TWI855277B - Active pixel sensor - Google Patents

Abstract

An active pixel sensor includes a photo sensor being conductive when radiated by X-ray; a driving compensation circuit connected to the photo sensor at a first node, for driving the photo sensor, before X-ray radiation, thedriving compensation circuit compensating a voltage of a second node inside the driving compensation circuit; a read circuit connected to the driving compensation circuit, for reading a read current through the driving compensation circuit; and a current-voltage conversion circuit connected to the read circuit for converting the read current from the read circuit into an output voltage.

Description

Main pixel sensing circuit

The present invention relates to a main motion pixel sensing circuit.

Active pixel sensor (APS) is commonly used in complementary metal oxide semiconductor photo sensors (CMOS sensors) as the driving circuit of photo detectors (PD). The main function of APS is to convert the light intensity detected by the photo sensor into a voltage value. APS has the advantages of increasing resolution and reducing external noise.

However, for existing APS, when exposed to X-rays, the critical voltage of the APS's internal transistors is prone to change, causing the APS's output current and output voltage to change accordingly. This in turn affects the accuracy and stability of the APS.

Therefore, how to design a new APS circuit so that even if it is exposed to X-rays, the output current and output voltage of the APS will not change, thus improving the accuracy and stability of the APS.

The present invention relates to a main motion pixel sensing circuit, comprising: a photo sensor, which is turned on when X-rays are irradiated; a drive compensation circuit connected to the photo sensor for driving the photo sensor, the drive compensation circuit and the photo sensor are connected to a first node, wherein before X-rays are irradiated, the drive compensation circuit compensates a second node A voltage of the drive compensation circuit is sensed, and the second node is located in the drive compensation circuit; a read circuit is connected to the drive compensation circuit, and the read circuit reads a read current flowing through the drive compensation circuit; and a current-voltage conversion circuit is connected to the read circuit, and the current-voltage conversion circuit is used to convert the read current read by the read circuit into an output voltage.

In order to better understand the above and other aspects of the present invention, the following is a specific example and a detailed description with the attached drawings as follows:

100, 400: Main pixel sensing circuit

110, 410: drive compensation circuit

120, 420: Light sensor

130, 430: Reading circuit

140, 440: Current-voltage conversion circuit

150, 450: Reset circuit

T11~T25: Transistor

D1, D2: Photodiodes

CPIN1, CPIN2: input capacitor

141, 441: Operational amplifier

C1, C2: capacitors

S1, S2: switch

S, B, A: nodes

P1~P4: Stage

L31, L32: Curve

I: Read current

Figure 1 shows a circuit diagram of an active pixel sensor (APS) according to the first embodiment of the present invention.

Figures 2A to 2D show the operation schematic diagram of the main dynamic pixel sensing circuit according to the first embodiment of the present invention.

Figure 3 shows the transfer curve according to the first embodiment of the present invention.

Figure 4 shows the circuit diagram of the main motion pixel sensing circuit according to the second embodiment of the present invention.

Figures 5A to 5D show the operation schematic diagram of the main dynamic pixel sensing circuit according to the second embodiment of the present invention.

The technical terms in this manual refer to the customary terms in this technical field. If this manual explains or defines some terms, the interpretation of these terms shall be based on the explanation or definition in this manual. Each embodiment disclosed in this disclosure has one or more technical features. Under the premise of possible implementation, a person with ordinary knowledge in this technical field can selectively implement some or all of the technical features in any embodiment, or selectively combine some or all of the technical features in these embodiments.

First embodiment

FIG. 1 shows a circuit diagram of an active pixel sensor (APS) according to the first embodiment of the present invention. As shown in FIG. 1, the active pixel sensor circuit 100 according to the first embodiment of the present invention includes: a drive compensation circuit 110, a photo sensor 120, a readout circuit 130, a current-voltage conversion circuit 140, and a reset circuit 150.

The drive compensation circuit 110 includes a first transistor T11, a second transistor T12 and a storage capacitor CST.

The first transistor T11 is a dual gate transistor, including: a first gate (e.g., a top gate) connected to the first node S; a second gate (e.g., a bottom gate) connected to the second node B; a drain connected to the third node A; and a source connected to the bias source VSS (also referred to as the first bias source). The driving compensation circuit 110 can drive the photo sensor 120 and compensate the voltage of the second node B.

