CN102931832B - PWM (pulse width modulation) type adjustable RC (resistance-capacitance) circuit and adjustment method thereof - Google Patents

Abstract

The invention discloses a PWM (pulse width modulation) type adjustable RC (resistance-capacitance) circuit and an adjustment method thereof. The PWM type adjustable RC circuit comprises a first-order RC circuit, an electronic switch, a PWM controller and a voltage-maintaining afterflow loop, wherein the electronic switch controls the switching on or switching off of a capacitor in the first-order RC circuit; and the voltage-maintaining afterflow loop controls the voltage at both ends of the capacitor in the first-order RC circuit, and ensures that the current in the circuit and the voltage at both ends of the capacitor are kept unchanged when the capacitor is not switched on in the circuit. The PWM type adjustable RC circuit designed by the invention is based on a PWM control technology, the control of the intermittent charging/discharging work states equivalently realizes the adjustment of the RC of the circuit, and the designed voltage-maintaining afterflow loop realizes that the voltage and current in the circuit are continuously and smoothly changed in the switching process of the switch.

Description

Pulse Width Modulation Adjustable Resistance-Capacitance Circuit and Its Adjustment Method

technical field

The invention relates to an adjustable resistance-capacitance circuit, in particular to a pulse width modulation (PWM) type adjustable resistance-capacitance circuit, and also relates to an adjustment method of the circuit.

Background technique

The most commonly used method of adjusting the capacitance of the circuit is to change the physical characteristics of the capacitance. However, the capacitance adjustment achieved by this method is often highly targeted, and the adjustable range is very small, and it does not have the ability to adjust the capacitance. Versatility. In addition, some scholars are also studying other methods of adjusting capacitance, such as Zhu Dongbai and Liu Ji from Harbin University of Science and Technology, etc., aiming at the problem that there is a difference in capacitance when switching capacitors in groups in current reactive power compensation devices and the number of capacitor banks required for compensation is large. , based on the PWM control principle, the switching of two sets of capacitors is controlled by a power electronic switch, and the working time of the two sets of capacitors is controlled by adjusting the duty ratio of the control pulse, which is equivalent to that the capacitance of the input system is adjustable. However, in this solution, there is a sudden change in the voltage at both ends of the capacitor and the current in the circuit at the moment of switching, which cannot achieve a smooth adjustment process. In addition, the first-order pulse width modulation load that the author of this article has studied realizes the adjustment of the resistance and capacitance by adjusting the resistance value in the first-order RC circuit. The harmonics that appear in the adjustment process of this method will be relatively large, and the stability is not good. good.

Contents of the invention

In view of this, in order to solve the above problems, the present invention provides a pulse width modulation adjustable resistance-capacitance circuit and its adjustment method, which ensures that the voltage and current in the circuit change continuously and smoothly during the switching process.

In order to achieve the stated purpose, the pulse width modulation type adjustable resistance-capacitance circuit designed by the present invention includes a first-order RC circuit, an electronic switch, a PWM controller and a pressure-holding freewheeling circuit; the PWM controller generates a pulse width modulation type square The wave signal controls the operation of the electronic switch, and the electronic switch controls the connection or disconnection of the capacitor in the first-order RC circuit; the pressure-holding and freewheeling circuit controls the voltage at both ends of the capacitor in the first-order RC circuit to ensure that the capacitor is not connected When in a circuit, the current in the circuit and the voltage across the capacitor remain unchanged.

Further, the pressure-holding freewheeling circuit is a circuit with high input impedance, low output impedance, and an amplification factor approximately equal to 1.

As the first technical solution of the present invention, the pressure maintaining and freewheeling circuit includes a first transistor T1, a second transistor T2, a first diode D1, a second diode D2, and a second resistor R2 and the third resistor R3. The collector of the first transistor T1 is connected to the anode of the first diode D1, the cathode of the first diode D1 is connected to the capacitor C terminal 4, the collector of the second transistor T2 is connected to the second diode The cathode of D2 is connected, and the anode of the second diode D2 is connected to terminal 4 of capacitor C. The base of the first transistor T1 is connected to the terminal 3 of the capacitor C through the second resistor R2, and the base of the second transistor T2 is connected to the terminal 3 of the capacitor C through the third resistor R3. The emitter of the first transistor T1 and the emitter of the second transistor T2 are connected to the electronic switch.

Further, the input and output currents of the pressure-holding freewheeling circuit are equal.

