CN207743952U - A kind of charge pump system for weakening charge and sharing effect - Google Patents

Abstract

The utility model discloses the charge pump systems that a kind of decrease charge shares effect.The circuit includes the starting module being sequentially connected electrically, bootstrap reference source module, mirror module, switch module and amplifier module.Starting module is used to control unlatching and the off-state of bootstrap reference source module.Bootstrap reference source module is exported for generating constant current signal to mirror module.For controlling electric current, mirror image exports mirror module according to a certain percentage.Switch module is used to control the generation of charging and discharging currents, and according to charging control signal or discharge control signal that external circuit provides, charge or discharge are carried out to output loading.Two voltage signals that amplifier module is used to control switch module output are identical, it is ensured that charging current is consistent with discharge current.A kind of above-mentioned charge pump system for weakening charge and sharing effect, eliminates charging and discharging currents asymmetrical effect, avoids charge and share effect, improve the stability of phaselocked loop.

Description

A Charge Pump System with Reduced Charge Sharing Effect

technical field

The utility model relates to the technical field of integrated circuits, in particular to a charge pump system which weakens the charge sharing effect.

Background technique

In the wireless transceiver system at this stage, the phase-locked loop, as a clock frequency multiplier, occupies an extremely important position in the entire system. The charge pump phase-locked loop has become the most widely used phase-locked loop structure due to its short frequency capture time and wide linear range of work. The charge-pump phase-locked loop mainly includes five modules: a frequency detector, a charge pump, a loop filter, a voltage-controlled oscillator, and a frequency divider. Among them, the charge pump is used as the interface between the digital circuit part and the analog circuit part in the phase-locked loop. Its main function is to convert the digital signal output by the frequency and phase detector into a corresponding analog signal, and then control the frequency change of the voltage-controlled oscillator. It can be seen that the charge pump circuit plays an important role in the performance of the entire phase-locked loop system.

When the phase-locked loop is in the locked state, the output signal of the charge pump must be stable at a certain fixed value to ensure the stability of the output frequency of the voltage-controlled oscillator. Because there is a certain capacitance in the mos tube, in the design of the charge pump phase-locked loop, there will inevitably be non-idealities such as different switching time delays, charge and discharge current mismatches, and charge sharing, making the output voltage unstable and causing voltage-controlled oscillations. The output frequency jitter of the converter reduces the stability of the overall system.

Utility model content

The purpose of the utility model is to provide a charge pump system that weakens the charge sharing effect, which has the advantages of good sensitivity, eliminates the asymmetric effect of charge and discharge current, avoids the charge sharing effect, and improves the stability of the phase-locked loop.

A charge pump system for weakening the charge sharing effect, comprising a start-up module, a bootstrap reference source module, a mirror module, a switch circuit module and an operational amplifier module electrically connected in sequence; wherein,

The startup module is used to control the on and off states of the bootstrap reference source module;

The bootstrap reference source module is used to generate a constant current signal and output it to the mirror image module;

The mirror image module is used to control the mirror output of the current according to a certain ratio;

The switch module is used to control the generation of charge and discharge current, and charge or discharge the output load according to the charge control signal or discharge control signal provided by the external circuit;

The operational amplifier module is used to control the two voltage signals output by the switch circuit to be the same, so as to ensure the consistency of the charging current and the discharging current.

In one of the embodiments, the startup module includes mos tubes M1, M2, M3 and M4; wherein,

The source of the mos tube M1 and the drain of the mos tube M4 are connected to the power supply voltage VDD; the gate of the mos tube M1 is respectively connected to the drain of the mos tube M1 and the source of the mos tube M2; the gates of the mos tube M2 are respectively It is connected to the drain of mos tube M2, the drain of mos tube M3, and the gate of mos tube M4; the gate of mos tube M3 is connected to the drain of mos tube M3; the source of mos tube M3 is grounded; the gate of mos tube M4 The source is connected to the bootstrap reference source module.

