CP sampling circuit of automobile AC charging pile

Abstract

The utility model provides a CP sampling circuit of an automobile alternating current charging pile, and relates to the technical field of CP sampling circuits, the technical scheme provided by the utility model adopts a double-path sampling design, CP signals in two forms of PWM and DC can be processed at the same time, and the signal processing precision is improved through multi-stage amplification and filtering. The overall structure of the circuit enables the circuit to quickly respond to the voltage change of the CP signal in different charging stages and accurately identify different voltage levels, so that the sampling speed and accuracy of the automobile AC charging pile to the CP signal can be improved.

Description

Technical Field

[0001] This application relates to the technical field of CP sampling circuits, and in particular to a CP sampling circuit for an AC charging pile for automobiles. Background Technology

[0002] With increasing environmental awareness and the development of new energy technologies, electric vehicles are rapidly becoming more widespread globally. As an indispensable infrastructure for electric vehicle use, the importance of AC charging stations is increasingly prominent. During the charging process, complex information exchange is required between the charging station and the vehicle, among which the CP (Charging Pilot) signal plays a crucial role. The CP signal is used to monitor the interaction between the charging station and the electric vehicle, including the identification of charging status, the setting of charging parameters, and safety protection.

[0003] In sampling the CP signal, related technologies often employ comparator circuits, using different thresholds to determine the voltage level of the CP signal. However, because the CP signal includes both PWM and DC forms, and it rapidly switches between different voltage levels during different charging stages, existing CP sampling circuits suffer from deficiencies in both signal acquisition speed and accuracy. Utility Model Content

[0004] In view of this, the purpose of this application is to provide a CP sampling circuit for an AC charging pile for automobiles, which can quickly and accurately sample the CP signal of the charging pile.

[0005] The above-mentioned inventive objective of this application is achieved through the following technical solutions:

[0006] This application provides a CP sampling circuit for an AC charging pile for automobiles. The circuit includes a PWM output sampling circuit and a DC output sampling circuit. The PWM output sampling circuit includes a first signal amplification unit and a current protection unit, and the DC output sampling circuit includes a second signal amplification unit and a filtering unit, wherein:

[0007] The input terminal of the first signal amplification unit is connected to the PWM output port of the microcontroller, and the output terminal of the first signal amplification unit is connected to the current protection unit, which outputs a PWM signal.

[0008] The input terminal of the second signal amplification unit is connected to the current protection unit, and the output terminal of the second signal amplification unit is connected to the filtering unit, which outputs a DC signal.

[0009] By adopting the above technical solution, the dual-channel sampling design can not only process both PWM and DC CP signals simultaneously, but also improve the accuracy of signal processing through multi-stage amplification and filtering. The overall circuit structure enables it to quickly respond to voltage changes in the CP signal at different charging stages and accurately identify different voltage levels, thereby improving the sampling speed and accuracy of the CP signal in the car AC charging pile.

[0010] In a preferred embodiment, this application can be further configured such that: the first signal amplification unit includes a first operational amplifier, a first resistor, a second resistor, and a third resistor, wherein:

[0011] The PWM output port of the microcontroller is connected to the non-inverting input of the first operational amplifier via the first resistor;

[0012] The inverting input terminal of the first operational amplifier is connected to the first terminal of the second resistor and the first terminal of the third resistor, respectively. The second terminal of the second resistor is connected to the power supply, and the second terminal of the third resistor is grounded.

[0013] By adopting the above technical solution, the PWM output port of the microcontroller is connected to the non-inverting input of the first operational amplifier through a first resistor. This configuration not only provides preliminary signal conditioning but also effectively suppresses potential high-frequency noise, improving the quality of the input signal. The inverting input of the first operational amplifier is connected to the second and third resistors, forming a precise feedback network. The second resistor is connected to the power supply, while the third resistor is grounded. This arrangement constitutes a voltage divider circuit, providing a suitable bias voltage for the amplifier. By setting the first signal amplification unit, the CP sampling circuit can more accurately capture and amplify the PWM signal, providing high-quality signal input for subsequent signal processing and analysis.

