CN104333239B - A kind of fully integrated AC DC transducer of high efficiency - Google Patents

Abstract

The invention discloses a high-efficiency fully integrated AC-DC converter, comprising a first AC signal input port, a second AC signal input port, a first linear voltage regulator, a second linear voltage regulator, a ground output port, A direct current output port and a self-controlled rectifier with a three-gate cross structure composed of the first transistor to the sixth transistor. The invention combines the rectifier and the voltage stabilizer, saves the large capacitor behind the rectifier, has small output capacitance, is easy to integrate and has small output ripple; it includes a self-controlled rectifier with a three-gate cross structure, and a self-controlled rectifier with a three-gate cross structure The method replaces the multiplexer, does not need the pulse wave signal generated by the multiplexer, solves the reverse leakage current problem of the active diode structure, and ensures the conversion efficiency. The invention can be widely applied in the technical field of integrated circuits.

Description

A High Efficiency Fully Integrated AC-DC Converter

technical field

The invention relates to the technical field of integrated circuits, in particular to a high-efficiency fully integrated AC-DC converter.

Background technique

Wireless power transmission technology based on inductive coupling is more and more widely used in systems where batteries are inconvenient to use or cannot be installed due to factors such as battery size, service life and cost control, such as biomedical implants, radio frequency identification tags and Short-range wireless communication equipment, etc. This energy self-excitation system based on inductive coupling usually includes three parts: reader, induction coil and transponder. The transponder acquires the carrier signal sent by the reader through the induction coil, and converts the AC signal into a DC voltage through the AC-DC converter. The block diagram of a traditional wireless power transmission system is shown in Figure 1. Usually, the AC-DC converter includes two parts: rectification and voltage regulation, and each part requires a large coupling capacitor.

A rectifier is a device used to convert an AC signal into a DC signal. The most basic rectifier is a diode rectifier, but whether it is a rectifier diode or a diode-connected MOSFET (metal-oxide-semiconductor-field-effect transistor), there will be a large conduction voltage drop, which limits the amplitude and voltage conversion efficiency, so conventional diode rectifiers are rarely used in integrated systems. In the past decade, full-wave rectifiers based on active diodes have become mainstream. This kind of full-wave rectifier uses four power transistors, by controlling their conduction and cut-off to realize the flow direction of current. Therefore, theoretically speaking, the difference between the output and input voltages is only determined by the on-resistance of the transistor and the load current, and has nothing to do with the threshold voltage of the power transistor, thereby significantly reducing the internal power consumption of the rectifier and greatly improving the conversion efficiency. In fact, one of the main problems affecting its efficiency under this structure is the synchronization problem of transistors: when the load is large, the transistor needs to be designed with a large size to meet the requirements of current density and efficiency, but the gate capacitance of a large-sized transistor is relatively large. Large, resulting in a long charge and discharge time, so the rectifier is easy to form a relatively long reverse conduction path at high frequency, which affects the efficiency of the rectifier. Therefore, how to avoid the reverse conduction path formed by simultaneous conduction of the transistors is the difficulty in the design of the rectifier.

The DC voltage output by the rectifier changes significantly due to factors such as the relative distance and angle of the induction coil, and the ripple is large, so a voltage regulator is needed to obtain a stable DC voltage after the transponder is rectified. For wireless energy transfer system circuits, the most basic requirement of a voltage regulator is low static power consumption. Coupled with cost issues, a linear regulator (LDO) that does not require an inductor and is easy to fully integrate has become the first choice for this type of application. However, the quiescent current of the LDO is extremely low, the transient performance is weak and the suppression ability of the power supply noise is low. In addition, in order to improve the integration level, the circuit of the wireless energy transmission system should minimize the large capacitance outside the chip. Therefore, no off-chip output capacitor, fast transient response, high power supply noise suppression capability, low static power consumption and stable LDO are the difficulties in voltage regulator design.