In one of the embodiments, the bootstrap reference source module includes mos transistors M5, M6, M7, M8, M9, M10 and resistor R; wherein,

The source of the mos tube M5 and one end of the resistor R are grounded; the gate of the mos tube M5 is respectively connected to the source of the mos tube M6 and the other end of the resistor R; the drain of the mos tube M5 is respectively connected to the gate of the mos tube M6 , the source of the mos tube M7 is connected to the startup module; the grid of the mos tube M6 is connected to the grid of the mos tube M8; the grid of the mos tube M7 is respectively connected to the grid of the mos tube M8 and the mirror image module; The source of the mos tube M7 is connected to the drain of the mos tube M10; the source of the mos tube M8 is connected to the drain of the mos tube M9; the gate of the mos tube M9 is respectively connected to the gate of the mos tube M10 and the mirror module connected; the source of the mos tube M9 and the source of the mos tube M10 are connected to the power supply voltage.

In one of the embodiments, the mirror module includes mos tubes M11, M12, M13, M14, M15, M16, M17, M18, M19, M20 and M21; wherein,

The source of the mos tube M11, the source of the mos tube M12, the source of the mos tube M17, the source of the mos tube M18, and the source of the mos tube M26 are all grounded; the source of the mos tube M14 and the source of the mos tube M15 , the source of the mos tube M16, the source of the mos tube M20, and the source of the mos tube M21 are all connected to the power supply voltage VDD;

The gate of the mos tube M11 is respectively connected to the gate of the mos tube M12, the drain of the mos tube M11 and the drain of the mos tube M13; the drain of the mos tube M12 is respectively connected to the drain of the mos tube M15 and the gate of the mos tube M15 pole and the gate of the mos tube M16; the gate of the mos tube M13 is connected to the bootstrap reference source module; the source of the mos tube M13 is connected to the drain of the mos tube M14; the gate of the mos tube M14 is connected to the The bootstrap reference source module is connected; the drain of the mos tube M16 is respectively connected to the drain of the mos tube M17, the gate of the mos tube M26 and the operational amplifier module; the gate of the mos tube M17 is connected to the gate of the mos tube M18 ; MOS tube M18 is connected to the source of M19; the gate of MOS tube M19 is connected to the drain of MOS tube M19; the drain of MOS tube M19 is respectively connected to the drain of MOS tube M20, the gate of MOS tube M20 and the mos The gate of the tube M21 is connected; the drain of the mos tube M26 is connected with the switch module;

In one of the embodiments, the switch module includes: MOS tubes M22, M23, M24 and M25; wherein,

The mos tube M22 and the mos tube M23 form a charging switch, which provides a pull-up current for the output load according to the charging control signal, and charges the output load; the mos tube M24 and the mos tube M25 form a discharge switch, which provides a pull-down current for the output load according to the discharge control signal , to discharge the output load; the source of the mos tube M22 and the source of the mos tube M23 are connected to the mirror module; the gate of the mos tube M22 is connected to the reverse logic of the charging control signal UP; the drain of the mos tube M22 poles are respectively connected to the drain of the mos tube M24 and the operational amplifier module; the gate of the mos tube M23 is connected to the charging control signal UP; the drain of the mos tube M23 is respectively connected to the drain of the mos tube M25 and the operational amplifier module connected; the gate of the mos tube M24 is connected to the reverse logic of the discharge control signal DN; the gate of the mos tube M25 is connected to the discharge control signal DN; the source of the mos tube M24 and the source of the mos tube M25 are connected to the mirror module connected.

In one of the embodiments, the operational amplifier module includes: mos tubes M27, M28, M29, M30 and M31; wherein,

The source of the mos tube M27 is grounded; the gate of the mos tube M27 is connected to the mirror module; the drain of the mos tube M27 is respectively connected to the source of the mos tube M28 and the source of the mos tube M29; the gate of the mos tube M28 It is connected with the switch module; the drain of mos tube M28 is connected with the drain of mos tube M30; the gate of mos tube M29 is connected with the drain of mos tube M31 and the output load respectively; the drain of mos tube M29 is connected with the drain of mos tube M31 respectively; The drain of the transistor M31 is connected; the gate of the mos transistor M30 is connected to the gate of the mos transistor M31 and the drain of the mos transistor M31 respectively; the source of the mos transistor M30 and the source of the mos transistor M31 are both connected to the power supply voltage VDD.