Further, the triode adopts an emitter follower structure.

As the second technical solution of the present invention, the pressure-holding freewheeling circuit includes a third triode T3, a fourth triode T4, a third diode D3, a fourth diode D4, and a sixth resistor R6 Seventh resistor R7 and operational amplifier A. The collector of the third transistor T3 is connected to the anode of the third diode D3, and the cathode of the third diode D3 is connected to the terminal 4 of the capacitor C; the collector of the third transistor T4 is connected to the fourth and second The cathode of the diode D4 is connected, and the anode of the fourth diode D4 is connected to the terminal 4 of the capacitor C. The base of the third transistor T3 is connected to the output terminal of the operational amplifier A through the sixth resistor R6, the base of the fourth transistor T4 is connected to the output terminal of the operational amplifier A through the seventh resistor R7, and the positive terminal of the operational amplifier A The input terminal is connected to the terminal 3 of the capacitor C, and the inverting input terminal of the operational amplifier A is connected to the output terminal.

Further, the triode adopts an emitter follower structure.

The method for adjusting a pulse width modulation adjustable resistance-capacitance circuit is characterized in that it comprises the following steps:

Step 1: When the electronic switch connects the capacitor in the first-order RC circuit, the capacitor C is in the charging or discharging state;

Step 2: When the electronic switch disconnects the capacitor in the first-order RC circuit, the pressure-holding freewheeling circuit keeps the voltage at both ends of the capacitor C constant, and at the same time keeps the current in the circuit constant;

Step 3: Repeat Step 1 and Step 2.

Further, the voltage across the capacitor C in the second step is the period of the electronic switch controlled by the square wave signal generated by the PWM controller and duty cycle To control the charging and discharging time of the capacitor, it is equivalent to changing the time constant of the first-order RC circuit, so as to realize the adjustment of the circuit resistance and capacitance, and , the smaller the duty cycle, the larger the time constant of the equivalent first-order RC circuit.

The present invention has the following advantages:

1, the pulse width modulation (PWM) type adjustable resistance-capacitance circuit of the present invention's design, comprises first-order RC circuit, electronic switch, PWM controller, pressure-holding freewheeling circuit; connected or disconnected; the pressure-holding freewheeling circuit controls the voltage across the capacitor in the first-order RC circuit to ensure that the current in the circuit and the voltage across the capacitor remain unchanged when the capacitor is not connected to the circuit. The PWM controller generates a pulse width modulated square wave signal to control the operation of the electronic switch, and controls the conduction time of the electronic switch by adjusting the pulse width duty cycle, thereby controlling the charging and discharging speed of the capacitor to achieve equivalent adjustment of the resistance and capacitance. The pressure-holding freewheeling circuit controls the intermittent charging and discharging working state of the capacitor, which is equivalent to the change of the capacitor capacity to realize the adjustment of the circuit resistance and capacitance. The pulse width modulation adjustable resistance-capacitance circuit has a large adjustable range and has universality for capacitance adjustment;

2. The pressure-holding freewheeling circuit designed by the present invention realizes continuous and stable changes in the voltage and current in the circuit during the switching process.

Description of drawings

In order to make the purpose, technical scheme and beneficial effect of the present invention clearer, the present invention provides the following drawings for illustration:

Fig. 1 is the working principle diagram of the pulse width modulation type adjustable resistance-capacitance circuit of the present invention;

Fig. 2 is a circuit diagram of Embodiment 1 of a pulse width modulation adjustable resistance-capacitance circuit provided by an embodiment of the present invention;

Fig. 3 is a circuit diagram of the second embodiment of the pulse width modulation adjustable resistance-capacitance circuit provided by the embodiment of the present invention;

Fig. 4 is a circuit diagram of the third embodiment of the pulse width modulation adjustable resistance-capacitance circuit provided by the embodiment of the present invention;

Fig. 5 is a circuit diagram of the fourth embodiment of the pulse width modulation adjustable resistance-capacitance circuit provided by the embodiment of the present invention;

Fig. 6 is a circuit diagram of the fifth embodiment of the pulse width modulation adjustable resistance-capacitance circuit provided by the embodiment of the present invention;

Fig. 7 is a circuit diagram of the sixth embodiment of the pulse width modulation adjustable resistance-capacitance circuit provided by the embodiment of the present invention;

Fig. 8 is a circuit diagram of Embodiment 7 of the pulse width modulation adjustable resistance-capacitance circuit provided by the embodiment of the present invention;

Fig. 9 is a schematic diagram of a circuit voltage step response curve of the present invention;

Fig. 10 is the circuit current step response curve of the present invention

FIG. 11 is a schematic diagram of voltage step response curves of the circuit with different duty ratios according to the present invention.