For biomedical implants that require full integration and low cost, improving the performance of the AC-DC converter and saving area are two key issues that need to be considered. The indicators to measure the performance of AC-DC converters are the ability to convert AC signals into DC signals and the ability to provide voltage and current, specifically, power conversion efficiency (PCE) and voltage conversion efficiency and output ripple. The current rectifiers generally can only achieve a power conversion efficiency of less than 82%, and the power conversion efficiency of an LDO can reach about 95%, so the PCE of the entire AC-DC converter is . The voltage conversion efficiency (VCR) is related to the size of the output impedance and the voltage drop of each component of the signal transmission loop. The voltage drop of the entire AC-DC converter includes the rectification voltage drop and the voltage regulation voltage drop. The output ripple is related to the output capacitor. The rectifier generally has a large storage capacitor, and the voltage regulator also needs a large voltage stabilizing capacitor, and these two output capacitors are generally in the order of μF, which takes up too much layout area. , is not conducive to integration. If the output capacitance is reduced, stability issues and ripple suppression methods need to be considered.

Aiming at the problem of large output capacitance, T.J Sun proposed a new circuit structure - a rectifier regulator (rectigulator), which combines the rectifier and the regulator to directly convert the AC signal into a stable DC signal output, thereby saving Remove the large capacitor behind the rectifier, but it still needs a μF capacitor, as shown in Figure 2. Figure 3 shows the specific circuit schematic diagram of the rectigulator proposed by T.JSun. The circuit consists of four parts: four transistors, two operational amplifiers, two two-to-one multiplexers, and a DC voltage divider circuit. Among them, four transistors (M1, M2, M3, and M4) are the main path of the AC-DC converter. The gates of transistors M1 and M2 are driven by operational amplifiers, and they are alternately turned on, passing the signal from the AC input to the DC output. ; Transistors M3 and M4 are gate cross-gate (cross-gate) structure, alternate conduction, through the DC signal from the ground to the AC input. This structure can be regarded as an active diode rectifier driven by a general operational amplifier and a pair of transistors shared by an LDO to stabilize voltage while rectifying. Moreover, only one capacitor is needed at the output end of this structure, and this capacitor simultaneously serves as the storage capacitor of the rectifier and the stabilizing capacitor of the voltage regulator. However, in this structure, the input signal of the operational amplifier is a pulse wave signal generated by a multiplexer, which is similar to the driving signal of a switch tube of a DC-DC converter, so the output will have relatively large noise. Secondly, the structure is essentially an active diode driven by an operational amplifier. When the frequency is high, the operational amplifier has an offset (offset), which leads to the formation of leakage current and further reduces the conversion efficiency.

Contents of the invention

In order to solve the above technical problems, the object of the present invention is to provide a fully integrated AC-DC converter with high efficiency, which is easy to integrate, has small output ripple and no reverse leakage current.

The technical solution adopted by the present invention to solve its technical problems is:

A high-efficiency fully integrated AC-DC converter, including a first AC signal input port, a second AC signal input port, a first linear voltage regulator, a second linear voltage regulator, a ground output port, a DC output port and A self-controlled rectifier with a three-gate cross structure composed of the first transistor to the sixth transistor, the first AC signal input port is connected to the source of the first transistor, the gate of the second transistor, the drain of the third transistor, and the first transistor respectively. The gates of the four transistors are connected, the source of the third transistor is connected to the source of the fourth transistor, the gate of the third transistor and the drain of the fourth transistor are connected to the DC output port, and the first The linear regulator is connected in parallel between the source and the drain of the fourth transistor; the second AC signal input port is respectively connected to the gate of the first transistor, the source of the second transistor, the drain of the fifth transistor and The gate of the sixth transistor is connected, the source of the fifth transistor is connected to the source of the sixth transistor, the gate of the fifth transistor and the drain of the sixth transistor are both connected to the DC output port, the first The two linear regulators are connected in parallel between the source and drain of the sixth transistor; the drain of the first transistor and the drain of the second transistor are both connected to the ground output port.

Further, the relationship between the first transistor and the sixth transistor satisfies: if the voltage of the first AC signal input port is greater than the voltage of the DC output port, the second transistor and the third transistor are both turned on, and the first transistor and the fourth transistor and the fifth transistor are all off; if the voltage of the first AC signal input port is less than the voltage of the DC output port, the second transistor and the third transistor are all off, and the first transistor and the fourth transistor are all on; if the second AC signal The voltage of the input port is greater than the voltage of the DC output port, the first transistor and the fifth transistor are all turned on, and the second transistor, the third transistor and the sixth transistor are all turned off; if the voltage of the second AC signal input port is lower than the DC output port voltage, the first transistor and the fifth transistor are all turned off, and the second transistor and the sixth transistor are both turned on.