[0014] In a preferred embodiment, this application can be further configured such that the circuit also includes a first capacitor, wherein:

[0015] The first terminal of the first capacitor is connected to the PWM output port of the microcontroller, and the second terminal of the first capacitor is grounded.

[0016] In a preferred embodiment, this application can be further configured such that the current protection unit also includes a first diode, a second diode, a Zener diode, a fourth resistor, and a fifth resistor, wherein:

[0017] The output terminal of the first signal amplification unit is connected to the negative terminal of the first diode and the positive terminal of the second diode, respectively. The positive terminal of the first diode is connected to a positive voltage source, and the negative terminal of the second diode is connected to a negative voltage source.

[0018] The output terminal of the first signal amplification unit is connected in parallel to the input terminal of the second signal amplification unit via the fourth resistor and the fifth resistor, and the input terminal of the second signal amplification unit is grounded via the Zener diode.

[0019] By adopting the above technical solution, the output terminal of the first signal amplification unit is simultaneously connected to the negative terminal of the first diode and the positive terminal of the second diode, forming a bidirectional voltage clamping circuit. The first diode is connected to the positive voltage source, and the second diode is connected to the negative voltage source. This configuration effectively limits the voltage swing of the signal, preventing excessively high or low voltages from damaging subsequent circuits. Grounding the Zener diode provides another layer of voltage protection; it quickly conducts when the voltage exceeds its breakdown voltage, guiding the excess voltage to ground, thereby protecting the subsequent amplification circuit from overvoltage damage.

[0020] In a preferred embodiment, this application can be further configured such that the circuit also includes a second capacitor, wherein:

[0021] The first end of the second capacitor is connected to the input end of the second signal amplification unit, and the second end of the second capacitor is grounded.

[0022] In a preferred embodiment, the Zener diode may be further configured as a TVS bidirectional Zener diode.

[0023] In a preferred embodiment, this application can be further configured such that: the second signal amplification unit includes a second operational amplifier, a sixth resistor, a seventh resistor, an eighth resistor, a ninth resistor, a tenth resistor, an eleventh resistor, a twelfth resistor, a thirteenth resistor, a third capacitor, and a fourth capacitor, wherein:

[0024] The circuit protection unit is connected to the inverting input terminal of the second operational amplifier via the sixth resistor and the seventh resistor. The connection point of the sixth resistor and the seventh resistor is grounded via the third capacitor, and the connection point of the sixth resistor and the seventh resistor is grounded via the eighth resistor.

[0025] The non-inverting input terminal of the second operational amplifier is connected to the first terminal of the ninth resistor and the first terminal of the tenth resistor, respectively. The second terminal of the ninth resistor is connected to the power supply, and the second terminal of the tenth resistor is grounded.

[0026] The inverting input terminal of the second operational amplifier is connected to the output terminal of the second operational amplifier via the eleventh resistor, and the inverting input terminal of the second operational amplifier is connected to the output terminal of the second operational amplifier via the fourth capacitor.

[0027] The output of the second operational amplifier is grounded via the twelfth resistor and the thirteenth resistor in series.

[0028] By adopting the above technical solution, the non-inverting input of the second operational amplifier forms a precise voltage divider through the ninth and tenth resistors, providing a stable reference voltage for the operational amplifier. This allows it to adapt to voltage changes in the CP signal under different charging states, ensuring the amplifier always operates in the linear region. The feedback network, consisting of the eleventh resistor and the fourth capacitor connected in parallel, forms the core of the inverting amplifier. It not only determines the amplifier's basic gain but also forms a low-pass filter characteristic through the fourth capacitor, further suppressing high-frequency noise and improving circuit stability. The output stage, consisting of the twelfth and thirteenth resistors connected in series and grounded, forms an output voltage divider structure, providing final signal conditioning and converting the amplified signal level to a range suitable for subsequent processing circuits. The design of the multi-stage filtering and stabilization network greatly improves the circuit's resistance to external interference, ensuring signal stability and reliability in complex charging environments.

[0029] In a preferred embodiment, this application can be further configured such that the filtering unit includes a fifth capacitor and a sixth capacitor, wherein:

[0030] The output terminal of the second signal amplification unit is grounded through the fifth capacitor, and the output terminal of the second signal amplification unit is grounded through the sixth capacitor.

