switching transistor T2 is turned on, the source S of the drive transistor T5 drops to Vss. Since Vss is lower than the threshold voltage Vthel of the light emitting element EL, no current flows through the light emitting element EL and a non-light emitting period starts. The potential difference between Vofs and Vss is larger than the threshold voltage Vth of the drive transistor T5. In this way, the threshold voltage correction operation is prepared by setting the potential across the pixel capacitor C1.

  At timing J2, the switching transistor T2 is turned off. As a result, the source S of the drive transistor T5 is disconnected from Vss and starts to rise. When current flows from the drive transistor T5 to the pixel capacitor C1, and the voltage Vgs at both ends is just equal to the threshold voltage Vth of the drive transistor T5, it is cut off. As a result, a voltage corresponding to the threshold voltage Vth of the drive transistor T5 is written to both ends of the pixel capacitor C1. The threshold cancellation operation is performed as described above.

  The switching transistor T4 is turned off at timing J3, and the switching transistor T3 is also turned off at timing J4. At this point, all the transistors T1 to T4 are turned off.

  At timing J5, the sampling transistor T1 is turned on, and the video signal Vsig supplied from the signal line SL is written to the gate G of the drive transistor T5. The sampling transistor T1 is turned off at timing J6 when the horizontal scanning period (1H) assigned to the pixel circuit 2 elapses. Signal writing was performed during this period J5-J6.

  Thereafter, the process proceeds to timing J7 and the switching transistor T4 is turned on. As a result, the drive transistor T5 is connected to the power supply Vcc and supplies an output current. The value of this output current is controlled to be constant by the input voltage Vgs held in the pixel capacitor C1. Light emission starts when the potential of the source S of the drive transistor T5 starts to rise and exceeds the threshold voltage Vthel of the light emitting element EL. Due to the bootstrap effect, the gate potential of the drive transistor T5 also rises in conjunction with the rise of the source potential. The gate / source voltage Vgs of the drive transistor T5 is always held constant by the pixel capacitor C1.

  Hereinafter, the operation of the pixel circuit according to the prior development shown in FIGS. 5 and 6 will be described in detail with reference to FIGS. First, the light emitting state of the light emitting element EL is a state in which only the switching transistor T4 is turned on as shown in FIG. At this time, since the drive transistor T5 is set to operate in the saturation region, the current Ids flowing through the light emitting element EL takes a value represented by the characteristic equation 1 according to the gate-source voltage Vgs of the drive transistor T5.

  Next, the switching transistor T3 and the switching transistor T2 are turned on in the non-light emitting period. At this time, the gate voltage of the drive transistor T5 is charged to Vofs and the source voltage is charged to Vss. The gate-source voltage of the drive transistor T5 takes a value of Vofs−Vss, and a corresponding current Ids ′ flows from Vcc to Vss. (FIG. 8) Here, in order to make the light emitting element EL emit no light, the voltage Vofs and Vss are set so that the voltage Vel applied to the light emitting element EL becomes smaller than the sum of the threshold voltage Vthel and the cathode voltage Vcat of the light emitting element EL. Need to be set. Either the switching transistor T3 or the switching transistor T2 may be turned on first.

  Further, the switching transistor T2 is turned off (FIG. 9). Since the equivalent circuit of the light emitting element EL is represented by a diode Tel and a capacitor Cel as shown in FIG. 10, as long as Vel ≦ Vcat + Vthel (the leakage current of the light emitting element EL is considerably smaller than the current flowing through the drive transistor T5). The current of the drive transistor T5 is used to charge the pixel capacitor C1 and the light emitting element capacitor Cel. At this time, the anode voltage Vel of the light emitting element (that is, the source voltage of the drive transistor) rises with time as shown in FIG. After a predetermined time has elapsed, the gate-source voltage of the drive transistor T5 takes a value of Vth. At this time, Vel = Vofs−Vth ≦ Vcat + Vthel.

After the threshold cancel operation is finished, the switching transistors T4 and T3 are turned off. By turning off the switching transistor T4 prior to the switching transistor T3, it is possible to suppress fluctuations in the gate voltage of the drive transistor T5. Next, the sampling transistor T1 is turned on and the gate voltage of the drive transistor T5 is set to the signal voltage Vsig (FIG. 12). At this time, the gate-source voltage of the drive transistor T5 is determined by the pixel capacitance C1, the parasitic capacitance Cel of the light emitting element EL, and the parasitic capacitance C2 of the drive transistor T5 as shown in the following Expression 2. However, since the light emitting element capacitance Cel is larger than the pixel capacitance C1 and the parasitic capacitance C2, the gate / source voltage Vgs of the drive transistor T5 is approximately Vsig + Vth. However, for convenience, Vofs = 0.

  After the writing is completed, the switching transistor T4 is turned on to raise the drain voltage of the drive transistor T5 to the power supply voltage Vcc. Since the gate-source voltage of the drive transistor T5 is constant, the drive transistor T5 causes a constant current Ids ″ to flow through the light emitting element EL, and Vel rises to a voltage Vx at which a current of Ids ″ flows through the light emitting element EL. The element EL emits light (FIG. 13).

  In this circuit as well, the IV characteristic of the light emitting element EL changes as the light emission time becomes longer. Therefore, the potential at point B in the figure also changes. However, since the gate / source voltage of the drive transistor T5 is maintained at a constant value, the current flowing through the light emitting element EL does not change. Therefore, even if the IV characteristic of the light emitting element EL deteriorates, the constant current Ids always flows, and the luminance of the light emitting element EL does not change.

  The pixel circuit of the reference example described above includes five transistors T1 to T5. The number of power supply lines and scanning lines (gate lines) connected to these transistors increases according to the number of transistor elements. The pixel circuit according to the reference example of FIG. 5 includes 12 power lines VCG, Vofs, Vss, and Vcat for 3 RGB pixels and 4 gate lines WS, AZ, AZ2, and DS. This increases the proportion of power supply lines and gate lines with respect to the pixels, which is difficult in terms of higher definition and higher yield of the panel.

  The present invention aims to simplify and rationalize the pixel circuit in order to cope with the above problems. FIG. 14 is a circuit diagram showing a first embodiment of a pixel circuit according to the present invention. As shown in the figure, this pixel circuit 2 is composed of four transistors T1, T2, T4, T5 and one pixel capacitor C1 per one light emitting element EL. In the reference example shown in FIG. In comparison, one transistor is simplified. Further, as a result of rationalization of driving of the pixel circuit 2, the number of wirings can be constituted by three gate lines and six power supply lines per three RGB pixels. For ease of understanding, the first embodiment shown in FIG. 14 is denoted by reference numerals corresponding to those of the reference example shown in FIG.

  As shown in FIG. 14, the pixel circuit 2 includes two switching transistors T2 and T4 in addition to the sampling transistor T1, the drive transistor T5, the pixel capacitor C1, and the light emitting element EL. All of the transistors T1, T2, T4, and T5 are N-channel type, and can be integrally formed on the panel with, for example, a polysilicon thin film transistor or an amorphous silicon thin film transistor.

  The switching transistor T4 is connected between the power supply Vcc and the drain D of the drive transistor T5. The gate of the switching transistor T4 is connected to the drive scanner 5 through the scanning line DS. This drive scanner 5 is provided for line-sequentially switching on and off the switching transistor T4. The other switching transistor T2 is connected between the signal line SL and the source S of the drive transistor T5. The gate of the switching transistor T2 is connected to the correction scanner 7 through the scanning line AZ. The correction scanner 7 is used for on / off control of the switching transistor T2 in accordance with line sequential scanning. The pixel capacitor C1 is connected between the gate G and the source S of the drive transistor T5. The anode of the light emitting element EL is connected to the source S of the drive transistor T5, and the cathode is connected to a predetermined cathode potential Vcat.