Further, the first linear voltage regulator or the second linear voltage regulator includes a first-stage amplifier circuit, a second-stage amplifier circuit, a third-stage amplifier circuit, a feedforward diode, a high-pass filter, and a Miller compensation capacitor, so The first-stage amplifying circuit includes a bias current mirror, a differential input circuit and a current mirror load, the second-stage amplifying circuit includes a common-source amplifying transistor and a current amplifying current mirror, and the third-stage amplifying circuit includes a seventh transistor and Load, the source of the seventh transistor is respectively connected to the cathode of the feedforward diode, the current amplification current mirror and one end of the bias current mirror, and the other end of the bias current mirror is connected to the differential input circuit, and the differential input One input terminal of the circuit is connected to the load, the other input terminal of the differential input circuit is connected to the reference voltage, the differential input circuit is also connected to the current mirror load, and the current mirror load is also connected to the Miller compensation capacitor and the common source The amplifying transistor is connected, and the Miller compensation capacitor is connected to the DC output port; the grid of the seventh transistor is respectively connected to the anode of the feedforward diode, the current amplification current mirror and the high-pass filter, and the current mirror load, common source Both the amplifying transistor and the high-pass filter are connected to the ground output port, and both the current amplifying current mirror and the high-pass filter are also connected to the input voltage; the drain of the seventh transistor is respectively connected to the load and the DC output port.

Further, the bias current mirror is a PMOS current mirror, the current mirror load is an NMOS current mirror, and the differential input circuit is a PMOS differential pair transistor input circuit.

Further, the first transistor and the second transistor are all NMOS transistors, and the third transistor, the fourth transistor, the fifth transistor, the sixth transistor and the seventh transistor are all PMOS transistors.

The beneficial effects of the present invention are: combining the rectifier and the voltage stabilizer, eliminating the large capacitor behind the rectifier, the output capacitor is small, easy to integrate and the output ripple is small; The self-control method of the crossover structure replaces the multiplexer without the pulse wave signal generated by the multiplexer, which solves the reverse leakage current problem of the active diode structure and ensures the conversion efficiency. Further, the linear regulator includes a differential input amplifier stage, a high-gain stage, a high-pass filter, a feed-forward diode, a Miller compensation capacitor, and a seventh transistor, and the linearity is improved through a cooperative feed-forward method of the high-pass filter and the diode-connected transistor The regulator's power supply noise rejection capability further reduces output ripple.

Description of drawings

The present invention will be further described below in conjunction with drawings and embodiments.

Figure 1 is a schematic block diagram of a traditional wireless power transfer system;

Fig. 2 is a functional block diagram of a wireless power transmission system based on a rectigulator structure;

Fig. 3 is a schematic circuit diagram of the rectigulator structure;

Fig. 4 is the circuit schematic diagram of a kind of high-efficiency fully integrated AC-DC converter of the present invention;

FIG. 5 is a schematic circuit diagram of the first linear voltage regulator or the second linear voltage regulator of the present invention.

detailed description

Referring to FIG. 4, a high-efficiency fully integrated AC-DC converter includes a first AC signal input port AC1, a second AC signal input port AC2, a first linear voltage regulator LLDO, a second linear voltage regulator RLDO, A ground output port, a DC output port DC Output, and a three-gate cross-structure self-controlled rectifier composed of the first transistor to the sixth transistor M1-M6, the first AC signal input port AC1 is respectively connected to the source of the first transistor M1, The gate of the second transistor M2, the drain of the third transistor M3 are connected to the gate of the fourth transistor M4, the source of the third transistor M3 is connected to the source of the fourth transistor M4, and the third transistor M3 The gate of the gate and the drain of the fourth transistor M4 are both connected to the DC output port DCOutput, and the first linear voltage regulator LLDO is connected in parallel between the source and the drain of the fourth transistor M4; the second AC signal The input port AC2 is respectively connected to the gate of the first transistor M1, the source of the second transistor M2, the drain of the fifth transistor M5 and the gate of the sixth transistor M6, and the source of the fifth transistor M5 is connected to the gate of the sixth transistor M6. The source of the transistor M6 is connected, the gate of the fifth transistor M5 and the drain of the sixth transistor M6 are both connected to the DC output port DC Output, and the second linear regulator RLDO is connected in parallel to the sixth transistor M6 Between the source and the drain; the drain of the first transistor M1 and the drain of the second transistor M2 are both connected to the ground output port.