The above-mentioned charge pump system that weakens the charge sharing effect includes a startup module, a bootstrap reference source module, a mirror module, a switch circuit module, and an operational amplifier module that are electrically connected in sequence; the startup module is used to control the bootstrap reference source The on and off states of the module; the bootstrap reference source module is used to generate a constant current signal and output to the mirror module; the mirror module is used to control the current mirror output in a certain ratio; the switch The module is used to control the generation of charge and discharge current; the operational amplifier module is used to control the two voltage signals output by the switch circuit to be the same, so as to ensure the consistency of the charging current and the discharging current; in the embodiment of the utility model, a kind of weakening charge sharing effect The advanced charge pump system has the advantages of good sensitivity, weakening the asymmetric effect of charge and discharge current, avoiding the charge sharing effect, and improving the stability of the phase-locked loop.

Description of drawings

Fig. 1 is a schematic structural diagram of a charge pump system that weakens the charge sharing effect provided by an embodiment of the present invention;

FIG. 2 is a circuit diagram of a charge pump system for reducing the charge sharing effect provided by an embodiment of the present invention.

Detailed ways

In order to facilitate understanding of the utility model, the utility model will be further described in detail below in conjunction with the accompanying drawings. But the utility model is not limited to the embodiments shown in the accompanying drawings, and can be realized in many forms, and other embodiments in different forms all belong to the protection scope of the utility model.

As shown in Figure 1, it is a structural block diagram of a charge pump system that weakens the charge sharing effect in an embodiment of the present invention, including a startup module (100), a bootstrap reference source module (101), and a mirror module ( 102), a switch circuit module (103) and an operational amplifier module (104).

In one embodiment, as shown in FIG. 2, the startup module (100) includes mos tubes M1, M2, M3, and M4; wherein, the mos tubes M1 and M2 are P-channel mos tubes (Pmos tubes); Tube M3 and mos tube M4 are N-channel mos tubes (Nmos tubes);

The source of the mos tube M1 and the drain of the mos tube M4 are connected to the power supply voltage VDD; the gate of the mos tube M1 is respectively connected to the drain of the mos tube M1 and the source of the mos tube M2; the gates of the mos tube M2 are respectively It is connected to the drain of mos tube M2, the drain of mos tube M3, and the gate of mos tube M4; the gate of mos tube M3 is connected to the drain of mos tube M3; the source of mos tube M3 is grounded; the gate of mos tube M4 The source is connected to the bootstrap reference source module (101);

When the power is turned on, mos tube M1, mos tube M2, and mos tube M3 are all in a saturated state and form a DC path due to the gate-drain short circuit; the gate of mos tube M4 has a conduction voltage V _g4 The tube M4 is turned on and injects current into the bootstrap reference source module (101), breaking the zero state, and the bootstrap reference source module (101) enters the working state; as the current continues to increase, the source of the mos tube M4 The voltage V _S4 rises, which eventually leads to the cut-off of the mos tube M4, and the startup module (100) no longer affects the bootstrap reference source module (101).

In one embodiment, the bootstrap reference source module (101) includes mos tubes M5, M6, M7, M8, M9, M10 and resistor R; wherein, mos tube M7, mos tube M8, mos tube M9, and mos tube M10 are P-channel mos tube (Pmos tube); mos tube M5 and mos tube M6 are N-channel mos tubes (Nmos tube);

The source of the mos tube M5 and one end of the resistor R are grounded; the gate of the mos tube M5 is respectively connected to the source of the mos tube M6 and the other end of the resistor R; the drain of the mos tube M5 is respectively connected to the gate of the mos tube M6 , the source of the mos tube M7 is connected to the startup module (100); the gate of the mos tube M6 is connected to the gate of the mos tube M8; the gate of the mos tube M7 is respectively connected to the gate of the mos tube M8 and the mirror image The modules (102) are connected; the source of the mos tube M7 is connected to the drain of the mos tube M10; the source of the mos tube M8 is connected to the drain of the mos tube M9; the gate of the mos tube M9 is respectively connected to the gate of the mos tube M10 Pole and the mirror module (102) are connected; the source of the mos tube M9 and the source of the mos tube M10 are connected to the power supply voltage;

Mos tube M7 and mos tube M8, mos tube M9 and mos tube M10 constitute a cascode current mirror, mos tube M7, mos tube M8 are strictly equal, mos tube M9, mos tube M10 are strictly equal, in the actual circuit, in the common source Under the action of the common gate current mirror, the current _I1 flowing through the mos tube M5 is equal to the current _I2 flowing through the mos tube M6, and the voltage drop generated by the current flowing through the mos tube M5 and the resistor R is equal.