The second transistor T12 includes: a gate receiving the second scanning signal SCAN2; a drain connected to the second node B; and a source connected to the third node A.

The storage capacitor CST is connected between the third bias source VCOM2 and the second node B.

The photo sensor 120 is connected to the drive compensation circuit 110. The photo sensor 120 includes a photo diode D1 and an input capacitor CPIN1 connected in parallel. The photo diode D1 and the input capacitor CPIN1 are connected between a bias source VCOM (also referred to as a second bias source) and a first node S. When X-rays are irradiated, the photo diode D1 (photo sensor 120) is turned on. Here, the photo diode is used as an example for explanation, but in other embodiments of the present case, other photoelectric conversion elements can also be used instead.

The read circuit 130 is connected to the drive compensation circuit 110. The read circuit 130 reads a read current flowing through the drive compensation circuit 110. The read circuit 130 includes, for example but not limited to, a third transistor T13. The third transistor T13 includes: a gate electrode receiving a read signal READ; a drain electrode connected to the current-voltage conversion circuit 140; and a source electrode connected to the third node A.

The current-voltage conversion circuit 140 is connected to the read circuit 130. The current-voltage conversion circuit 140 is used to convert the read current read by the read circuit 130 into an output voltage VO. The current-voltage conversion circuit 140 includes: an operational amplifier 141, a capacitor C1 and a switch S1. The operational amplifier 141 includes: a positive input terminal connected to the second reference voltage VREF2; a negative input terminal connected to the drain of the third transistor T13; an output terminal, generating an output voltage VO. The capacitor C1 is connected in parallel to the switch S1. The capacitor C1 and the switch S1 are connected between the negative input terminal and the output terminal of the operational amplifier 141. The switch S1 is controlled by the control signal CLR.

The reset circuit 150 is connected to the drive compensation circuit 110. The reset circuit 150 resets the first node S. The reset circuit 150 includes, for example but not limited to, a fourth transistor T14. The fourth transistor T14 includes: a gate receiving a first scanning signal SCAN1; a drain connected to the first node S; and a source connected to the first reference voltage VREF1.

Figures 2A to 2D show the operation schematic diagrams of the main dynamic pixel sensing circuit 100 according to the first embodiment of the present invention. Figure 2A shows the operation schematic diagram in the initial stage P1. Figure 2B shows the operation schematic diagram in the compensation stage P2. Figure 2C shows the operation schematic diagram in the illumination stage P3. Figure 2D shows the operation schematic diagram in the reading stage P4.

As shown in Figure 2A, in the initial stage P1, the first scan signal SCAN1, the second scan signal SCAN2 and the read signal READ make the second transistor T12, the third transistor T13 and the fourth transistor T14 conductive. Since the second transistor T12 and the third transistor T13 are conductive, the potential of the second node B and the third node A is equal to the second reference voltage VREF2; and, since the fourth transistor T14 is conductive, the potential of the first node S is equal to the first reference voltage VREF1. Since the potential of the first node S is equal to the first reference voltage VREF1, the potential of the first node S turns on the first transistor T11. That is, in the initial stage P1, the potential of the first node S is reset to the first reference voltage VREF1.

As shown in Figure 2B, in the compensation phase P2, the potential of the first node S controls the first transistor T11 to be turned on, the second scan signal SCAN2 controls the second transistor T12 to remain turned on, but the read signal READ controls the third transistor T13 to be turned off, and the first scan signal SCAN1 controls the fourth transistor T14 to be turned off. The voltage of the second node B is discharged to the bias source VSS through the second transistor T12 and the first transistor T11 until the first transistor T11 is turned off. When the first transistor T11 is turned off, the voltage of the third node A is equal to f(VTH) and the voltage of the second node B is equal to f(VTH), where VTH represents the critical voltage of the first transistor T11. f(VTH) represents a function with critical voltage VTH as a variable.