As a further preferred embodiment, the relationship between the first transistor M1 and the sixth transistor M6 satisfies: if the voltage of the first AC signal input port AC1 is greater than the voltage of the DC output port DC Output, the second transistor M2 and the third transistor M3 are all turned on, and the first transistor M1, the fourth transistor M4 and the fifth transistor M5 are all turned off; if the voltage of the first AC signal input port AC1 is less than the voltage of the DC output port DC Output, the second transistor M2 and the third transistor Both M3 are turned off, and both the first transistor M1 and the fourth transistor M4 are turned on; if the voltage of the second AC signal input port AC2 is greater than the voltage of the direct current output port DC Output, the first transistor M1 and the fifth transistor M5 are both turned on, The second transistor M2, the third transistor M3 and the sixth transistor M6 are all turned off; if the voltage of the second AC signal input port AC2 is lower than the voltage of the direct current output port DC Output, the first transistor M1 and the fifth transistor M5 are all turned off, and the second transistor M5 is turned off. Both the second transistor M2 and the sixth transistor M6 are turned on.

Referring to Fig. 5, further as a preferred embodiment, the first linear voltage regulator or the second linear voltage regulator includes a first-stage amplifying circuit 1, a second-stage amplifying circuit 2, a third-stage amplifying circuit 3, a feedforward Diode 4, high-pass filter 5 and Miller compensation capacitor Cm, the first-stage amplifying circuit 1 includes a bias current mirror 11, a differential input circuit 12 and a current mirror load 13, and the second-stage amplifying circuit includes a common source amplifier A transistor 21 and a current amplifying current mirror 22, the third stage amplifying circuit includes a seventh transistor M7 and a load load, the source of the seventh transistor M7 is respectively connected to the cathode of the feedforward diode 4, the current amplifying current mirror 22 and the bias One end of the current mirror 11 is connected, the other end of the bias current mirror 11 is connected to the differential input circuit 12, one input end of the differential input circuit 12 is connected to the load load, and the other input of the differential input circuit 12 The terminal is connected to the reference voltage Vref, and the differential input circuit 12 is also connected to the current mirror load 13, and the current mirror load 13 is also respectively connected to the Miller compensation capacitor Cm and the common source amplifier transistor 21, and the Miller compensation capacitor Cm is connected to the common source amplifier transistor 21. The DC output port DCOutput is connected; the gate of the seventh transistor M7 is connected with the anode of the feedforward diode 4, the current amplification current mirror 22 and the high-pass filter 5, the current mirror load 13, the common source amplification transistor 21 and the high-pass filter The filters 5 are all connected to the ground output port, and the common current amplifying current mirror 22 and the high-pass filter 5 are both connected to the input voltage VIN; the drains of the seventh transistor M7 are respectively connected to the load load and the DC output port DC Output .

Wherein, the first amplifying circuit and the second amplifying circuit constitute a two-stage operational amplifier for amplifying the feedback signal of the load at the output end. The Miller compensation capacitor Cm is used to ensure the stability of the entire linear regulator loop.

As a further preferred embodiment, the bias current mirror 11 is a PMOS current mirror, the current mirror load 13 is an NMOS current mirror, and the differential input circuit 12 is a PMOS differential pair transistor input circuit.

As a further preferred embodiment, the first transistor M1 and the second transistor M2 are both NMOS transistors, and the third transistor M3, the fourth transistor M4, the fifth transistor M5, the sixth transistor M6 and the seventh transistor M7 are all For the PMOS tube.

The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

Embodiment one

Referring to Fig. 4, the first embodiment of the present invention:

The AC-DC converter of the present invention is a simple two-port network, wherein AC1 and AC2 are input ports, and DCOutput and GND output ports are output ports. The whole AC-DC converter consists of two main parts: a self-controlled rectifier consisting of three gate crossover structures and a linear regulator with high supply voltage rejection ratio. These two parts share an output capacitor, and the size only needs 4nF, which is easy to integrate on-chip.

Among them, the core of the rectifier is a three-gate cross structure. Transistors M3 and M5 are the main paths of the AC signal from the input end to the output end, while transistors M1 and M2 are the main path of the DC signal from the input end to the ground. The transistors M4 and M6 are switches for realizing self-control, so as to avoid using a multiplexer to generate an external pulse signal to control the working path of the rectifier as in the rectigulator structure.