Detailed ways

Embodiments of the present invention will be described in detail below with reference to the accompanying drawings. It should be understood that the examples are only to illustrate the present invention, not to limit the protection scope of the present invention.

The pulse width modulation adjustable resistance-capacitance circuit designed by the present invention includes a first-order RC circuit, a PWM controller, an electronic switch and a pressure-holding freewheeling circuit. The PWM controller generates a pulse width modulated square wave signal to control the operation of the electronic switch, and the electronic switch controls the connection or disconnection of the capacitor in the first-order RC circuit; the pressure-holding freewheeling circuit controls the first-order RC circuit The voltage across the capacitor ensures that when the capacitor is not connected to the circuit, the current in the circuit and the voltage across the capacitor remain unchanged. The pressure-holding freewheeling circuit is a circuit with high input impedance (resistance value less than 2MΩ and greater than 1MΩ), low output impedance (less than 40Ω), and amplification factor approximately equal to 1.

According to the circuit diagram 2, the present invention can design a pulse width modulation adjustable resistance-capacitance circuit. As the first embodiment of the present invention, the pressure-holding and freewheeling circuit includes triodes in two directions (the first triode T1 and the second triode T2 ), diodes in two directions (the first diode D1 With the second diode D2) and two resistors (the second resistor R2 and the third resistor R3), the triode adopts an emitter follower structure. The collector of the first triode T1 is connected to the anode of the first diode D1, the cathode of the first diode D1 is connected to the terminal 4 of the capacitor C, and the collector of the second triode T2 is connected to the second two The cathode of the diode D2 is connected, and the anode of the second diode D2 is connected to the terminal 4 of the capacitor C. The base of the first transistor T1 is connected to the terminal 3 of the capacitor C through the second resistor R2, and the base of the second transistor T2 is connected to the terminal 3 of the capacitor C through the third resistor R3. The emitter of the first transistor T1 and the emitter of the second transistor T2 are connected to the electronic switch. When the electronic switch is turned on 1, the capacitor C is in the charging or discharging state; when the switch is turned on 2, the current flows through the pressure-holding and freewheeling circuit. Considering the positive and negative voltage signals, the triodes T1 and T2 in two directions are configured, and the diodes D1 and D2 in the circuit ensure that only one triode works at the same time in the freewheeling circuit. When the electronic switch is switched from 1 to 2, the emitter voltage U _O is basically the same as the voltage U _C across the capacitor, and the voltage on the resistor remains unchanged before and after the switch is switched, so the current i flowing through the first resistor R1 remains constant The change ensures that the voltage and current in the circuit during the switching process are continuously and smoothly changed.

The present invention can also design a pulse width modulation adjustable resistance-capacitance circuit according to FIG. 3 . As the second embodiment of the present invention, the pressure-holding and freewheeling circuit includes triodes in two directions (the first triode T1 and the second triode T2) and two resistors (the second resistor R2 and the third resistor R3), the triode adopts an emitter follower structure. The collector of the first transistor T1 is connected to the terminal 4 of the capacitor C; the collector of the second transistor T2 is connected to the terminal 4 of the capacitor C. The base of the first transistor T1 is connected to the terminal 3 of the capacitor C through the second resistor R2, and the base of the second transistor T2 is connected to the terminal 3 of the capacitor C through the third resistor R3. The emitter of the first transistor T1 and the emitter of the second transistor T2 are connected to the electronic switch. When the electronic switch is turned on 1, the capacitor C is in the charging or discharging state; when the switch is turned on 2, the current flows through the pressure-holding and freewheeling circuit. In consideration of the positive and negative voltage signals, transistors T3 and T4 in two directions are configured. When the electronic switch is switched from 1 to 2, the emitter voltage U _O is basically the same as the voltage U _C across the capacitor, and the voltage on the resistor remains unchanged before and after the switch is switched, so the current i flowing through the first resistor R1 remains constant The change ensures that the voltage and current in the circuit during the switching process are continuously and smoothly changed.