FIG. 3 shows the transfer curve according to the first embodiment of the present invention. In the first embodiment of the present invention, before illumination, the transfer curve is L32 (representing that VTH has not changed); after illumination, the transfer curve changes to L31. The critical voltage VTH change is -4V as an example for explanation, but it should be known that the present invention is not limited to this. If the critical voltage VTH change caused by illumination is -4V, in the first embodiment of the present invention, after illumination, the second gate electrode of the first transistor T11 is made equal to -4V (that is, f(VTH)=-4V), then the transfer curve can be changed from L31 to L32. That is, after compensation, no matter which transfer curve is changed during irradiation, it can be compensated back to the transfer curve (i.e. L32) where the first transistor is considered to have no critical voltage change under the same T11_VGS (gate-source voltage difference of the first transistor T11).

As shown in Figure 2C, in the illumination stage P3, the main dynamic pixel sensing circuit 100 is exposed to X-rays. When exposed to X-rays, the voltage of the first node S will be discharged to the bias source VCOM through the photodiode D1. Assuming that after the illumination stage P3 ends, the voltage of the first node S decreases by △V, then after the illumination stage P3 ends, the voltage of the first node S can be expressed as: S=VREF1-△V.

As shown in Figure 2D, in the reading phase P4, the direction of the reading current I is shown in Figure 2D. The reading current I flows from the operational amplifier 141 through the third transistor T13 and the first transistor T11 to the bias source VSS.

The read current I is the conduction current of the first transistor T11. In the first embodiment of the present case, the transfer curve has been compensated to a transfer curve without critical voltage change during the compensation stage. Therefore, even if the critical voltage of the first transistor T11 changes due to X-ray irradiation, this critical voltage change will not affect the conduction current of the first transistor T11, that is, it will not affect the read current I. Therefore, when the current-voltage conversion circuit 140 converts the read current into the output voltage VO, the output voltage VO is not affected by the change of the critical voltage of the first transistor T11.

In the first embodiment of the present case, transistors T11~T13 can be NMOS transistors or PMOS transistors, both of which are within the scope of the present case. In addition, the first transistor T11 has a critical voltage compensation function regardless of whether it is an enhancement type or a depletion type element.

From the above, it can be seen that in the first embodiment of the present case, through compensation, the output voltage VO can be made not affected by the change of the critical voltage of the bipolar gate transistor (caused by X-ray irradiation). Therefore, the main dynamic pixel sensing circuit 100 of the first embodiment of the present case has better accuracy and stability.

Second embodiment

FIG. 4 shows a circuit diagram of a main dynamic pixel sensing circuit according to the second embodiment of the present invention. As shown in FIG. 4, the main dynamic pixel sensing circuit 400 according to the second embodiment of the present invention includes: a drive compensation circuit 410, a light sensor 420, a read circuit 430, a current-voltage conversion circuit 440 and a reset circuit 450.

The drive compensation circuit 410 includes a first transistor T21, a second transistor T22, a third transistor T23 and a storage capacitor CST.

The first transistor T21 is a double gate transistor, including: a first gate (e.g., a top gate), connected to the first node S; a second gate (e.g., a bottom gate), connected to the second node B; a drain, connected to the third node A; and a source, connected to the bias source VSS. The drive compensation circuit 410 can drive the photo sensor 420 and compensate the voltage of the second node B.

The second transistor T22 includes: a gate, receiving the second scanning signal SCAN2; a drain, connected to the second node B; and a source, connected to the third node A.

The third transistor T23 includes: a gate receiving the third scanning signal SCAN3; a drain connected to its own gate; and a source connected to the third node A. In the second embodiment of the present case, the critical voltage compensation range can be increased (that is, a larger critical voltage variation range can be tolerated) through the third transistor T23 and the third scanning signal SCAN3, and the reason will be described below.

The storage capacitor CST is connected between the third bias source VCOM2 and the second node B.

The photo sensor 420 is connected to the drive compensation circuit 110. The photo sensor 420 includes a photo diode D2 and an input capacitor CPIN2 connected in parallel. The photo diode D2 and the input capacitor CPIN2 are connected between the bias source VCOM and the first node S. When the X-ray is irradiated, the photo diode D2 (photo sensor 420) is turned on. Here, the photo diode is used as an example for explanation, but in other embodiments of the present case, other photoelectric conversion elements can also be used instead.

The read circuit 430 is connected to the drive compensation circuit 410. The read circuit 430 reads a read current flowing through the drive compensation circuit 410. The read circuit 430 includes, for example but not limited to, a fourth transistor T24. The fourth transistor T24 includes: A gate receiving a read signal READ; a drain connected to the current-voltage conversion circuit 440; and, a source connected to the third node A.