In one embodiment, the mirror module (102) includes mos tubes M11, M12, M13, M14, M15, M16, M17, M18, M19, M20, and M21; wherein, mos tubes M13, mos tubes M14, mos tubes M15, mos tube M16, mos tube M20, mos tube M21 are P channel mos tubes (Pmos tube); mos tube M11, mos tube M12, mos tube M17, mos tube M18, mos tube M19, mos tube M26 are N channel The mos tube (Nmos tube); the source of the mos tube M11, the source of the mos tube M12, the source of the mos tube M17, the source of the mos tube M18, the source of the mos tube M26 are all grounded; the source of the mos tube M14 pole, the source of the mos tube M15, the source of the mos tube M16, the source of the mos tube M20, and the source of the mos tube M21 are all connected to the power supply voltage VDD;

The gate of the mos tube M11 is respectively connected to the gate of the mos tube M12, the drain of the mos tube M11 and the drain of the mos tube M13; the drain of the mos tube M12 is respectively connected to the drain of the mos tube M15 and the gate of the mos tube M15 pole and the gate of mos transistor M16; the gate of mos transistor M13 is connected to the bootstrap reference source module (101); the source of mos transistor M13 is connected to the drain of mos transistor M14; the gate of mos transistor M14 Connected to the bootstrap reference source module (101); the drain of the mos tube M16 is respectively connected to the drain of the mos tube M17, the gate of the mos tube M26 and the operational amplifier module (104); the gate of the mos tube M17 The pole is connected to the gate of the mos tube M18; the source of the mos tube M18 is connected to the source of the M19; the gate of the mos tube M19 is connected to the drain of the mos tube M19; the drain of the mos tube M19 is respectively connected to the drain of the mos tube M20 , the gate of the mos tube M20 is connected to the gate of the mos tube M21; the drain of the mos tube M26 is connected to the switch module;

Among them, mos tube M11, mos tube M12 constitute the first current mirror pair, mos tube M15, mos tube M16 constitute the second current mirror pair, mos tube M17, mos tube M18, mos tube M26, mos tube M27 constitute the third current mirror Yes, the mos tube M20 and the mos tube M21 constitute the fourth current mirror pair; in the third current mirror pair, the mos tube M18 is the first mirror image of the mos tube M17, the mos tube M26 is the second mirror image of the mos tube M17, and the mos tube M27 is the third mirror image of mos tube M17; the reference current output by the bootstrap reference source module passes through the first current mirror pair, and after the second current mirror pair, it is divided into three branches by the third current mirror pair, one of which passes through the mos tube M181, the fourth current mirror pair is amplified as the pull-up current source of the switch module (103), one is amplified by the mos tube M26 and used as a pull-down current source to the switch module (103), and one is amplified by the mos tube M27 Afterwards, a bias current is provided for the operational amplifier module (104), and the mos tube M19 is a dummy tube, which increases the voltage drop of an equivalent switch and improves the matching of charge and discharge currents.

In one embodiment, the switch module (103) includes: MOS tubes M22, M23, M24, and M25; wherein, the MOS tubes M22 and M23 are P-channel MOS tubes (Pmos tubes); the MOS tubes M24, MOS tubes M25 is an N-channel mos tube (Nmos tube);

The mos tube M22 and the mos tube M23 constitute a charging switch, which provides a pull-up current for the output load according to the charging control signal UP, and charges the output load; Pulling down the current to discharge the output load; the source of the mos tube M22 and the source of the mos tube M23 are connected to the mirror module (102); the gate of the mos tube M22 is connected to the reverse logic UPN of the charging control signal; The drain of the mos tube M22 is respectively connected to the drain of the mos tube M24 and the operational amplifier module (104); the gate of the mos tube M23 is connected to the charging control signal UP; the drain of the mos tube M23 is respectively connected to the drain of the mos tube M25 The drain is connected to the operational amplifier module (104); the gate of the mos transistor M24 is connected to the reverse logic DNN of the discharge control signal; the gate of the mos transistor M25 is connected to the discharge control signal DN; the source of the mos transistor M24, the mos The sources of the tubes M25 are all connected to the mirror module (102).