The working process of the whole circuit of the present invention is as follows:

In a half cycle, when the voltage of the input terminal AC1 is higher than the output voltage of the DC output port, M3 is turned on, and the current passes through LLDO from M3 to the DC output terminal, and at the same time, M2 also turns on the current through the ground to AC1, while M5 is turned off. to avoid reverse leakage current from the DC output to ground. At this time, M4 is turned off, and M1 is also turned off. The main signal path is shown by the dotted line and the arrow in Figure 4. The converter mainly uses LLDO to stabilize the voltage. In this state, the DC voltage output by the converter is , among them, Vref is the reference voltage, R1 and R2 are the divider resistors of the converter. And when AC1 is smaller than the signal at the output end, M3 is turned off, and the main signal path is turned off. At this time, LLDO does not work, and M2 is also turned off. Because the transient response of LLDO is very fast, its off time is very short, so it can effectively avoid the passage of reverse leakage current. At this time, M4 is turned on, and the voltage stabilization is realized by charging and discharging the storage capacitor at the DC output end; M1 is also turned on, so that the circuits on the left and right sides work alternately. The working process of the input terminal AC2 is similar to that of the input terminal AC1.

In order to further improve the conversion efficiency of the converter, the present invention also requires an LDO with small output capacitance, high transient response and strong power ripple suppression capability. FIG. 5 is a specific circuit schematic diagram of the LDO of the present invention. The present invention adopts a PMOS tube differential input and a two-stage operational amplifier (first-stage amplifying circuit and second-stage amplifying circuit) of N-tube active amplification to amplify the feedback signal of the load. Among them, the first-stage amplifying circuit is used for the common-source amplification of the differential input tube, and the load current mirror 13 is used as its load; the second-stage amplifying circuit (high gain stage) is the main circuit for increasing the gain, and the current amplifying current mirror 22 takes the first The output current of the second stage is increased by 10 times, thereby increasing the transconductance, reducing the output impedance, and increasing the bandwidth. Such an output stage can also increase the swing of the output voltage. In the present invention, the feed-forward diode and the high-pass filter together form a feed-forward path, and feed forward the power supply ripple to the gate of the transistor M7. The diode-connected PMOSFET is mainly used to feed-forward the low-frequency ripple, and the high-pass filter is mainly used to feed-forward the mid-high frequency ripple. These two signals are superimposed at the gate of transistor M7. The feed-forward path improves the power supply noise suppression capability of the mid-frequency band and reduces output ripple. In addition, the present invention also uses a 1pF Miller compensation capacitor Cm to ensure the stability of the entire loop.

The present invention proposes a new AC-DC conversion circuit structure, which is similar to the rectigulator structure idea, combining the rectifier and the voltage stabilizer to save the large capacitor behind the rectifier. However, different from the rectigulator structure, the present invention adopts a three-gate cross-gate structure self-control method to avoid the use of multiplexers, and solves the reverse leakage current problem of the active diode structure. The MOS tube connected with the filter and the diode cooperates with the feed-forward method to improve the power supply noise suppression ability of the LDO and reduce the output ripple. The AC-DC converter of the present invention is suitable for a low-cost wireless power transmission system, is simple and practical, easy to integrate and has high performance, and is especially suitable for use with biomedical implant devices.

The above is a specific description of the preferred implementation of the present invention, but the invention is not limited to the described embodiments, and those skilled in the art can also make various equivalent deformations or replacements without violating the spirit of the present invention. , these equivalent modifications or replacements are all within the scope defined by the claims of the present application.

Claims (4)