The current-voltage conversion circuit 440 is connected to the read circuit 430. The current-voltage conversion circuit 440 is used to convert the read current read by the read circuit 430 into an output voltage VO. The current-voltage conversion circuit 440 includes: an operational amplifier 441, a capacitor C2 and a switch S2. The operational amplifier 441 includes: a positive input terminal connected to the second reference voltage VREF2; a negative input terminal connected to the drain of the fourth transistor T24; an output terminal, generating an output voltage VO. The capacitor C2 is connected in parallel to the switch S2. The capacitor C2 and the switch S2 are connected between the negative input terminal and the output terminal of the operational amplifier 441. The switch S2 is controlled by the control signal CLR.

The reset circuit 450 is connected to the drive compensation circuit 410. The reset circuit 450 resets the first node S. The reset circuit 450 includes, for example but not limited to, a fifth transistor T25. The fifth transistor T25 includes: a gate receiving a first scan signal SCAN1; a drain connected to the first node S; and a source connected to the first reference voltage VREF1.

Figures 5A to 5D show the operation schematic diagrams of the main dynamic pixel sensing circuit 400 according to the second embodiment of the present invention. Figure 5A shows the operation schematic diagram in the initial stage P1. Figure 5B shows the operation schematic diagram in the compensation stage P2. Figure 5C shows the operation schematic diagram in the illumination stage P3. Figure 5D shows the operation schematic diagram in the reading stage P4.

As shown in FIG. 5A, in the initial stage P1, the first scanning signal SCAN1, the second scanning signal SCAN2 and the third scanning signal SCAN3 make the second transistor T22, the third transistor T23 and the fifth transistor T25 conductive. Since the second transistor T22 and the third transistor T23 are conductive, the potentials of the second node B and the third node A are equal to the potential VGH (which represents the logical high level of the third scanning signal SCAN3); and, since the fifth transistor T25 is conductive, the potential of the first node S is equal to the first reference voltage VREF1. Since the potential of the first node S is equal to the first reference voltage VREF1, the potential of the first node S turns on the first transistor T21. That is to say, in the initial stage P1, the potential of the first node S is reset to the first reference voltage VREF1.

In the second embodiment of the present case, in the initial stage P1, the potentials of the second node B and the third node A are equal to the potential VGH (which represents the logical high level of the third scanning signal SCAN3). Therefore, by adjusting the potential VGH (which represents the logical high level of the third scanning signal SCAN3), the potentials of the second node B and the third node A can be adjusted arbitrarily, thereby obtaining a larger compensation range.

As shown in FIG. 5B , in the compensation phase P2, the potential of the first node S controls the first transistor T21 to be turned on, the second scan signal SCAN2 controls the second transistor T22 to be turned on, the third scan signal SCAN3 controls the third transistor T23 to be turned off, the read signal READ controls the fourth transistor T24 to be turned off, and the first scan signal SCAN1 controls the fifth transistor T25 to be turned off. The voltage of the second node B is discharged to the bias source VSS through the second transistor T22 and the first transistor T21 until the first transistor T21 is turned off. When the first transistor T21 is turned off, the voltage of the third node A is equal to f(VTH) and the voltage of the second node B is equal to f(VTH), where VTH represents the critical voltage of the first transistor T21. f(VTH) represents a function with the critical voltage VTH as a variable.

FIG. 3 is also applicable to the second embodiment. Similar to the first embodiment, in the second embodiment of the present invention, before illumination, the transfer curve is L32 (representing that VTH has not changed); after illumination, the transfer curve changes to L31. The critical voltage VTH change is -4V as an example for explanation, but it should be known that the present invention is not limited to this. If the critical voltage VTH change is -4V, in the second embodiment of the present invention, after illumination, the second gate voltage of the first transistor T21 is made equal to -4V, then the transfer curve can change from L31 to L32. That is, after compensation, no matter which transfer curve is changed during irradiation, it can be compensated back to the transfer curve (i.e. L32) where the first transistor is considered to have no critical voltage change under the same T21_VGS (gate-source voltage difference of the first transistor T21).