In the switch module (103), the charge and discharge control signals come from the two outputs of the frequency and phase detector of the previous circuit in the phase-locked loop, and the charge and discharge control signals are used to provide the output load with pull-up/pull-down current to realize the The charge and discharge control of the output load; when the charge control signal UP is at high level and the discharge control signal is at DN low level, mos tube M23 and mos tube M24 are disconnected, mos tube M22 and mos tube M25 are both turned on, and the pull-down current I ₃ flows through the mos tube M25 to discharge the output load, and the output voltage V _OUT drops; when the charge control signal UP is at low level and the discharge control signal DN is at high level, both mos tube M22 and mos tube M25 are disconnected, and the mos tube Both M23 and mos tube M24 are turned on; the pull-up current I ₄ flows through the mos tube M23 to charge the output load, and the output voltage V _OUT rises; when the charge control signal UP and the discharge control signal DN are both at high level, the mos tubes M22, Both mos tubes M24 are turned on, mos tubes M23 and mos tubes M25 are both disconnected, at this time, branch a is disconnected, and the output voltage V _OUT remains unchanged; when the charge control signal UP and discharge control signal DN are both low level , the mos tube M23 and the mos tube M25 are both turned on, and the mos tube M22 and the mos tube M24 are both turned off. At this time, the pull-up current _I4 flows through the mos tube M23, and the pull-down current _I3 flows through the mos tube M25, and the two currents If they are equal, they cancel each other out, and the current flowing into the output load is zero, that is, the charge and discharge currents are symmetrical, and the output voltage V _OUT remains unchanged.

In one embodiment, the operational amplifier module (104) includes: mos tubes M27, M28, M29, M30, and M31; wherein, the mos tubes M30 and the mos tubes M31 are P-channel mos tubes (Pmos tubes); the mos tube M28 , MOS tube M29, and MOS tube M27 are N-channel MOS tubes (Nmos tubes);

The source of the mos tube M27 is grounded; the gate of the mos tube M27 is connected to the mirror module (102); the drain of the mos tube M27 is respectively connected to the source of the mos tube M28 and the source of the mos tube M29; the mos tube M28 The gate of the mos tube M28 is connected to the switch module (103); the drain of the mos tube M28 is connected to the drain of the mos tube M30; the gate of the mos tube M29 is respectively connected to the drain of the mos tube M31 and the output load; the mos tube M29 The drain of the mos tube M31 is respectively connected to the drain; the gate of the mos tube M30 is respectively connected to the gate of the mos tube M31 and the drain of the mos tube M31; the source of the mos tube M30 and the source of the mos tube M31 are both Connected to the power supply voltage VDD, it can suppress the asymmetric effect of charge and discharge current in a wide voltage range.

The above-mentioned embodiment is the best implementation mode of the utility model, which describes the principle of the utility model in detail. However, the utility model is not limited to the above-mentioned embodiments, and on the premise of the concept of the utility model, any other form of embodiment belongs to the protection scope of the utility model. Therefore, the protection scope of the present utility model should be based on the appended claims.

Claims (6)