  FIG. 15 is a timing chart for explaining the operation of the pixel circuit according to the present invention shown in FIG. This timing chart represents on-off changes of the sampling transistor T1, the switching transistor T2, and the switching transistor T4 along the time axis J. In accordance with this, a change in the signal voltage supplied to the signal line SL is also shown. As illustrated, the signal line switches between a signal voltage Vsig representing a video signal, a first fixed voltage Vofs fixed at the first level, and a second fixed voltage Vss fixed at the second level. Supply. In addition, this timing chart also shows potential changes of the gate G and the source S of the drive transistor T5.

  Until the time point J1, the switching transistor T4 is on. Therefore, the drive transistor T5 is connected to the power supply Vcc, and supplies the drain current Ids corresponding to the gate voltage Vgs to the light emitting element EL. Therefore, the light emitting element EL is in the light emission period.

  At time J1, since the switching transistor T4 is turned off, the drain current Ids stops flowing, and the light emitting element EL enters a non-light emitting period. Since no current flows through the light emitting element EL, the source potential of the drive transistor T5 is lowered to Vcat + Vthel. Vthel is a threshold voltage of the light emitting element EL. In conjunction with this, the gate potential of the drive transistor T5 also decreases.

  At time J2, both the sampling transistor T1 and the switching transistor T2 are turned on. At this time, the signal voltage is at the fixed potential Vss. By connecting the source S of the drive transistor T5 to the signal line, the source potential is lowered to Vss. Further, since the gate G of the drive transistor T5 is also connected to the signal line SL, the gate potential is also lowered to Vss.

  The signal voltage is switched from Vss to Vofs at time J3 after T2 is turned off. At this time, since the sampling transistor T1 is still in the on state, the gate potential of the drive transistor T5 rises to Vofs.

  Immediately after this, at time point J4, the switching transistor T4 is turned on. As a result, the drain current Ids flows, but since the light emitting element EL is in the reverse bias state, the potential of the source S rises. When the difference between the potential of the gate G and the potential of the source S reaches the threshold voltage Vth, the drain current Ids stops flowing.

  After the drive transistor T5 is cut off, the switching transistor T4 is turned off at time J5. As a result, the threshold voltage Vth is written to the pixel capacitor C1 connected between the gate G and the source S of the drive transistor T5. The time J4-J5 during which the threshold voltage Vth is detected and written in this way is called a threshold cancellation period.

  Thereafter, the signal voltage changes from the fixed potential Vofs to the signal potential Vsig. At this time, since the sampling transistor T1 is continuously on, the video signal potential Vsig is written into the pixel capacitor C1, and the potential of the gate G of the drive transistor T5 becomes Vsig. Since the writing of the signal potential Vsig is added to the threshold voltage Vth, Vgs is Vsig + Vth. Time J6-J7 is called a signal writing period.

  Thereafter, the sampling transistor T1 is turned off at time J7, and the switching transistor T4 is turned on again at time T8. As a result, the output current Ids flows into the light emitting element EL and enters the light emission period. When the drain current Ids flows through the light emitting element EL, the potential of the source S rises. In conjunction with this, the potential of the gate G also rises. The input voltage Vgs to the drive transistor T5 is kept constant during the light emission period.

  As is apparent from the timing chart of FIG. 15, the period J2-J7 in which the sampling transistor T1 is on corresponds to approximately one horizontal scanning period (1H). During this time, the signal voltage changes from Vss to Vofs and further changes to Vsig. Further, the threshold cancel period J4-J5 and the signal writing period J6-J7 are included in the one horizontal scanning period J2-J7. In other words, the present invention performs the threshold cancellation operation and the signal writing operation in a short time of one horizontal scanning period.

  The operation of the pixel circuit according to the present invention shown in FIG. 14 will be described again with reference to FIGS. FIG. 16 shows the state of the pixel circuit 2 before the time point J1. Prior to time J1, only the switching transistor T4 is on. At this time, since the drive transistor T5 is designed to operate in the saturation region, the current Ids flowing through the light emitting element EL is expressed by the above-described equation 1 in accordance with the gate / source voltage Vgs of the drive transistor T5. Take the value.

  FIG. 17 shows the state of the pixel circuit 2 at time J1-J2. Here, the switching transistor T4 is turned off. By turning off the switching transistor T4, no current is supplied from the power source Vcc to the cathode of the light emitting element EL, so that the light emitting element EL is extinguished. The source voltage of the drive transistor T5 is the sum of the cathode voltage Vcat and the threshold voltage Vthel of the light emitting element EL, that is, a value of Vcat + Vthel.

  FIG. 18 shows the state of the pixel circuit 2 at time J2-J3. Here, the sampling transistor T1 and the switching transistor T2 are turned on. When the sampling transistor T1 is turned on, the gate G of the drive transistor T5 is charged with the fixed potential Vss. When the switching transistor T2 is turned on, the source S of the drive transistor T5 is charged with the fixed potential Vss from the signal line SL. Here, Vss is set smaller than the sum Vcat + Vthel of the cathode voltage Vcat and the threshold voltage Vthel of the light emitting element EL. In other words, the light emitting element EL does not emit light because it is in a reverse bias state of Vss <Vthel + Vcat.

FIG. 19 shows the state of the pixel circuit 2 at time J3-J4. Here, the switching transistor T2 is turned off, and the signal potential on the signal line SL is switched from Vss to Vofs. As a result, Vofs is charged in the gate G of the drive transistor T5. Since the equivalent circuit of the light emitting element EL is represented by a diode-connected transistor Tel and a capacitor Cel, the source voltage of the drive transistor T5 is determined by the pixel capacitor C1, the parasitic capacitor Cel of the light emitting element EL, and the parasitic capacitor C2 of the drive transistor T5. , And is determined as shown in Equation 3 below. Therefore, the gate / source voltage Vgs is as follows.

  FIG. 20 shows the state of the pixel circuit 2 at time J4-J5. In this state, the switching transistor T4 is turned on to start a threshold voltage canceling operation (threshold voltage correcting operation). Since the gate / source voltage Vgs of the drive transistor T5 expressed by the above equation 2 is larger than the threshold voltage Vth of the drive transistor T5, a current flows from the power source Vcc through the drive transistor T5 as shown in the figure, and the pixel capacitance Start charging C1. As described above, the equivalent circuit of the light emitting element EL is represented by the parallel connection of the diode Tel and the capacitor Cel. As long as the anode voltage Vel of the light emitting element EL is in the reverse bias state smaller than Vcat + Vthel, the leakage current flowing through the light emitting element EL is almost negligible, and the current of the drive transistor T5 is almost all of the pixel capacitance C1 and the light emitting element capacitance Cel. Used for charging. The anode voltage Vel of the light emitting element EL is equal to the source voltage of the drive transistor T5. By this charging, the anode voltage Vel rises with time. After a predetermined time has elapsed, the gate-source voltage of the drive transistor T5 takes a value of Vth and the drive transistor T5 is cut off. At this time, Vel = Vofs−Vth <Vcat + Vthel.

  FIG. 21 is a graph showing changes in the anode voltage Vel that increases with time. Since the anode voltage Vel is the source voltage of the drive transistor T5, the graph of FIG. 12 takes time on the horizontal axis, while the vertical axis shows the source voltage of the drive transistor T5 instead of the anode voltage Vel. As shown in the figure, the source voltage rises with the charging of the pixel capacitor C1, and stops when it reaches Vofs−Vth. In other words, the drive transistor T5 is cut off when the gate / source voltage Vgs of the drive transistor T5 is just equal to the threshold voltage Vth. In this way, in the cancel period J4-J5, the detection of the threshold voltage Vth of the drive transistor T5 and the writing of Vth to the pixel capacitor C1 are performed.

  FIG. 22 shows the state of the pixel circuit 2 at time J5-J7. During this period J5-J7, the video signal voltage Vsig is written. That is, after the threshold voltage cancel operation is completed, the switching transistor T4 is turned off, the voltage on the signal line SL is set to the signal voltage Vsig, and the desired signal voltage Vsig is written to the gate G of the drive transistor T5. At this time, the gate / source voltage Vgs of the drive transistor T5 is determined by the pixel capacitance C1, the parasitic capacitance Cel of the light emitting element EL, and the parasitic capacitance C2 of the drive transistor T5 as shown in Equation 2 above. However, since Cel is larger than C1 and C2, the gate / source voltage Vgs is approximately Vsig + Vth.