The present invention can also design a pulse width modulation adjustable resistance-capacitance circuit according to FIG. 4 . As the third embodiment of the present invention, the pressure-holding freewheeling circuit includes triodes in two directions (the first triode T1 and the second triode T2) and four resistors (the second resistor R2, the third resistor R3 , the fourth resistor R4 and the fifth resistor R5), the triode adopts an emitter follower structure. The collector of the first transistor T1 is connected to the capacitor C terminal 4 through the fourth resistor R4, and the collector of the second transistor T2 is connected to the capacitor C terminal 4 through the fifth resistor R5. The base of the first transistor T1 is connected to the terminal 3 of the capacitor C through the second resistor R2, and the base of the second transistor T2 is connected to the terminal 3 of the capacitor C through the third resistor R3. The emitter of the first transistor T1 and the emitter of the second transistor T2 are connected to the electronic switch. When the electronic switch is turned on 1, the capacitor C is in the charging or discharging state; when the switch is turned on 2, the current flows through the pressure-holding and freewheeling circuit. In consideration of the positive and negative voltage signals, transistors T1 and T2 in two directions are configured. When the electronic switch is switched from 1 to 2, the emitter voltage U _O is basically the same as the voltage U _C across the capacitor, and the voltage on the resistor remains unchanged before and after the switch is switched, so the current i flowing through the first resistor R1 remains constant The change ensures that the voltage and current in the circuit during the switching process are continuously and smoothly changed.

The present invention can also design a pulse width modulation adjustable resistance-capacitance circuit according to FIG. 5 . As the fourth embodiment of the present invention, the pressure-holding freewheeling circuit includes triodes in two directions (third triode T3 and third triode T4 ), diodes in two directions (third diode D3 With the fourth diode D4), two resistors (sixth resistor R6 and seventh resistor R7) and an operational amplifier A, the triode adopts an emitter follower structure. The collector of the third transistor T3 is connected to the anode of the third diode D3, and the cathode of the third diode D3 is connected to the terminal 4 of the capacitor C; the collector of the third transistor T4 is connected to the fourth and second The cathode of the diode D4 is connected, and the anode of the fourth diode D4 is connected to the terminal 4 of the capacitor C. The base of the third transistor T3 is connected to the output terminal of the operational amplifier A through the sixth resistor R6, the base of the fourth transistor T4 is connected to the output terminal of the operational amplifier A through the seventh resistor R7, and the positive terminal of the operational amplifier A The input terminal is connected to the terminal 3 of the capacitor C, and the inverting input terminal of the operational amplifier A is connected to the output terminal. The emitter of the third transistor T3 and the emitter of the fourth transistor T4 are connected to the electronic switch. When the electronic switch is turned on 1, the capacitor C is in the charging or discharging state; when the switch is turned on 2, the current flows through the pressure-holding and freewheeling circuit. Considering the positive and negative voltage signals, the triodes T3 and T4 in two directions are configured, and the diodes D3 and D4 in the circuit ensure that only one triode works at the same time in the freewheeling circuit. When the electronic switch is switched from 1 to 2, the voltage U _O at the emitter is basically the same as the voltage U _C at both ends of the capacitor, and the voltage on the resistor remains unchanged before and after the switch, so the current i flowing through the first resistor R1 remains constant The change ensures that the voltage and current in the circuit during the switching process are continuously and smoothly changed.

The present invention can also design a pulse width modulation adjustable resistance-capacitance circuit according to FIG. 6 . As the fifth embodiment of the present invention, the pressure-holding and freewheeling circuit includes triodes in two directions (third triode T3 and third triode T4 ), diodes in two directions (third diode D3 With the fourth diode D4), two resistors (the eighth resistor R8 and the ninth resistor R9) and an operational amplifier A, the triode adopts an emitter follower structure. The collector of the third transistor T3 is connected to the anode of the third diode D3, and the cathode of the third diode D3 is connected to the terminal 4 of the capacitor C; the collector of the fourth transistor T4 is connected to the fourth two The cathode of the diode D4 is connected, and the anode of the fourth diode D4 is connected to the terminal 4 of the capacitor C. The base of the third transistor T3 is connected to the output terminal of the operational amplifier A, the base of the fourth transistor T4 is connected to the output terminal of the operational amplifier A, and the positive input terminal of the operational amplifier A is connected to the The terminal 3 of the capacitor C, the inverting input terminal of the operational amplifier A is connected to the output terminal through the ninth resistor R9. The emitter of the third transistor T3 and the emitter of the fourth transistor T4 are connected to the electronic switch. When the electronic switch is turned on 1, the capacitor C is in the charging or discharging state; when the switch is turned on 2, the current flows through the pressure-holding and freewheeling circuit. Considering the positive and negative voltage signals, the triodes T3 and T4 in two directions are configured, and the diodes D3 and D4 in the circuit ensure that only one triode works at the same time in the freewheeling circuit. When the electronic switch is switched from 1 to 2, the voltage U _O at the emitter is basically the same as the voltage U _C at both ends of the capacitor, and the voltage on the resistor remains unchanged before and after the switch, so the current i flowing through the first resistor R1 remains constant The change ensures that the voltage and current in the circuit during the switching process are continuously and smoothly changed.