As shown in Figure 5C, in the illumination stage P3, the main dynamic pixel sensing circuit 400 is exposed to X-rays. When exposed to X-rays, the voltage of the first node S will be discharged to the bias source VCOM through the photodiode D2. Assuming that after the illumination stage P3 ends, the voltage of the first node S decreases by △V, then after the illumination stage P3 ends, the voltage of the first node S can be expressed as: S=VREF1-△V.

As shown in Figure 5D, in the read phase P4, the direction of the read current I is as shown in Figure 5D. The read current I flows from the operational amplifier 441 through the fourth transistor T24 and the first transistor T21 to the bias source VSS.

The read current I is the conduction current of the first transistor T21. In the second embodiment of the present case, the transfer curve has been compensated to a transfer curve without critical voltage change during the compensation stage. Therefore, even if the critical voltage of the first transistor T21 changes due to X-ray irradiation, this critical voltage change will not affect the conduction current of the first transistor T21, that is, it will not affect the read current I. Therefore, when the current-voltage conversion circuit 440 converts the read current into the output voltage VO, the output voltage VO is not affected by the change of the critical voltage of the first transistor T21.

In the second embodiment of the present case, transistors T21~T25 can be NMOS transistors or PMOS transistors, both of which are within the scope of the present case. In addition, the first transistor T21 has a critical voltage compensation function regardless of whether it is an enhancement type or a depletion type device.

From the above, it can be seen that in the second embodiment of the present case, through compensation, the output voltage VO can be made not affected by the change of the critical voltage of the bipolar gate transistor (caused by X-ray irradiation). Therefore, the main dynamic pixel sensing circuit 100 of the second embodiment of the present case has better accuracy and stability.

In summary, although the present invention has been disclosed as above by the embodiments, it is not intended to limit the present invention. Those with common knowledge in the technical field to which the present invention belongs can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention shall be subject to the scope defined in the attached patent application.

100: Main pixel sensing circuit

110: Drive compensation circuit

120: Light sensor

130: Reading circuit

140: Current-voltage conversion circuit

150: Reset circuit

T11~T14: Transistor

D1: Photodiode

CPIN1: Input capacitor

141: Operational amplifier

C1: Capacitor

S1: switch

S, B, A: nodes

Claims (6)