  FIG. 23 shows the state of the pixel circuit 2 after the time point J8. After the writing of the video signal potential is completed, the sampling transistor T1 is turned off, while the switching transistor T4 is turned on to raise the drain voltage of the drive transistor T5 to the power supply voltage Vcc. Since the gate / source voltage Vgs of the drive transistor T5 is constant at Vsig + Vth, the drive transistor T5 supplies a constant current Ids ″ to the light emitting element EL. As a result, the anode voltage Vel rises to the voltage Vx through which the current Ids ″ flows in the light emitting element EL, and the light emitting element EL emits light. Also in the pixel circuit according to this reference example, the IV characteristic of the light emitting element EL changes as the light emission time becomes longer. Therefore, the source potential of drive transistor T5 shown in FIG. 23 also changes. However, since the gate / source voltage Vgs of the drive transistor T5 is kept constant, the current flowing through the light emitting element EL does not change. Therefore, even if the IV characteristic of the light emitting element EL deteriorates, the constant current Ids always flows, and the luminance of the light emitting element EL does not change. Further, since the gate-source voltage Vgs is previously added to the signal voltage Vsig, the influence of the threshold voltage Vth of the drive transistor T5 is cancelled.

  FIG. 24 is a circuit diagram showing a modification of the pixel circuit according to the first embodiment shown in FIG. For easy understanding, portions corresponding to those of the pixel circuit shown in FIG. 14 are denoted by corresponding reference numerals. The difference is that one end of the switching transistor T2 is connected not to the signal line SL but to the gate G of the drive transistor T5. Hereinafter, this modification will be described in detail with reference to FIG.

  The pixel circuit 2 is disposed at a portion where the signal line SL and the required number of scanning lines WS, DS, AZ intersect, and includes a light emitting element EL and a drive transistor T5 for driving the light emitting element EL. A pixel capacitor C1 is connected between the gate G and source S of the drive transistor T5. A light emitting element EL is connected between the source S of the drive transistor T5 and a predetermined cathode potential Vcat. A sampling transistor T1 is connected between the gate G of the drive transistor T5 and the signal line SL. A switching transistor T4 is connected between the drain D of the drive transistor T5 and the power supply Vcc. Another switching transistor T2 is connected between the source S and the gate G of the drive transistor T5.

  The sampling transistor T1 conducts in the horizontal scanning period (1H) and samples the video signal supplied from the signal line SL into the pixel capacitor C1. The pixel capacitor C1 applies the input voltage Vsig to the gate G of the drive transistor T5 in accordance with the sampled video signal. The drive transistor T5 supplies an output current Ids corresponding to the input voltage Vsig to the light emitting element EL. This output current Ids is dependent on the threshold voltage Vth of the drive transistor T5. The light emitting element EL emits light with luminance according to the signal potential Vsig of the video signal by the output current Ids supplied from the drive transistor T5. As a feature of the present invention, the two switching transistors T2 and T4 operate before the video signal is sampled to the pixel capacitor C1, detect the threshold voltage Vth of the drive transistor T5, and write it to the pixel capacitor C1. Thus, the dependence of the output current Ids on the threshold voltage Vth is corrected.

  In the present embodiment, the signal line SL switches the signal voltage Vsig representing the video signal, the first fixed voltage Vofs fixed to the first level, and the second fixed voltage Vss fixed to the second level. Supply. The signal voltage Vsig is applied to the gate G of the drive transistor T5 when sampling the video signal, the first fixed voltage Vofs is applied to the gate G of the drive transistor T5 when correcting the threshold voltage Vth, and the second fixed voltage Vss is the threshold voltage. This is applied to the source S of the drive transistor T5 in a preparatory stage before correcting Vth. The two switching transistors T2 and T4 operate in the horizontal scanning period (1H), detect the threshold voltage Vth of the drive transistor T5, and write it in the pixel capacitor C1.

  FIG. 25 is a timing chart for explaining the operation of the pixel circuit shown in FIG. Along the time axis J, ON / OFF changes of the transistors T1, T2, and T4 are shown. Further, it represents a potential change appearing on the signal line SL. In addition, the potential changes of the gate G and source S of the drive transistor T5 are also shown. As shown in the drawing, the light emission period is from the timing J1 to the timing J8, and the light emission period is from J1 to J8. This non-emission period includes a horizontal scanning period (1H), where a correction preparation operation, a threshold cancellation operation, and a signal writing operation are performed. At the timing J1, the switching transistor T4 is turned off, and a non-light emission period starts. The source potential of the drive transistor T5 drops to a level obtained by adding the cathode voltage Vcat to the threshold voltage Vthel of the light emitting element EL. Subsequently, at timing J2, the sampling transistor T1 is turned on and the switching transistor T2 is also turned on. When the switching transistor T2 is turned on, the source potential of the drive transistor T5 is lowered to Vss. Note that the potential Vss is supplied from the signal line SL. Thereby, a threshold voltage correction preparation operation is performed. After the transistor T2 is turned off at timing Ja, the potential of the signal line SL is switched from Vss to Vofs at timing J3. Further, at timing J4, the switching transistor T4 is turned on, and the threshold cancel operation starts. When the switching transistor T4 is turned on, an output current flows and the drive transistor T5 is cut off when the source potential of the drive transistor T5 is just raised to Vth. Thereafter, the switching transistor T4 is turned off at timing J5. The above-described timing J4 to timing J5 is the threshold cancellation period. Thereafter, at timing J6, the potential of the signal line SL changes from the fixed potential Vofs to the signal potential Vsig, and signal writing is performed. At timing J7, the sampling transistor T1 is turned off, and then at timing J8, the switching transistor T4 is turned on again to enter the light emission period.

  Hereinafter, the operation of the pixel circuit 24 shown in FIG. 24 will be described in more detail with reference to FIGS. First, in the light emission period before the timing J1, only the switching transistor T4 is turned on (FIG. 26).

  Next, in a non-light emitting period, the switching transistor T4 is turned off. By turning off the switching transistor T4, no current flows through the light emitting element EL, so the light emitting element EL is extinguished and its anode voltage becomes Vcat + Vthel (FIG. 27).

  Thereafter, the sampling transistor T1 and the switching transistor T2 are turned on, the signal line potential is set to Vss, and the gate G voltage and the source S voltage of the drive transistor T5 are set to Vss (FIG. 28). Here, since Vss is smaller than the sum of the threshold voltage Vthel and the cathode voltage Vcat of the light emitting element EL, that is, since Vss ≦ Vcat + Vthel, the light emitting element EL does not emit light. Either the sampling transistor T1 or the switching transistor T2 may be turned on first.

Next, the switching transistor T2 is turned off, the signal line potential is set to Vofs, and the gate potential of the drive transistor T5 is charged with a value Vofs (FIG. 29). At this time, the gate-source voltage Vgs of the drive transistor T5 has a value represented by the following equation.
Vgs = {(Cel / (Cel + C1 + C2)} × (Vofs−Vss)

  After the writing of Vofs is completed, the switching transistor T4 is turned on to start the threshold value correcting operation (FIG. 30). Since the switching transistor T4 is turned on, a current flows from Vcc to charge the pixel capacitor C1 and the light emitting element capacitor Cel. As described above, the source voltage of the drive transistor T5 becomes Vofs−Vth after a predetermined time has elapsed.

  After the source voltage of the drive transistor T5 becomes Vofs−Vth, the switching transistor T4 is turned off and the threshold value correcting operation is finished. Thereafter, the signal line potential is set to Vsig, and the gate voltage of the drive transistor T5 is set to Vsig (FIG. 31).