1. A charge pump system that weakens the charge sharing effect, is characterized in that, comprises the startup module, bootstrap reference source module, mirror image module, switch module and operational amplifier module that are electrically connected in sequence; Wherein, The startup module is used to control the on and off states of the bootstrap reference source module; The bootstrap reference source module is used to generate a constant current signal and output it to the mirror image module; The mirror image module is used to control the mirror output of the current according to a certain ratio; The switch module is used to control the generation of charge and discharge current, and charge or discharge the output load according to the charge control signal or discharge control signal provided by the external circuit; The operational amplifier module is used to control the two voltage signals output by the switch circuit to be the same, so as to ensure the consistency of the charging current and the discharging current. 2. A charge pump system that weakens the charge sharing effect according to claim 1, wherein the start-up module includes mos tubes M1, M2, M3 and M4; wherein, The source of the mos tube M1 and the drain of the mos tube M4 are connected to the power supply voltage VDD; the gate of the mos tube M1 is respectively connected to the drain of the mos tube M1 and the source of the mos tube M2; the gates of the mos tube M2 are respectively It is connected to the drain of mos tube M2, the drain of mos tube M3, and the gate of mos tube M4; the gate of mos tube M3 is connected to the drain of mos tube M3; the source of mos tube M3 is grounded; the gate of mos tube M4 The source is connected to the bootstrap reference source module. 3. A kind of charge pump system that weakens charge sharing effect according to claim 1, is characterized in that, described bootstrap reference source module comprises MOS tube M5, M6, M7, M8, M9, M10 and resistance R; Wherein , The source of the mos tube M5 and one end of the resistor R are grounded; the gate of the mos tube M5 is respectively connected to the source of the mos tube M6 and the other end of the resistor R; the drain of the mos tube M5 is respectively connected to the gate of the mos tube M6 , the source of the mos tube M7 is connected to the startup module; the grid of the mos tube M6 is connected to the grid of the mos tube M8; the grid of the mos tube M7 is respectively connected to the grid of the mos tube M8 and the mirror image module; The source of the mos tube M7 is connected to the drain of the mos tube M10; the source of the mos tube M8 is connected to the drain of the mos tube M9; the gate of the mos tube M9 is respectively connected to the gate of the mos tube M10 and the mirror module connected; the source of the mos tube M9 and the source of the mos tube M10 are connected to the power supply voltage. 4. A charge pump system that weakens the charge sharing effect according to claim 1, wherein the mirror image module includes mos transistors M11, M12, M13, M14, M15, M16, M17, M18, M19, M20 and M21; where, The source of the mos tube M11, the source of the mos tube M12, the source of the mos tube M17, the source of the mos tube M18, and the source of the mos tube M26 are all grounded; the source of the mos tube M14 and the source of the mos tube M15 , the source of the mos tube M16, the source of the mos tube M20, and the source of the mos tube M21 are all connected to the power supply voltage VDD; The gate of the mos tube M11 is respectively connected to the gate of the mos tube M12, the drain of the mos tube M11 and the drain of the mos tube M13; the drain of the mos tube M12 is respectively connected to the drain of the mos tube M15 and the gate of the mos tube M15 pole and the gate of the mos tube M16; the gate of the mos tube M13 is connected to the bootstrap reference source module; the source of the mos tube M13 is connected to the drain of the mos tube M14; the gate of the mos tube M14 is connected to the The bootstrap reference source module is connected; the drain of the mos tube M16 is respectively connected to the drain of the mos tube M17, the gate of the mos tube M26 and the operational amplifier module; the gate of the mos tube M17 is connected to the gate of the mos tube M18 ; MOS tube M18 is connected to the source of M19; the gate of MOS tube M19 is connected to the drain of MOS tube M19; the drain of MOS tube M19 is respectively connected to the drain of MOS tube M20, the gate of MOS tube M20 and the mos The gate of the transistor M21 is connected; the drain of the mos transistor M26 is connected to the switch module. 5. A charge pump system for weakening the charge sharing effect according to claim 1, wherein the switch module comprises: mos transistors M22, M23, M24 and M25; wherein, The mos tube M22 and the mos tube M23 form a charging switch, which provides a pull-up current for the output load according to the charging control signal, and charges the output load; the mos tube M24 and the mos tube M25 form a discharge switch, which provides a pull-down current for the output load according to the discharge control signal , to discharge the output load; the source of the mos tube M22 and the source of the mos tube M23 are connected to the mirror module; the gate of the mos tube M22 is connected to the reverse logic of the charging control signal UP; the drain of the mos tube M22 poles are respectively connected to the drain of the mos tube M24 and the operational amplifier module; the gate of the mos tube M23 is connected to the charging control signal UP; the drain of the mos tube M23 is respectively connected to the drain of the mos tube M25 and the operational amplifier module connected; the gate of the mos tube M24 is connected to the reverse logic of the discharge control signal DN; the gate of the mos tube M25 is connected to the discharge control signal DN; the source of the mos tube M24 and the source of the mos tube M25 are connected to the mirror module connected. 6. A charge pump system for weakening the charge sharing effect according to claim 1, wherein the operational amplifier module comprises: mos transistors M27, M28, M29, M30 and M31; wherein, The source of the mos tube M27 is grounded; the gate of the mos tube M27 is connected to the mirror module; the drain of the mos tube M27 is respectively connected to the source of the mos tube M28 and the source of the mos tube M29; the gate of the mos tube M28 It is connected with the switch module; the drain of mos tube M28 is connected with the drain of mos tube M30; the gate of mos tube M29 is connected with the drain of mos tube M31 and the output load respectively; the drain of mos tube M29 is connected with the drain of mos tube M31 respectively; The drain of the transistor M31 is connected; the gate of the mos transistor M30 is connected to the gate of the mos transistor M31 and the drain of the mos transistor M31 respectively; the source of the mos transistor M30 and the source of the mos transistor M31 are both connected to the power supply voltage VDD.