A main motion pixel sensing circuit includes: a photo sensor, which is turned on when X-rays are irradiated; a drive compensation circuit connected to the photo sensor to drive the photo sensor, the drive compensation circuit and the photo sensor are connected to a first node, wherein before the X-rays are irradiated, the drive compensation circuit compensates a voltage of a second node according to a critical voltage function, and the second node is located in the drive compensation circuit; a read circuit connected to the drive compensation circuit, the read circuit A circuit reads a read current flowing through a first transistor of the drive compensation circuit, the read current being related to the voltage of the second node; and a current-to-voltage conversion circuit connected to the read circuit, the current-to-voltage conversion circuit being used to convert the read current read by the read circuit into an output voltage, wherein the drive compensation circuit comprises: the first transistor, a second transistor and a storage capacitor; the first transistor comprises: a first gate connected to the first node; a second A gate connected to the second node; a drain connected to a third node; and a source connected to a first bias source; the second transistor includes: a gate receiving a second scanning signal; a drain connected to the second node; and a source connected to the third node; the storage capacitor is connected between a third bias source and the second node; the read circuit includes a third transistor, the third transistor includes: a gate receiving a read signal; a drain connected to the A current-to-voltage conversion circuit; and a source connected to the third node; and the current-to-voltage conversion circuit includes an operational amplifier, a first capacitor and a first switch, the operational amplifier includes: a first terminal connected to a second reference voltage; a second terminal connected to the drain of the third transistor; an output terminal, generating the output voltage, the first capacitor is connected in parallel to the first switch, and the first capacitor and the first switch are connected between the second terminal of the operational amplifier and the output terminal. The main dynamic pixel sensing circuit as described in claim 1 further includes: a reset circuit connected to the drive compensation circuit, resetting a potential of the first node to a first reference voltage, the reset circuit includes a fourth transistor, the fourth transistor includes: a gate, receiving a first scanning signal; a drain, connected to the first node; and, a source, connected to the first reference voltage. The main dynamic pixel sensing circuit as described in claim 2, wherein, in an initial stage, the first scanning signal, the second scanning signal and the read signal make the fourth transistor, the second transistor and the third transistor conductive, the potentials of the second node and the third node equal to the second reference voltage, and the potential of the first node equal to the first reference voltage; in a compensation stage after the initial stage, the potential of the first node controls the first transistor to be conductive, the second scanning signal controls the second transistor to be conductive, the read signal controls the third transistor to be closed, the first scanning signal controls the fourth transistor to be closed, and the first scanning signal controls the fourth transistor to be closed. The voltage of the second node is discharged to the first bias source through the second transistor and the first transistor until the first transistor is turned off. When the first transistor is turned off, the voltage of the second node is equal to a critical voltage related to the first transistor. In an illumination phase after the compensation phase, after X-ray irradiation, the voltage of the first node is discharged to a second bias source through the photo sensor. After the illumination phase ends, the voltage of the first node is related to the first reference voltage and a voltage drop variation. In a reading phase after the illumination phase, the current-voltage conversion circuit converts the read current into the output voltage. A main motion pixel sensing circuit includes: a photo sensor, which is turned on when X-rays are irradiated; a drive compensation circuit connected to the photo sensor for driving the photo sensor, the drive compensation circuit and the photo sensor are connected to a first node, wherein before the X-rays are irradiated, the drive compensation circuit compensates a voltage of a second node according to a critical voltage function, and the second node is located in the drive compensation circuit; a reading circuit connected to the drive compensation circuit, the reading circuit reads a second node flowing through the drive compensation circuit A read current of a transistor, the read current is related to the voltage of the second node; and a current-voltage conversion circuit connected to the read circuit, the current-voltage conversion circuit is used to convert the read current read by the read circuit into an output voltage, wherein, the drive compensation circuit includes: the first transistor, a second transistor, a third transistor and a storage capacitor; the first transistor includes: a first gate connected to the first node; a second gate connected to the second node; a drain connected to a third node ; and, a source connected to a first bias source; the second transistor includes: a gate receiving a second scanning signal; a drain connected to the second node; and, a source connected to the third node; the third transistor includes: a gate receiving a third scanning signal; a drain connected to the gate of the third transistor; and, a source connected to the third node; the storage capacitor is connected between a third bias source and the second node; the read circuit includes a fourth transistor, the fourth transistor includes: a gate, Receive a read signal; a drain connected to the current-voltage conversion circuit; and a source connected to the third node; and the current-voltage conversion circuit includes an operational amplifier, a second capacitor and a second switch, the operational amplifier includes: a first end connected to a second reference voltage; a second end connected to the drain of the fourth transistor; an output end, generating the output voltage, the second capacitor is connected in parallel to the second switch, and the second capacitor and the second switch are connected between the second end of the operational amplifier and the output end. The main dynamic pixel sensing circuit as described in claim 4 further includes: a reset circuit connected to the drive compensation circuit, resetting a potential of the first node to a first reference voltage, the reset circuit includes a fifth transistor, the fifth transistor includes: a gate, receiving a first scanning signal; a drain, connected to the first node; and, a source, connected to the first reference voltage. The main dynamic pixel sensing circuit as described in claim 5, wherein, in an initial stage, the first scanning signal, the second scanning signal and the third scanning signal make the fifth transistor, the second transistor and the third transistor conductive, the potentials of the second node and the third node are equal to a logical high level of the third scanning signal, and the potential of the first node is equal to the first reference voltage; in a compensation stage after the initial stage, the potential of the first node controls the first transistor to be conductive, the second scanning signal controls the second transistor to be conductive, the third scanning signal controls the third transistor to be closed, the read signal controls the fourth transistor to be closed, and the first scanning signal controls the The fifth transistor is controlled to be turned off, and the voltage of the second node is discharged to the first bias source through the second transistor and the first transistor until the first transistor is turned off. When the first transistor is turned off, the voltage of the second node is equal to a critical voltage related to the first transistor. In an illumination phase after the compensation phase, at X After light irradiation, the voltage of the first node is discharged to a second bias source through the photo sensor. After the illumination phase ends, the voltage of the first node is related to the first reference voltage and a voltage drop variation. In a reading phase after the illumination phase, the current-voltage conversion circuit converts the read current into the output voltage.

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