The present invention can also design a pulse width modulation adjustable resistance-capacitance circuit according to FIG. 7 . As the sixth embodiment of the present invention, the pressure-holding and freewheeling circuit includes triodes in two directions (third triode T3 and third triode T4 ), triodes in two directions (third triode T3 With the third triode T4) and an operational amplifier A, the triode adopts an emitter follower structure. The collector of the third transistor T3 is connected to the anode of the third diode D3, and the cathode of the third diode D3 is connected to the terminal 4 of the capacitor C; the collector of the fourth transistor T4 is connected to the fourth diode The cathode of D4 is connected, and the anode of the fourth diode D4 is connected to terminal 4 of capacitor C. The base of the third transistor T3 is connected to the output terminal of the operational amplifier A, the base of the fourth transistor T4 is connected to the output terminal of the operational amplifier A, and the positive input terminal of the operational amplifier A is connected to the terminal 3 of the capacitor C , the inverting input of the operational amplifier A is connected to the output. The emitter of the third transistor T3 and the emitter of the fourth transistor T4 are connected to the electronic switch. When the electronic switch is turned on 1, the capacitor C is in the charging or discharging state; when the switch is turned on 2, the current flows through the pressure-holding and freewheeling circuit. Considering the positive and negative voltage signals, the triodes T3 and T4 in two directions are configured, and the diodes D3 and D4 in the circuit ensure that only one triode works at the same time in the freewheeling circuit. When the electronic switch is switched from 1 to 2, the emitter voltage U _O is basically the same as the voltage U _C across the capacitor, and the voltage on the resistor remains unchanged before and after the switch is switched, so the current i flowing through the first resistor R1 remains constant The change ensures that the voltage and current in the circuit during the switching process are continuously and smoothly changed.

The present invention can also design a pulse width modulation adjustable resistance-capacitance circuit according to FIG. 8 . As the seventh embodiment of the present invention, the pressure-holding and freewheeling circuit includes triodes in two directions (the third triode T3 and the third triode T4), two resistors (the eighth resistor R8 and the ninth resistor R9) and an operational amplifier A, the triode adopts an emitter follower structure. The collector of the third transistor T3 is connected to the terminal 4 of the capacitor C; the collector of the fourth transistor T4 is connected to the terminal 4 of the capacitor C. The base of the third transistor T3 is connected to the output terminal of the operational amplifier A, the base of the fourth transistor T4 is connected to the output terminal of the operational amplifier A, and the positive input terminal of the operational amplifier A is connected to the The terminal 3 of the capacitor C, the inverting input terminal of the operational amplifier A is connected to the output terminal through the ninth resistor R9. The emitter of the third transistor T3 and the emitter of the fourth transistor T4 are connected to the electronic switch. When the electronic switch is turned on 1, the capacitor C is in the charging or discharging state; when the switch is turned on 2, the current flows through the pressure-holding and freewheeling circuit. Considering the positive and negative voltage signals, the triodes T3 and T4 in two directions are configured. When the electronic switch is switched from 1 to 2, the voltage U _O of the emitter is basically the same as the voltage U _C at both ends of the capacitor, and the voltage on the resistor is at The switch remains unchanged before and after switching, so the current i flowing through the first resistor R1 remains unchanged, which ensures that the voltage and current in the circuit during the switching process are continuously and smoothly changed.

Based on the above embodiments, the adjustment method of the pulse width modulation type adjustable resistance-capacitance circuit of the present invention includes the following steps:

Step 1: When the electronic switch connects the capacitor in the first-order RC circuit, the capacitor C is in the charging or discharging state;

Step 2: When the electronic switch disconnects the capacitor in the first-order RC circuit, the pressure-holding freewheeling circuit keeps the voltage at both ends of the capacitor C constant, and at the same time keeps the current in the circuit constant;

Step 3: Repeat Step 1 and Step 2.