[0031] By adopting the above technical solution, the parallel design of the two capacitors enhances the filtering effect, better suppressing various electromagnetic interferences and noises from the charging environment, such as high-frequency ripple generated by the switching power supply or electromagnetic radiation generated by other devices. Simultaneously, this structure effectively smooths the PWM signal, converting it into a stable DC signal, facilitating accurate voltage level determination by subsequent circuits. Attached Figure Description

[0032] Figure 1 A schematic diagram of the CP sampling circuit of an AC charging pile for automobiles provided in this application embodiment;

[0033] Figure 2 This is a schematic diagram of the structure of a PWM output sampling circuit provided in an embodiment of this application;

[0034] Figure 3 This is a schematic diagram of a DC output sampling circuit provided in an embodiment of this application.

[0035] Reference numerals in the attached diagram: R1, first resistor; R2, second resistor; R3, third resistor; R4, fourth resistor; R5, fifth resistor; R6, sixth resistor; R7, seventh resistor; R8, eighth resistor; R9, ninth resistor; R10, tenth resistor; R11, eleventh resistor; R12, twelfth resistor; R13, thirteenth resistor; C1, first capacitor; C2, second capacitor; C3, third capacitor; C4, fourth capacitor; C5, fifth capacitor; C6, sixth capacitor; U1, first operational amplifier; U2, second operational amplifier; D1, first diode; D2, second diode; D3, Zener diode. Detailed Implementation

[0036] The present application is further described in detail below with reference to the accompanying drawings.

[0037] Reference Figure 1 This is a schematic diagram of the CP sampling circuit of an AC charging pile for automobiles provided in an embodiment of this application. The circuit includes a PWM output sampling circuit and a DC output sampling circuit. The PWM output sampling circuit includes a first signal amplification unit and a current protection unit, and the DC output sampling circuit includes a second signal amplification unit and a filtering unit, wherein:

[0038] The input terminal of the first signal amplification unit is connected to the PWM output port of the microcontroller, and the output terminal of the first signal amplification unit is connected to the current protection unit, which outputs a PWM signal.

[0039] The input terminal of the second signal amplification unit is connected to the current protection unit, and the output terminal of the second signal amplification unit is connected to the filter unit, which outputs a DC signal.

[0040] In this embodiment, CP stands for Charging Control Guidance, used to monitor the interaction between the charging pile and the electric vehicle. The CP signal has two states: DC output and PWM output, with the PWM period being 1kHz. Based on voltage levels, there are three states: 12V, 9V, and 6V. Specifically, DC / PWM 12V, DC / PWM 9V, and DC / PWM 6V represent the following states: the charging plug is not inserted into the electric vehicle socket; the charging plug is inserted into the electric vehicle socket; and the electric vehicle is ready and requesting AC power output, respectively. The charging pile needs to identify the above CP signal voltage states to determine the charging status.

[0041] In this embodiment, the CP sampling circuit includes a PWM output sampling circuit and a DC output sampling circuit. The PWM output sampling circuit can accurately capture a 1kHz PWM signal, while the DC output sampling circuit can accurately measure the static voltage level. Specifically, the first signal amplification unit receives the PWM output signal from the microcontroller. This unit is designed to accurately capture PWM signals at three voltage levels: 12V, 9V, and 6V. By carefully adjusting the amplifier's gain and input range, accurate identification of these three voltage levels is ensured. The current protection unit protects the circuit while maintaining signal integrity. This is particularly important for charging pile systems that operate stably for extended periods, as they require continuous monitoring of CP signal changes. The DC output sampling circuit, composed of the second signal amplification unit and a filtering unit, is specifically designed to process DC signals. When the CP signal is in a DC state, this part of the circuit can accurately measure the voltage level, thereby determining the current charging state. The filtering unit is designed not only to smooth the PWM signal to obtain an equivalent DC value but also to effectively suppress potential grid interference or other high-frequency noise, improving measurement accuracy.

[0042] See Figure 2 The diagram shows a PWM output sampling circuit provided in an embodiment of this application, which specifically includes a first signal amplification unit and a current protection unit.