  After the writing is completed, the sampling transistor T1 is turned off and the switching transistor T4 is turned on to raise the drain voltage of the drive transistor T5 to the power supply voltage Vcc (FIG. 32). Since the gate / source voltage Vgs of the drive transistor T5 is constant, the drive transistor T5 causes a constant current Ids ″ to flow through the light emitting element EL, and the anode voltage Vel of the light emitting element EL causes a current of Ids ″ to flow through the light emitting element EL. The voltage rises to the voltage Vx, and the light emitting element EL emits light.

  In this pixel circuit as well, the light emission element EL changes its IV characteristic as the light emission time becomes longer. However, since the gate-source voltage Vgs of the drive transistor T5 is maintained at a constant value, the light emission element EL The flowing current does not change. Therefore, even if the IV characteristic of the light emitting element EL deteriorates, the constant current Ids always flows, and the luminance of the light emitting element EL does not change.

  According to the present invention, since the threshold variation of the drive transistor can be suppressed, a uniform image quality without unevenness and roughness can be obtained. Since the pixel circuit of the present invention is composed of three gate lines and six power supply lines per RGB trio, the ratio of the power supply and the gate line to the pixel can be reduced, resulting in higher definition and higher performance. Yield can be expected. According to the present invention, since it is not necessary to pulse the power supply, the number of external drive circuits and drivers can be reduced, and the cost can be reduced. According to the present invention, since the gate / source voltage of the drive transistor is maintained at a constant value, the current flowing through the light emitting element EL does not change. Therefore, even if the IV characteristic of the light emitting element EL deteriorates, the constant current Ids always flows, and the luminance of the light emitting element EL does not change.

  FIG. 33 is a circuit diagram showing a second embodiment of the pixel circuit according to the present invention. This pixel circuit is also composed of four transistors T1, T3, T4, T5 and one pixel capacitor C1 per light emitting element EL. The number of wirings is 4 gate lines (scanning lines) and 6 power supply lines per 3 RGB pixels, and the wiring is simplified.

  As shown in the figure, the pixel circuit 2 includes a light emitting element EL and a drive transistor T5 that drives the signal line SL and a required number of scanning lines WS, DS, and AZ. A pixel capacitor C1 is connected between the gate G and source S of the drive transistor T5. A light emitting element EL is connected between the source S of the drive transistor T5 and a predetermined cathode potential Vcat. A sampling transistor T1 is connected between the gate G of the drive transistor T5 and the signal line SL. A switching transistor T4 is connected between the drain D of the drive transistor T5 and the variable power supply (Vcc). Another switching transistor T3 is connected between the gate G of the drive transistor T5 and the fixed power source (Vofs). The sampling transistor T1 conducts in the horizontal scanning period (1H) and samples the video signal Vsig supplied from the signal line SL into the pixel capacitor C1. The pixel capacitor C1 applies the input voltage Vgs to the gate G of the drive transistor T5 in accordance with the sampled video signal Vsig. The drive transistor T5 supplies an output current Ids corresponding to the input voltage Vgs to the light emitting element EL. This output current Ids is dependent on the threshold voltage Vth of the drive transistor T5. The light emitting element EL emits light with luminance corresponding to the video signal Vsig by the output current Ids supplied from the drive transistor T5.

  As a feature of the present invention, the two switching transistors T3 and T4 operate before the video signal Vsig is sampled to the pixel capacitor C1, detect the threshold voltage Vth of the drive transistor T5, and write it to the pixel capacitor C1. Thus, the dependency of the output current Ids on the threshold voltage Vth is corrected. The variable power supply Vcc is supplied from the power supply line DL. By taking the binary value of the high voltage Vcc_high and the low voltage Vcc_low, the voltage Vgs between the gate G and the source S of the drive transistor T5 is set to the threshold voltage Vth or higher. The correction of the threshold voltage Vth is started by applying Vcc_high, and the correction of the threshold voltage Vth is ended when the switching transistor T4 connected between the drive transistor T5 and the variable power supply Vcc is turned off. The power supply line DL is connected to the power supply line scanner 9 in order to switch the power supply Vcc between high and low in synchronization with the scanning timing. The power line scanner 9 is a driver IC having a built-in shift register like the write scanner 4, drive scanner 5, and correction scanner 7.

  The two switching transistors T3 and T4 operate in a time width longer than one horizontal scanning period (1H) and can write the threshold voltage Vth of the drive transistor T5 into the pixel capacitor C1. Before the light emission of the light emitting element EL is finished, the switching transistor T3 connected between the gate G of the drive transistor T5 and the fixed power supply Vofs is turned on, and the drain D of the drive transistor T5 and the variable power supply Vcc are connected. The drive transistor T5 is negatively biased by turning off the switching transistor T4 connected therebetween. The negative bias applied to the drive transistor T5 has an effect of suppressing fluctuations in the threshold voltage Vth of the drive transistor T5. The fixed power supply Vofs is set to have a power supply voltage smaller than the sum of the cathode voltage Vcat applied to the cathode of the light emitting element EL and the threshold voltage Vthel of the light emitting element EL.

  FIG. 34 is a timing chart for explaining the operation of the pixel circuit 2 shown in FIG. Along with the time axis J, ON / OFF changes of the transistors T1, T3, T4, a voltage change of the variable power supply Vcc, and potential changes of the gate G and the source S of the drive transistor T5 are shown. At timing J1, the transistor T3 is turned on and a reverse bias is applied to the drive transistor T5. At timing J2-J4, the variable power supply Vcc is switched to the low level Vcc_low, and preparation for threshold voltage correction is performed. Next, at timing J4-J5, the variable power supply Vcc is switched to the high level Vcc_high, and the threshold cancel operation is performed. Thereafter, the sampling transistor T1 is turned on at timings J6-J7, and signal writing is performed. Further, at timing J8, the switching transistor T4 is turned on to enter the light emission period.

  The operation of the pixel circuit 2 shown in FIG. 33 will be described in more detail with reference to FIGS. First, before the timing J1, the light emitting state of the light emitting element EL is a state in which only the switching transistor T4 is turned on as shown in FIG. At this time, since the drive transistor T5 is designed to operate in the saturation region, the value of the current flowing through the light emitting element EL takes the value represented by the characteristic equation 1 according to the gate-source voltage Vgs of the drive transistor T5. .

  Next, the switching transistor T3 is turned on in the non-light emitting period. By turning on the switching transistor T3, the gate of the drive transistor T5 takes a value of Vofs. Here, since Vofs is smaller than the sum of the threshold voltage Vthel and cathode voltage Vcat of the light emitting element EL and the threshold value Vth of the drive transistor T5, that is, Vofs ≦ Vcat + Vthel + Vth, no current flows through the light emitting element EL and the light is extinguished. Furthermore, since Vofs is smaller than Vthel + Vcat, the drive transistor T5 is reverse-biased, and the variation in the threshold voltage of the drive transistor T5 can be suppressed to a low level (FIG. 36).

  Thereafter, the power supply voltage Vcc is set to a low level (Vcc_low) (FIG. 37). At this time, since Vcc_low is smaller than the potential at point A in the drawing, that is, Vcat + Vthel, and Vofs−Vcc_low> Vth, point B in the drawing becomes the source of the drive transistor T5. As a result, point A in the figure becomes the drain, and current flows as shown in the figure. By this operation, the potential at the point A becomes Vcc_low. Either the on timing of the switching transistor T3 or the switching timing of the power supply voltage may be first, but by turning on the switching transistor T3 first, a reverse bias is applied to the drive transistor T5 as described above, thereby realizing high reliability. Is possible.