In the second step, the voltage across the capacitor C is the period of the electronic switch controlled by the square wave signal generated by the PWM controller and duty cycle To control the charging and discharging time of the capacitor, it is equivalent to changing the time constant of the first-order RC circuit, so as to realize the adjustment of the circuit resistance and capacitance, and , the smaller the duty cycle, the larger the time constant of the equivalent first-order RC circuit.

In Figure 9, the curve ① is the voltage response curve when the switch is always at position 1 in the design circuit, and the time constant of the RC circuit at this time is , ②The curve is the electronic switch according to the cycle , the duty cycle The voltage response curve during operation, the time constant of the RC circuit at this time is .

Figure 10 The curve is the current step response curve when the switch is on 1, The curve is the current step response curve when the switch switches between 1 and 2.

Figure 11, The curve is δ=1, that is, the voltage step response curve when the switch is only in position 1, and the curve ⑥ is =0.5 when the voltage step response curve, ⑦ curve is =0.1, the voltage step response curve can be seen .

In summary, it can be seen that the adjustment of the resistance and capacitance of the circuit is realized through pulse width modulation, and the adjustment process is carried out continuously and smoothly.

Finally, it should be noted that the above preferred embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail through the above preferred embodiments, those skilled in the art should understand that it can be described in terms of form and Various changes may be made in the details without departing from the scope of the invention defined by the claims.

Claims (4)

1. The pulse width modulation type adjustable resistance-capacitance circuit is characterized in that: it comprises a first-order RC circuit, an electronic switch, a PWM controller and a pressurized freewheeling circuit; the PWM controller generates a pulse width modulation type square wave signal to control the electronics The work of the switch; the electronic switch controls the connection or disconnection of the capacitor in the first-order RC circuit; the pressure-holding and freewheeling circuit controls the voltage at both ends of the capacitor in the first-order RC circuit to ensure that the capacitor is not connected to the circuit , the current in the circuit and the voltage across the capacitor remain unchanged; The pressure-holding and freewheeling circuit includes a first transistor (T1), a second transistor (T2), a first diode (D1), a second diode (D2), and a second resistor (R2) and a third resistor (R3), the collector of the first transistor (T1) is connected to the anode of the first diode (D1), and the cathode of the first diode (D1) is connected to the capacitor (C) At one end, the collector of the second triode (T2) is connected to the cathode of the second diode (D2), and the anode of the second diode (D2) is connected to the cathode of the first diode; the first triode The base of the tube (T1) is connected to the other end of the capacitor (C) through the second resistor (R2), and the base of the second triode (T2) is connected to the common connection between the capacitor and the second resistor through the third resistor (R3). end; or the pressurized freewheeling circuit includes a third triode (T3), a fourth triode (T4), a third diode (D3), a fourth diode (D4), a sixth resistor (R6), the seventh resistor (R7) and operational amplifier (A); the collector of the third triode (T3) is connected to the anode of the third diode (D3), and the third diode (D3) ) is connected to one end of the capacitor (C); the collector of the fourth triode (T4) is connected to the cathode of the fourth diode (D4), and the anode of the fourth diode (D4) is connected to the capacitor ( C) and the common end of the third diode (D3); the base of the third transistor (T3) is connected to the output terminal of the operational amplifier (A) through the sixth resistor (R6), and the fourth transistor (T4 ) base is connected to the output of the operational amplifier (A) through the seventh resistor (R7), the positive input of the operational amplifier (A) is connected to the other end of the capacitor (C), and the reverse of the operational amplifier (A) The input terminal is connected with the output terminal; the emitter of the third triode (T3) is connected with the electronic switch, and the emitter of the fourth triode (T4) is connected with the electronic switch. 2 . The pulse width modulation adjustable resistance-capacitance circuit according to claim 1 , wherein the pressure-holding freewheeling circuit is a circuit with high input impedance, low output impedance, and an amplification factor approximately equal to 1. 3 . 3 . The pulse width modulation adjustable resistance-capacitance circuit according to claim 1 , wherein the input and output currents of the pressure-holding freewheeling circuit are equal. 4 . 4. The pulse width modulation adjustable resistance-capacitance circuit according to claim 1, characterized in that: the triode adopts an emitter follower structure.