[0043] Specifically, the first signal amplification unit includes a first operational amplifier, a first resistor, a second resistor, and a third resistor, wherein:

[0044] The PWM output port of the microcontroller is connected to the non-inverting input of the first operational amplifier via the first resistor;

[0045] The inverting input terminal of the first operational amplifier is connected to the first terminal of the second resistor and the first terminal of the third resistor, respectively. The second terminal of the second resistor is connected to the power supply, and the second terminal of the third resistor is grounded.

[0046] In practice, the microcontroller's PWM output port is connected to the non-inverting input of the first operational amplifier via a first resistor. This first resistor serves as a current limiter and provides preliminary filtering, effectively suppressing potential high-frequency noise and improving signal quality. Simultaneously, due to the high impedance of the operational amplifier's non-inverting input, this connection method preserves the characteristics of the original PWM signal to the greatest extent possible, ensuring that signal integrity is not compromised.

[0047] The inverting input of the first operational amplifier is connected to the second and third resistors, forming a precise feedback network. The second resistor is connected to the power supply, while the third resistor is grounded; this configuration constitutes a voltage divider circuit. This voltage divider circuit is the core of the entire amplification unit; it not only determines the amplifier's gain but also provides appropriate bias for the signal. By precisely selecting the values ​​of the second and third resistors, the amplifier's operating point can be flexibly adjusted, enabling it to accurately handle CP signals of different voltage levels (12V, 9V, 6V).

[0048] The first signal amplification unit is configured as a non-inverting amplifier, with its gain determined by the ratio of the second and third resistors. The advantage of this structure is that it can maintain the signal phase while providing the necessary amplification. Furthermore, by appropriately selecting the resistor values, linear amplification can be achieved, ensuring a good linear relationship between the output and input signals.

[0049] Based on the above embodiments, as an optional implementation, the circuit further includes a first capacitor, wherein:

[0050] The first terminal of the first capacitor is connected to the PWM output port of the microcontroller, and the second terminal of the first capacitor is grounded.

[0051] The first capacitor is placed between the microcontroller's PWM output port and ground, providing a low-impedance path to ground for high-frequency components, forming a low-pass filter structure. The main purpose of this configuration is to filter out potential high-frequency noise and interference in the PWM signal while preserving its basic characteristics. In complex electromagnetic environments such as electric vehicle charging systems, various high-frequency interference sources (such as switching power supplies and motor drivers) can affect signal quality. By adding the first capacitor, the impact of these interferences on the CP signal can be effectively reduced.

[0052] Specifically, the current protection unit also includes a first diode, a second diode, a Zener diode, a fourth resistor, and a fifth resistor, wherein:

[0053] The output terminal of the first signal amplification unit is connected to the negative terminal of the first diode and the positive terminal of the second diode, respectively. The positive terminal of the first diode is connected to a positive voltage source, and the negative terminal of the second diode is connected to a negative voltage source.

[0054] The output of the first signal amplification unit is connected to the input of the second signal amplification unit via a fourth resistor and a fifth resistor in parallel. The input of the second signal amplification unit is grounded via a Zener diode.

[0055] In this embodiment, the output of the first signal amplification unit is simultaneously connected to the cathode of the first diode and the anode of the second diode, forming a bidirectional voltage clamping circuit. The anode of the first diode is connected to a positive voltage source, while the cathode of the second diode is connected to a negative voltage source. The main purpose of this configuration is to limit the voltage swing of the signal and prevent excessively high or low voltages from damaging subsequent circuits. When the signal voltage exceeds a preset safe range, the diode will conduct, clamping the excess voltage to a safe level. This design is particularly suitable for handling CP signals, as CP signals may exhibit different voltage levels (12V, 9V, 6V) under different conditions, and this protection mechanism can effectively cope with various possible abnormal situations, such as voltage spikes or unexpected high voltage inputs.

[0056] The output of the first signal amplification unit is connected to the input of the second signal amplification unit through a parallel network of the fourth and fifth resistors. The parallel resistors provide a voltage divider function, which can adjust the input signal to a suitable operating range for the second signal amplification unit. At the same time, the parallel resistor configuration increases the redundancy and reliability of the circuit, so that the circuit can still maintain its basic function even if one of the resistors fails.