  After the potential at the point A becomes Vcc_low, the power supply voltage Vcc is again set to Vcc_high (FIG. 38). By this operation, the point A becomes the source of the drive transistor T5 again, and the gate / source voltage Vgs of the drive transistor T5 becomes Vofs−Vcc_low and becomes larger than Vth of the drive transistor T5. As shown in FIG. 38, the equivalent circuit of the light emitting element EL is represented by a diode Tel and a capacitor Cel, so that as long as Vel ≦ Vcat + Vthel (the leakage current of the light emitting element EL is considerably smaller than the current flowing through the drive transistor T5). The current of the drive transistor T5 is used to charge the pixel capacitor C1 and the light emitting element capacitor Cel. At this time, the anode voltage Vel of the light emitting element, that is, the source voltage of the drive transistor T5 rises with time as shown in FIG. After a certain time has elapsed, the gate / source voltage Vgs of the drive transistor T5 takes a value of Vth. At this time, if Vel = Vofs−Vth ≦ Vcat + Vthel, the light-emitting element EL performs the Vth cancel operation without emitting light.

  After the threshold cancel operation is finished, the switching transistors T4 and T3 are turned off. By turning off the switching transistor T4 prior to the switching transistor T3, it is possible to suppress fluctuations in the gate voltage of the drive transistor T5. Next, the sampling transistor T1 is turned on and the gate voltage of the drive transistor T5 is set to the signal voltage Vsig (FIG. 40). At this time, the gate-source voltage of the drive transistor T5 is determined by the pixel capacitance C1, the parasitic capacitance Cel of the light emitting element EL, and the parasitic capacitance C2 of the drive transistor T5 as shown in Equation 2 above. However, since Cel is larger than C1 and C2, the gate / source voltage Vgs of the drive transistor T5 is approximately Vsig + Vth.

  After the writing of the signal voltage Vsig is completed, the switching transistor T4 is turned on to raise the drain voltage of the drive transistor T5 to the power supply voltage Vcc_high. Since the gate / source voltage Vgs of the drive transistor T5 is constant, the drive transistor T5 passes a constant current Ids ″ to the light emitting element EL, and Vel rises to a voltage Vx at which a current of Ids ″ flows through the light emitting element EL. The light emitting element EL emits light (FIG. 41). Also in this pixel circuit, the light-emitting element EL changes its IV characteristic when the light emission time becomes long. Therefore, the potential at point A in the figure also changes. However, since the gate / source voltage Vgs of the drive transistor T5 is maintained at a constant value, the current flowing through the light emitting element EL does not change. Therefore, even if the IV characteristic of the light emitting element EL deteriorates, the constant current Ids always flows, and the luminance of the light emitting element EL does not change. Further, in the present invention, since the power supply line is a binary pulse, it is not necessary to newly develop a gate driver, and an existing one can be used, so that the cost can be reduced.

  According to the present invention, the threshold correction period of the drive transistor T5 can be as long as 1H or more, and uniform image quality without unevenness can be obtained even in black display. Since the pixel circuit of the present invention is composed of four transistors and one capacitor per pixel, four gate lines and six power supply lines per RGB pixel set, the ratio of the power supply and gate line to the pixels is determined. The size can be reduced, and high definition and high yield can be expected. According to the present invention, fluctuations in the threshold value of the drive transistor T5 can be kept small, so that the lifetime of the pixel can be extended. According to the present invention, since the power line is a pulse having a binary value, it is not necessary to newly develop a gate driver, and an existing one can be used, so that the cost can be reduced. According to the present invention, since the gate-source voltage of the drive transistor T5 is maintained at a constant value, the current flowing through the light emitting element EL does not change. Therefore, even if the IV characteristic of the light emitting element EL deteriorates, the constant current Ids always flows, and the luminance of the light emitting element EL does not change.

  FIG. 42 is a circuit diagram showing a third embodiment of the pixel circuit according to the present invention. In order to facilitate understanding, portions corresponding to those of the first embodiment and the second embodiment described above are denoted by corresponding reference numerals. The pixel circuit 2 is also composed of four transistors, one pixel capacitor, and one light emitting element. In this embodiment, before performing Vth correction, the gate potential of the drive transistor is once written as a high voltage and then lowered to a low voltage (voltage at the time of threshold voltage correction). By this voltage change, the potential of the source S of the drive transistor can be coupled to the cut-off operation point of the light emitting element EL. With this series of operations, preparation for threshold voltage correction operation is performed. Thus, the number of transistors can be reduced. In addition, the number of power lines and gate lines could be reduced, leading to improved panel yield.

  As shown in FIG. 42, the pixel circuit 2 is disposed at a portion where the signal line SL and the required number of scanning lines WS, DS, AZ intersect, and includes a light emitting element EL and a drive transistor T5 for driving the light emitting element EL. The pixel capacitor C1 is connected between the gate G and the source S of the drive transistor T5, the light emitting element EL is connected between the source S of the drive transistor T5 and a predetermined cathode potential, and the gate G of the drive transistor T5 and the signal line SL are connected. A sampling transistor T1 is connected between them, a first switching transistor T4 is connected between the drain D of the drive transistor T5 and the fixed power supply Vcc, and a second switching transistor is connected between the gate G of the drive transistor T5 and the variable power supply Vss. T3 is connected. The sampling transistor T1 conducts in the horizontal scanning period (1H) and samples the video signal supplied from the signal line SL into the pixel capacitor C1. The pixel capacitor C1 applies the input voltage Vgs to the gate G of the drive transistor T5 in accordance with the sampled video signal. The drive transistor T5 supplies an output current Ids corresponding to the input voltage Vgs to the light emitting element EL. This output current Ids is dependent on the threshold voltage Vth of the drive transistor T5. The light emitting element EL emits light with luminance according to the video signal by the output current Ids supplied from the drive transistor T5.

  As a feature of the present invention, the first switching transistor T4 and the second switching transistor T3 detect the threshold voltage Vth of the drive transistor T5 and write it to the pixel capacitor C1 before the video signal is sampled to the pixel capacitor C1. The operation is performed, thereby correcting the dependence of the output current Ids on the threshold voltage Vth. At that time, the variable power supply Vss switches its power supply voltage before the correction operation, and changes the gate voltage of the drive transistor T5 from the high voltage VssH to the low voltage VssL via the second switching transistor T3. By performing coupling to the source voltage of T5, a preparatory operation before entering the correction operation is performed. As a result of this preparatory operation, after coupling, the voltage between the gate G and the source S of the drive transistor T5 becomes larger than the threshold voltage Vth of the drive transistor T5, and the source potential of the drive transistor T5 determines the operating point of the light emitting element EL. Set to be below.

  In the drive transistor T5, the output current Ids depends on the carrier mobility in addition to the threshold voltage Vth of the channel region. The first switching transistor T4 operates in a part of the horizontal scanning period (1H) to cancel the dependence of the output current Ids on the carrier mobility μ, and the output current Ids from the drive transistor T5 in a state where the video signal is sampled. And a mobility correction operation for correcting the input voltage Vgs by performing negative feedback to the pixel capacitor C1. In order to accurately perform this mobility correction operation, the source potential of the drive transistor T5 is set in advance to be lower than the operating point of the light emitting element EL.

  FIG. 43 is a schematic diagram showing a portion of the pixel circuit 2 extracted from the pixel array 1 shown in FIG. In order to facilitate understanding, the video signal Vsig sampled by the sampling transistor T1, the input voltage Vgs and output current Ids of the drive transistor T5, and the capacitance component Cel included in the light emitting element EL are added. The scanning lines WS, DS, and AZ connected to the gates of the transistors are also written. FIG. 44 is a timing chart of the pixel circuit shown in FIGS. The operation of the pixel circuit shown in FIGS. 42 and 43 will be described specifically and in detail with reference to FIG.

  FIG. 44 shows the waveforms of control signals applied to the scanning lines WS, AZ, and DS along the time axis J. In order to simplify the notation, the control signals are also denoted by the same reference numerals as the corresponding scanning lines. In addition, the potential change of the variable power supply Vss is also shown along the time axis J. As shown in the figure, the variable power supply Vss becomes a high potential VssH in the first half of each horizontal scanning period H and becomes a low potential VssL in the second half. Since the transistors T1 and T3 are N-channel type, they are turned on when the scanning lines WS and AZ are at a high level and turned off when the scanning lines WS and AZ are at a low level. On the other hand, since the transistor T4 is a P-channel type, it is turned off when the scanning line DS is at a high level and turned on when it is at a low level. This timing chart also shows the change in the potential of the gate G and the change in the potential of the source S of the drive transistor T5, along with the waveforms of the control signals WS, AZ, DS and the waveform of the variable power supply Vss.