1. A high-efficiency fully integrated AC-DC converter, characterized in that it includes a first AC signal input port (AC1), a second AC signal input port (AC2), a first linear voltage regulator (LLDO), The second linear voltage regulator (RLDO), the ground output port, the DC output port (DC Output) and the three-gate cross-structure self-controlled rectifier composed of the first transistor to the sixth transistor (M1-M6), the first AC The signal input port (AC1) is respectively connected to the source of the first transistor (M1), the gate of the second transistor (M2), the drain of the third transistor (M3) and the gate of the fourth transistor (M4), so The source of the third transistor (M3) is connected to the source of the fourth transistor (M4), and the gate of the third transistor (M3) and the drain of the fourth transistor (M4) are both connected to the DC output port (DC Output) connection, the first linear voltage regulator (LLDO) is connected in parallel between the source and drain of the fourth transistor (M4); the second AC signal input port (AC2) is connected to the first transistor ( The gate of M1), the source of the second transistor (M2), the drain of the fifth transistor (M5) and the gate of the sixth transistor (M6) are connected, and the source of the fifth transistor (M5) is connected to the gate of the sixth transistor (M5). The sources of the six transistors (M6) are connected, the gate of the fifth transistor (M5) and the drain of the sixth transistor (M6) are both connected to the DC output port (DC Output), and the second linear voltage regulator (RLDO) connected in parallel between the source and drain of the sixth transistor (M6); the drain of the first transistor (M1) and the drain of the second transistor (M2) are both connected to the ground output port; The relationship between the first transistor (M1) and the sixth transistor (M6) satisfies: if the voltage of the first AC signal input port (AC1) is greater than the voltage of the DC output port (DC Output), the second transistor (M2) and The third transistor (M3) is all turned on, and the first transistor (M1), the fourth transistor (M4) and the fifth transistor (M5) are all turned off; if the voltage of the first AC signal input port (AC1) is lower than the DC output port ( DC Output) voltage, the second transistor (M2) and the third transistor (M3) are both off, the first transistor (M1) and the fourth transistor (M4) are both on; if the second AC signal input port (AC2) The voltage of the DC output port (DC Output) is greater than the voltage of the DC output port (DC Output), the first transistor (M1) and the fifth transistor (M5) are both turned on, the second transistor (M2), the third transistor (M3) and the sixth transistor (M6 ) are all cut off; if the voltage of the second AC signal input port (AC2) is lower than the voltage of the DC output port (DC Output), the first transistor (M1) and the fifth transistor (M5) are both cut off, and the second transistor (M2) and the sixth transistor (M6) are both turned on. 2. A high-efficiency fully integrated AC-DC converter according to claim 1, characterized in that: the first linear regulator (LLDO) or the second linear regulator (RLDO) comprises a first Stage amplifier circuit (1), second stage amplifier circuit (2), third stage amplifier circuit (3), feedforward diode (4), high pass filter (5) and Miller compensation capacitor (Cm), the first The primary amplifier circuit (1) includes a bias current mirror (11), a differential input circuit (12) and a current mirror load (13), and the second-stage amplifier circuit includes a common source amplifier NMOSFET (21) and an amplifier current mirror ( 22), the third-stage amplifying circuit includes a seventh transistor (M7) and a load (load), the source of the seventh transistor (M7) is respectively connected to the cathode of the feed-forward diode (4), and a common source amplifying NMOSFET ( 21) is connected to one end of the bias current mirror (11), the other end of the bias current mirror (11) is connected to the differential input circuit (12), and one input end of the differential input circuit (12) is connected to the load ( load), the other input terminal of the differential input circuit (12) is connected to the reference voltage, the differential input circuit (12) is also connected to the current mirror load (13), and the current mirror load (13) is also connected to the The Miller compensation capacitor (Cm) is connected to the amplifying current mirror (22), and the Miller compensation capacitor (Cm) is connected to the DC output port (DC Output); the gate of the seventh transistor (M7) is respectively connected to the feedforward The anode of the diode (4), the common-source amplifying NMOSFET (21) and the high-pass filter (5) are connected, and the current mirror load (13), the amplifying current mirror (22) and the high-pass filter (5) are all connected to the ground output port connected, the common-source amplifying NMOSFET (21) and the high-pass filter (5) are both connected to the input voltage VIN; the drain of the seventh transistor (M7) is connected to the load (load) and the DC output port (DCOutput) respectively . 3. A high-efficiency fully integrated AC-DC converter according to claim 2, characterized in that: the bias current mirror (11) is a PMOS current mirror, and the current mirror load (13) is an NMOS A current mirror, the differential input circuit (12) is a PMOS differential pair tube input circuit. 4. A high-efficiency fully integrated AC-DC converter according to claim 3, characterized in that: the first transistor (M1) and the second transistor (M2) are both NMOS transistors, and the third The transistor (M3), the fourth transistor (M4), the fifth transistor (M5), the sixth transistor (M6) and the seventh transistor (M7) are all PMOS transistors.