[0057] A Zener diode was added to the input of the second signal amplification unit. The Zener diode provides a layer of voltage protection. It can quickly conduct when the voltage exceeds its breakdown voltage, guiding the excess voltage to ground, thereby protecting the subsequent amplification circuit from overvoltage damage.

[0058] Based on the above embodiments, as an optional implementation, the circuit further includes a second capacitor, wherein:

[0059] The first terminal of the second capacitor is connected to the input terminal of the second signal amplification unit, and the second terminal of the second capacitor is grounded.

[0060] In practice, one end of the second capacitor is directly connected to the input of the second signal amplification unit, while the other end is grounded. This connection method forms a bypass channel connected in parallel to the signal path. The signal output from the first signal amplification unit is then filtered.

[0061] Based on the above embodiments, as an optional implementation method, the Zener diode is a TVS bidirectional Zener diode.

[0062] In practical implementation, compared to ordinary unidirectional Zener diodes, TVS (Transient Voltage Suppressor) bidirectional Zener diodes provide fast-response overvoltage protection in both positive and negative directions. Regardless of whether the signal voltage is excessively high in the positive or negative direction, the TVS diode responds quickly, clamping the excess voltage to a safe level. Specifically, when the signal voltage exceeds the breakdown voltage of the TVS diode, it quickly conducts, guiding the excess voltage to ground, thereby protecting subsequent amplifier circuits from damage.

[0063] See Figure 3 The figure shows a schematic diagram of a DC output sampling circuit provided in an embodiment of this application, which specifically includes a second signal amplification unit and a filtering unit.

[0064] The second signal amplification unit includes a second operational amplifier, a sixth resistor, a seventh resistor, an eighth resistor, a ninth resistor, a tenth resistor, an eleventh resistor, a twelfth resistor, a thirteenth resistor, a third capacitor, and a fourth capacitor, wherein:

[0065] The circuit protection unit is connected to the inverting input of the second operational amplifier via the sixth and seventh resistors. The connection point of the sixth and seventh resistors is grounded via the third capacitor, and the connection point of the sixth and seventh resistors is grounded via the eighth resistor.

[0066] The non-inverting input terminal of the second operational amplifier is connected to the first terminal of the ninth resistor and the first terminal of the tenth resistor, respectively. The second terminal of the ninth resistor is connected to the power supply, and the second terminal of the tenth resistor is grounded.

[0067] The inverting input terminal of the second operational amplifier is connected to the output terminal of the second operational amplifier via the eleventh resistor, and the inverting input terminal of the second operational amplifier is connected to the output terminal of the second operational amplifier via the fourth capacitor.

[0068] The output of the second operational amplifier is grounded via the twelfth and thirteenth resistors connected in series.

[0069] The circuit's input is connected to the inverting input of the second operational amplifier via a circuit protection unit, a sixth resistor, and a seventh resistor. This configuration forms a precise input voltage divider network, providing not only initial signal attenuation but also impedance matching, ensuring that the CP signal from the protection unit can be effectively processed by the operational amplifier. The connection point of the sixth and seventh resistors is grounded via a third capacitor, forming a low-pass filter to filter out high-frequency noise and improve signal purity. Simultaneously, this connection point is also grounded via an eighth resistor, forming a stable bias network to ensure the input signal is at the appropriate operating point.

[0070] The non-inverting input of the second operational amplifier forms a precise voltage divider through resistors nine and ten. Resistor nine is connected to a 3.3V power supply, while resistor ten is grounded. This configuration provides the operational amplifier with a stable reference voltage. By adjusting the ratio of these two resistors, the amplifier's bias point can be precisely controlled, allowing it to adapt to voltage variations in the CP signal under different charging states, ensuring that the amplifier always operates in the linear region.

[0071] The feedback network, consisting of the eleventh resistor and the fourth capacitor connected in parallel between the inverting input and output of the second operational amplifier, forms the core of the inverting amplifier. The eleventh resistor determines the amplifier's basic gain, which is -R11 / R7 (assuming R7 is the input resistance). The fourth capacitor, connected in parallel with the eleventh resistor, creates a low-pass filter characteristic, limiting the high-frequency gain and helping to further suppress high-frequency noise and improve circuit stability. The output stage consists of the twelfth and thirteenth resistors connected in series and grounded, forming an output voltage divider structure. This design provides final signal conditioning, converting the amplified signal level to the range required by the ADC.