  First, at timing J1, the switching transistor T4 is turned off to emit no light. At this time, the source potential of the drive transistor T5 is lowered to the cut-off voltage Vthel of the light emitting element EL because no power is supplied from Vcc. Next, at timing J2, the switching transistor T3 is turned on. At this time, the voltage of the power supply line Vss is set to the high voltage VssH. Thus, VssH is written as the gate potential of the drive transistor T5 by turning on the switching transistor T3. At this time, coupling enters the source potential via the pixel capacitor C1, and the source potential rises. Although the source S rises once, it is discharged through the light emitting element EL, so that the source voltage becomes Vthel again. At this time, the gate voltage remains VssH.

  Next, the voltage of the Vss line is changed to VssL at timing J3 while the switching transistor T3 is kept on. This potential change is coupled to the source potential via the pixel capacitor C1. The amount of coupling at this time is obtained by C1 / (C1 + Cel) × (VssH−VssL). At this time, the gate potential is expressed as VssL, and the source potential is expressed as Vthel−C1 / (C1 + Cel) × (VssH−VssL). Since a negative bias is applied here, the source voltage becomes lower than Vthel, and the light emitting element EL is cut off. Here, the source potential is desirably set to a potential at which the light emitting element EL continues to be cut off after the subsequent Vth correction or mobility correction. Further, by adding coupling so that Vgs> Vth, preparation for Vth correction can be made. As described above, Vth correction preparation can be performed even in a pixel circuit in which transistors, power supply lines, and gate lines are reduced.

  Thereafter, at timing J4, the switching transistor T4 is turned on, whereby a current flows through the drive transistor T5, and Vth correction is performed. A current flows until the drive transistor T5 is cut off. When the drive transistor T5 is cut off, the source potential of the drive transistor T5 becomes VssL-Vth. Here, it is necessary to satisfy VssL−Vth <Vthel. Thereafter, at timing J5, the switching transistor T4 is turned off, and then the switching transistor T3 is turned off to complete the Vth correction.

  After that, at timing J6, the sampling transistor T1 is turned on, and the input voltage Vgs is changed to a voltage based on the light emission luminance and written to the gate G. Finally, the switching transistor T4 is turned on at timing J7 to perform mobility correction, and then the sampling transistor T1 is turned off at timing J8 to enter the light emission period. Details of the mobility correction will be described later.

  As described above, in the present invention, the gate voltage is changed from a high voltage to a low voltage, and the Vth correction preparation is performed by using the coupling due to the voltage change. As a result, the power source for Vth correction, the switching transistor, and its gate line can be reduced, and the yield of the panel can be improved. In addition, high definition can be achieved by reducing the layout. In this embodiment, the mobility correction is performed by turning on the switching transistor T4 while the sampling transistor T1 is turned on. However, the mobility correction is not performed by making the sampling transistor T1 and the switching transistor T4 non-overlapping. In the Vth correction operation, the number of wirings and transistors can be similarly reduced. In the pixel circuit of this embodiment, the transistors other than the switching transistor T4 are N-channel type, but the characteristics of each transistor may be N-channel or P-channel.

  According to the present invention, in an organic EL panel that employs a threshold voltage correction method or a mobility correction method, the gate potential is changed from a high voltage to a low voltage, and a coupling voltage is added to the source potential of the drive transistor T5 so as to emit light. Is cut off so that Vgs> Vth. Thus, preparation for Vth correction is performed. According to the present invention, the number of power supply lines, transistors, and gate lines can be reduced and the wiring crossover can be reduced, thereby improving the yield. At the same time, high definition panels can be achieved.

  Here, the mobility correction will be described in detail. As described above, the mobility μ is corrected at timings J7 to J8. At timing J7 before the end of the sampling period J8, the control signal DS becomes low level and the switching transistor T4 is turned on. As a result, the drive transistor T5 is connected to the power supply Vcc, so that the pixel circuit proceeds from the non-light emitting period to the light emitting period. In this manner, the mobility of the drive transistor T5 is corrected during the period J7-J8 when the sampling transistor T1 is still in the on state and the switching transistor T4 is in the on state. That is, in the present embodiment, the mobility correction is performed in the period J7-J8 in which the rear part of the sampling period and the head part of the light emission period overlap. Note that, at the beginning of the light emission period in which the mobility correction is performed, the light emitting element EL is actually in a reverse bias state, and thus does not emit light. In the mobility correction period J7-J8, the drain current Ids flows through the drive transistor T5 while the gate G of the drive transistor T5 is fixed to the level of the video signal Vsig. Here, by setting VssL−Vth <Vthel, the light emitting element EL is placed in a reverse bias state, so that it exhibits simple capacitance characteristics instead of diode characteristics. Therefore, the current Ids flowing through the drive transistor T5 is written into the capacitor C = C1 + Cel obtained by combining both the pixel capacitor C1 and the equivalent capacitor Cel of the light emitting element EL. As a result, the source potential (S) of the drive transistor T5 rises. In the timing chart of FIG. 44, this increase is represented by ΔV. This increase ΔV is eventually subtracted from the gate / source voltage Vgs held in the pixel capacitor C1, so negative feedback is applied. In this way, the mobility μ can be corrected by negatively feeding back the output current Ids of the drive transistor T5 to the input voltage Vgs of the drive transistor T5. The negative feedback amount ΔV can be optimized by adjusting the time width t of the mobility correction period J7-J8.

At timing J8, the control signal WS becomes low level and the sampling transistor T1 is turned off. As a result, the gate G of the drive transistor T5 is disconnected from the signal line SL. Since the application of the video signal Vsig is cancelled, the gate potential (G) of the drive transistor T5 can be increased and increases with the source potential (S). Meanwhile, the gate / source voltage Vgs held in the pixel capacitor C1 maintains the value of (Vsig−ΔV + Vth). As the source potential (S) rises, the reverse bias state of the light emitting element EL is canceled, so that the light emitting element EL actually starts to emit light by the inflow of the output current Ids. The relationship between the drain current Ids and the gate voltage Vgs at this time is given by the following equation by substituting Vsig−ΔV + Vth into Vgs of the previous transistor characteristic equation 1.
Ids = kμ (Vgs−Vth) ² = kμ (Vsig−ΔV) ²
In the above formula, k = (1/2) (W / L) Cox. From this characteristic equation, it can be seen that the term of Vth is canceled and the output current Ids supplied to the light emitting element EL does not depend on the threshold voltage Vth of the drive transistor T5. Basically, the drain current Ids is determined by the signal voltage Vsig of the video signal. In other words, the light emitting element EL emits light with a luminance corresponding to the video signal Vsig. At that time, Vsig is corrected by the feedback amount ΔV. This correction amount ΔV works so as to cancel the effect of the mobility μ located in the coefficient part of the above characteristic equation. Therefore, the drain current Ids substantially depends only on the video signal Vsig.

  FIG. 45 is a circuit diagram showing a state of the pixel circuit 2 in the mobility correction period J7-J8. As illustrated, in the mobility correction period J7-J8, the sampling transistor T1 and the switching transistor T4 are turned on, while the remaining switching transistor T3 is turned off. In this state, the source potential of the drive transistor T5 is VssL-Vth. This source potential is also the anode potential of the light emitting element EL. Here, by setting VssL−Vth <Vthel, the light emitting element EL is placed in a reverse bias state, and exhibits simple capacitance characteristics instead of diode characteristics. Therefore, the current Ids flowing through the drive transistor T5 flows into the combined capacitance C = C1 + Cel of the pixel capacitance C1 and the equivalent capacitance Cel of the light emitting element EL. In other words, a part of the drain current Ids is negatively fed back to the pixel capacitor C1, and the mobility is corrected.

  FIG. 46 is a graph of the transistor characteristic equation described above, with Ids on the vertical axis and Vsig on the horizontal axis. This characteristic equation is also shown below the graph. The graph of FIG. 46 shows a characteristic curve in a state where the pixel 1 and the pixel 2 are compared. The mobility μ of the drive transistor of the pixel 1 is relatively large. Conversely, the mobility μ of the drive transistor included in the pixel 2 is relatively small. Thus, when the drive transistor is composed of a polysilicon thin film transistor or the like, it is inevitable that the mobility μ varies between pixels. For example, when the video signal Vsig of the same level is written in both the pixels 1 and 2, the output current Ids 1 ′ flowing in the pixel 1 having the high mobility μ is the pixel 2 having the low mobility μ unless the mobility is corrected. A large difference is generated as compared with the output current Ids2 'flowing through the current. In this way, a large difference occurs between the output currents Ids due to the variation in the mobility μ, so that the uniformity of the screen is impaired.

  Therefore, in the present invention, the variation in mobility is canceled by negatively feeding back the output current to the input voltage side. As is clear from the transistor characteristic equation, the drain current Ids increases when the mobility is large. Therefore, the negative feedback amount ΔV increases as the mobility increases. As shown in the graph of FIG. 46, the negative feedback amount ΔV1 of the pixel 1 having a high mobility μ is larger than the negative feedback amount ΔV2 of the pixel 2 having a low mobility. Therefore, the larger the mobility μ is, the more negative feedback is applied, and the variation can be suppressed. As shown in the figure, when ΔV1 is corrected in the pixel 1 having a high mobility μ, the output current greatly decreases from Ids1 ′ to Ids1. On the other hand, since the correction amount ΔV2 of the pixel 2 having the low mobility μ is small, the output current Ids2 ′ does not decrease so much to Ids2. As a result, Ids1 and Ids2 are substantially equal, and the variation in mobility is cancelled. Since the cancellation of the variation in mobility is performed in the entire range of Vsig from the black level to the white level, the uniformity of the screen becomes extremely high. In summary, when there are pixels 1 and 2 having different mobility, the correction amount ΔV1 of the pixel 1 having high mobility is smaller than the correction amount ΔV2 of the pixel 2 having low mobility. That is, as the mobility increases, ΔV increases and the decrease value of Ids increases. As a result, pixel current values having different mobilities are made uniform, and variations in mobility can be corrected.

For reference, a numerical analysis of the mobility correction described above is performed with reference to FIG. As shown in FIG. 47, the analysis is performed by taking the source potential of the drive transistor T5 as a variable V while the transistors T1 and T4 are turned on. Assuming that the source potential (S) of the drive transistor T5 is V, the drain current Ids flowing through the drive transistor T5 is as shown in Equation 4 below.

Further, due to the relationship between the drain current Ids and the capacitance C (= C1 + Cel), Ids = dQ / dt = CdV / dt is established as shown in Equation 5 below.

Both sides are integrated by substituting Equation 4 into Equation 5. Here, the initial state of the source voltage V is -Vth, and the mobility variation correction time (J7-J8) is t. When this differential equation is solved, the pixel current with respect to the mobility correction time t is given as in Equation 6 below.

  FIG. 48 is a graph of Expression 6, in which the vertical axis represents the output current Ids and the horizontal axis represents the video signal Vsig. As the parameters, mobility correction periods t = 0 us, 2.5 us, and 5 us are set. Further, when the mobility μ is a relatively large parameter, the parameter is 1.2 μ and the relatively small mobility is 0.8 μ. It can be seen that the mobility variation is sufficiently corrected at t = 2.5 us, compared to the case where the mobility correction is not substantially applied at t = 0 us. Without mobility correction, Ids with 40% variation can be reduced to 10% or less when mobility correction is applied. However, if the correction period is lengthened with t = 5 us, the variation in the output current Ids due to the difference in mobility μ is increased. Thus, in order to apply appropriate mobility correction, it is necessary to set t to an optimal value. In the graph shown in FIG. 48, the optimum value is around t = 2.5 us.

1 is a block diagram illustrating a general configuration of an active matrix display device. It is a circuit diagram which shows the prior art example of the pixel circuit integrated in the display apparatus shown in FIG. It is a graph which shows a time-dependent change of the IV characteristic of light emitting element EL. FIG. 10 is a circuit diagram illustrating another conventional example of a pixel circuit incorporated in the display device illustrated in FIG. 1. It is a circuit diagram which shows the reference example of the pixel circuit developed prior to this invention. 6 is a timing chart for explaining the operation of the pixel circuit shown in FIG. 5. It is a circuit diagram with which it uses for operation | movement description of the pixel circuit concerning a reference example. It is a circuit diagram similarly used for operation | movement description. It is a circuit diagram similarly used for operation | movement description. It is a circuit diagram similarly used for operation | movement description. It is a graph similarly provided for operation | movement description. It is a circuit diagram similarly used for operation | movement description. It is a circuit diagram similarly used for operation | movement description. 1 is a circuit diagram illustrating a first embodiment of a pixel circuit according to the present invention. FIG. It is a timing chart with which it uses for operation | movement description of 1st Embodiment. It is a circuit diagram with which it uses for operation | movement description of 1st Embodiment. It is a circuit diagram similarly used for operation | movement description. It is a circuit diagram similarly used for operation | movement description. It is a circuit diagram similarly used for operation | movement description. It is a circuit diagram similarly used for operation | movement description. It is a graph similarly provided for operation | movement description. It is a circuit diagram similarly used for operation | movement description. It is a circuit diagram similarly used for operation | movement description. It is a circuit diagram which shows the modification of 1st Embodiment of the pixel circuit concerning this invention. FIG. 25 is a timing chart for explaining the operation of the modification shown in FIG. 24. FIG. It is a circuit diagram with which it uses for operation | movement description of a modification. It is a circuit diagram similarly used for operation | movement description. It is a circuit diagram similarly used for operation | movement description. It is a circuit diagram similarly used for operation | movement description. It is a circuit diagram similarly used for operation | movement description. It is a circuit diagram similarly used for operation | movement description. It is a circuit diagram similarly used for operation | movement description. It is a circuit diagram which shows 2nd Embodiment of the pixel circuit concerning this invention. It is a timing chart with which it uses for operation | movement description of 2nd Embodiment. It is a circuit diagram with which it uses for operation | movement description of 2nd Embodiment. It is a circuit diagram similarly used for operation | movement description. It is a circuit diagram similarly used for operation | movement description. It is a circuit diagram similarly used for operation | movement description. It is a graph similarly provided for operation | movement description. It is a circuit diagram similarly used for operation | movement description. It is a circuit diagram similarly used for operation | movement description. It is a circuit diagram which shows 3rd Embodiment of the pixel circuit concerning this invention. It is a schematic diagram with which operation | movement description of 3rd Embodiment is provided. It is a timing chart used for operation | movement description of 3rd Embodiment. It is a circuit diagram with which it uses for operation | movement description of 3rd Embodiment. It is a graph similarly provided for operation | movement description. It is a circuit diagram similarly used for operation | movement description. It is a graph similarly provided for operation | movement description.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Pixel array, 2 ... Pixel circuit, 3 ... Horizontal selector, 4 ... Write scanner, 5 ... Drive scanner, 7 ... Correction scanner, 9 ... Power supply line scanner , T1 ... sampling transistor, T2 ... switching transistor, T3 ... switching transistor, T4 ... switching transistor, T5 ... drive transistor, C1 ... pixel capacitance, EL ... light emitting element

Claims (18)

A pixel circuit that is arranged at a portion where a signal line and a required number of scanning lines intersect and includes a light emitting element and a drive transistor that drives the light emitting element,
A pixel capacitor is connected between the gate and source of the drive transistor, the light emitting element is connected between the source of the drive transistor and a predetermined cathode potential, and a sampling transistor is connected between the gate of the drive transistor and the signal line A switching transistor is connected between the drain of the drive transistor and the power supply, and another switching transistor is connected between the source of the drive transistor and the signal line,
The sampling transistor conducts a horizontal scanning period and samples a video signal supplied from the signal line into the pixel capacitor,
The pixel capacitor applies an input voltage to the gate of the drive transistor according to the sampled video signal,
The drive transistor supplies an output current corresponding to the input voltage to the light emitting element, and the output current has a dependency on a threshold voltage of the drive transistor,
The light emitting element emits light with a luminance corresponding to the video signal by an output current supplied from the drive transistor,
The two switching transistors operate before the video signal is sampled to the pixel capacitor, detect the threshold voltage of the drive transistor and write to the pixel capacitor, and thus depend on the threshold voltage of the output current. A pixel circuit characterized by correcting the characteristics.   The signal line is configured to switch and supply a signal voltage representing a video signal, a first fixed voltage fixed to a first level, and a second fixed voltage fixed to a second level. The pixel circuit according to claim 1.   The signal voltage is applied to the gate of the drive transistor when sampling the video signal, the first fixed voltage is applied to the gate of the drive transistor when correcting the threshold voltage, and the second fixed voltage is applied before correcting the threshold voltage. 3. The pixel circuit according to claim 2, wherein the pixel circuit is supplied to the source of the drive transistor in the preparation stage.   2. The pixel circuit according to claim 1, wherein the two switching transistors operate during the horizontal scanning period, detect a threshold voltage of the drive transistor, and write to the pixel capacitor. A pixel circuit that is arranged at a portion where a signal line and a required number of scanning lines intersect and includes a light emitting element and a drive transistor that drives the light emitting element,
A pixel capacitor is connected between the gate and source of the drive transistor, the light emitting element is connected between the source of the drive transistor and a predetermined cathode potential, and a sampling transistor is connected between the gate of the drive transistor and the signal line A switching transistor is connected between the drain of the drive transistor and the power supply, and another switching transistor is connected between the source and gate of the drive transistor,
The sampling transistor conducts a horizontal scanning period and samples a video signal supplied from the signal line into the pixel capacitor,
The pixel capacitor applies an input voltage to the gate of the drive transistor according to the sampled video signal,
The drive transistor supplies an output current corresponding to the input voltage to the light emitting element, and the output current has a dependency on a threshold voltage of the drive transistor,
The light emitting element emits light with a luminance corresponding to the video signal by an output current supplied from the drive transistor,
The two switching transistors operate before the video signal is sampled to the pixel capacitor, detect the threshold voltage of the drive transistor and write to the pixel capacitor, and thus depend on the threshold voltage of the output current. A pixel circuit characterized by correcting the characteristics.   The signal line is configured to switch and supply a signal voltage representing a video signal, a first fixed voltage fixed to a first level, and a second fixed voltage fixed to a second level. The pixel circuit according to claim 5.   The signal voltage is applied to the gate of the drive transistor when sampling the video signal, the first fixed voltage is applied to the gate of the drive transistor when correcting the threshold voltage, and the second fixed voltage is applied before correcting the threshold voltage. The pixel circuit according to claim 6, wherein the pixel circuit is supplied to the source of the drive transistor in the preparation stage.   6. The pixel circuit according to claim 5, wherein the two switching transistors operate during the horizontal scanning period, detect a threshold voltage of the drive transistor, and write to the pixel capacitor. A pixel circuit that is arranged at a portion where a signal line and a required number of scanning lines intersect and includes a light emitting element and a drive transistor that drives the light emitting element,
A pixel capacitor is connected between the gate and source of the drive transistor, the light emitting element is connected between the source of the drive transistor and a predetermined cathode potential, and a sampling transistor is connected between the gate of the drive transistor and the signal line A switching transistor is connected between the drain of the drive transistor and the variable power source, and another switching transistor is connected between the gate of the drive transistor and the fixed power source,
The sampling transistor conducts a horizontal scanning period and samples a video signal supplied from the signal line into the pixel capacitor,
The pixel capacitor applies an input voltage to the gate of the drive transistor according to the sampled video signal,
The drive transistor supplies an output current corresponding to the input voltage to the light emitting element, and the output current has a dependency on a threshold voltage of the drive transistor,
The light emitting element emits light with a luminance corresponding to the video signal by an output current supplied from the drive transistor,
The two switching transistors operate before the video signal is sampled to the pixel capacitor, detect the threshold voltage of the drive transistor and write to the pixel capacitor, and thus depend on the threshold voltage of the output current. A pixel circuit characterized by correcting the characteristics.   The variable power supply takes a binary value of a high voltage and a low voltage so that the voltage between the gate and the source of the drive transistor exceeds the threshold voltage, and correction of the threshold voltage is started by the application of the high voltage. 10. The pixel circuit according to claim 9, wherein the correction of the threshold voltage is terminated when the switching transistor connected to the variable power source is turned off.   10. The pixel circuit according to claim 9, wherein the two switching transistors operate in a time width longer than one horizontal scanning period and can write a threshold voltage of the drive transistor to the pixel capacitor.   When the light emission of the light emitting element is finished, one switching transistor connected between the gate of the drive transistor and the fixed power source is turned on, and between the drain of the drive transistor and the variable power source. 10. The pixel circuit according to claim 9, wherein a negative bias is applied to the drive transistor by turning off the other connected switching transistor.   The pixel circuit according to claim 12, wherein the negative bias applied to the drive transistor suppresses a variation in a threshold voltage of the drive transistor.   14. The pixel circuit according to claim 13, wherein the fixed power supply is set to have a power supply voltage smaller than a sum of a cathode voltage applied to a cathode of the light emitting element and a threshold voltage of the light emitting element. A pixel circuit that is arranged at a portion where a signal line and a required number of scanning lines intersect and includes a light emitting element and a drive transistor that drives the light emitting element,
A pixel capacitor is connected between the gate and source of the drive transistor, the light emitting element is connected between the source of the drive transistor and a predetermined cathode potential, and a sampling transistor is connected between the gate of the drive transistor and the signal line A first switching transistor is connected between the drain of the drive transistor and the fixed power source, and a second switching transistor is connected between the gate of the drive transistor and the variable power source,
The sampling transistor conducts a horizontal scanning period and samples a video signal supplied from the signal line into the pixel capacitor,
The pixel capacitor applies an input voltage to the gate of the drive transistor according to the sampled video signal,
The drive transistor supplies an output current corresponding to the input voltage to the light emitting element, and the output current has a dependency on a threshold voltage of the drive transistor,
The light emitting element emits light with a luminance corresponding to the video signal by an output current supplied from the drive transistor,
The first switching transistor and the second switching transistor perform a correction operation of detecting a threshold voltage of the drive transistor and writing to the pixel capacitor before the video signal is sampled to the pixel capacitor, thereby performing the correction operation. Correct the dependency of the output current on the threshold voltage,
The variable power supply switches its power supply voltage before the correction operation, changes the gate voltage of the drive transistor from a high voltage to a low voltage via the second switching transistor, and changes the voltage change to the source of the drive transistor. A pixel circuit which performs a preparatory operation before entering the correction operation by coupling to a voltage.   As a result of the preparatory operation, after coupling, the voltage between the gate and source of the drive transistor is larger than the threshold voltage of the drive transistor, and the source potential of the drive transistor is less than the operating point of the light emitting element. The pixel circuit according to claim 15, wherein the pixel circuit is set. In the drive transistor, the output current depends on the carrier mobility in addition to the threshold voltage of the channel region,
The first switching transistor operates in a part of the horizontal scanning period to cancel the dependence of the output current on the carrier mobility, and extracts the output current from the drive transistor in a state where the video signal is sampled. 16. The pixel circuit according to claim 15, wherein a mobility correction operation for correcting the input voltage by performing negative feedback on the pixel capacitance is performed. 18. The pixel circuit according to claim 17, wherein a source potential of the drive transistor is set in advance to be lower than an operating point of the light emitting element in order to perform the mobility correction operation accurately.

Images