[0072] Specifically, the filter unit includes a fifth capacitor and a sixth capacitor, wherein:

[0073] The output of the second signal amplification unit is grounded through the fifth capacitor, and the output of the second signal amplification unit is grounded through the sixth capacitor.

[0074] In practice, the fifth and sixth capacitors are connected in parallel between the output of the second signal amplification unit and ground. This parallel configuration forms a second-order low-pass filter, which can more effectively attenuate high-frequency components. Ensuring that the ADC maintains high accuracy throughout the sampling process is crucial, especially when processing rapidly changing CP signals.

[0075] The embodiments described herein are preferred embodiments of this utility model and are not intended to limit the scope of protection of this utility model. Therefore, all equivalent changes made to the structure, shape, and principle of this utility model should be included within the scope of protection of this utility model.

Claims

1. A CP sampling circuit for an AC charging pile for automobiles, characterized in that, The circuit includes a PWM output sampling circuit and a DC output sampling circuit. The PWM output sampling circuit includes a first signal amplification unit and a current protection unit. The DC output sampling circuit includes a second signal amplification unit and a filtering unit, wherein: The input terminal of the first signal amplification unit is connected to the PWM output port of the microcontroller, and the output terminal of the first signal amplification unit is connected to the current protection unit, which outputs a PWM signal. The input terminal of the second signal amplification unit is connected to the current protection unit, and the output terminal of the second signal amplification unit is connected to the filtering unit, which outputs a DC signal.

2. The CP sampling circuit of the vehicle AC charging pile according to claim 1, characterized in that, The first signal amplification unit includes a first operational amplifier, a first resistor, a second resistor, and a third resistor, wherein: The PWM output port of the microcontroller is connected to the non-inverting input of the first operational amplifier via the first resistor; The inverting input terminal of the first operational amplifier is connected to the first terminal of the second resistor and the first terminal of the third resistor, respectively. The second terminal of the second resistor is connected to the power supply, and the second terminal of the third resistor is grounded.

3. The CP sampling circuit for an AC charging pile for automobiles according to claim 2, characterized in that, The circuit also includes a first capacitor, wherein: The first terminal of the first capacitor is connected to the PWM output port of the microcontroller, and the second terminal of the first capacitor is grounded.

4. The CP sampling circuit of the vehicle AC charging pile according to claim 1, characterized in that, The current protection unit further includes a first diode, a second diode, a Zener diode, a fourth resistor, and a fifth resistor, wherein: The output terminal of the first signal amplification unit is connected to the negative terminal of the first diode and the positive terminal of the second diode, respectively. The positive terminal of the first diode is connected to a positive voltage source, and the negative terminal of the second diode is connected to a negative voltage source. The output terminal of the first signal amplification unit is connected in parallel to the input terminal of the second signal amplification unit via the fourth resistor and the fifth resistor, and the input terminal of the second signal amplification unit is grounded via the Zener diode.

5. The CP sampling circuit of the car AC charging pile according to claim 4, characterized in that, The circuit also includes a second capacitor, wherein: The first end of the second capacitor is connected to the input end of the second signal amplification unit, and the second end of the second capacitor is grounded.

6. The CP sampling circuit of the car AC charging pile according to claim 4, characterized in that, The Zener diode is a TVS bidirectional Zener diode.

7. The CP sampling circuit of the vehicle AC charging pile according to claim 1, characterized in that, The second signal amplification unit includes a second operational amplifier, a sixth resistor, a seventh resistor, an eighth resistor, a ninth resistor, a tenth resistor, an eleventh resistor, a twelfth resistor, a thirteenth resistor, a third capacitor, and a fourth capacitor, wherein: The circuit protection unit is connected to the inverting input terminal of the second operational amplifier via the sixth resistor and the seventh resistor. The connection point of the sixth resistor and the seventh resistor is grounded via the third capacitor, and the connection point of the sixth resistor and the seventh resistor is grounded via the eighth resistor. The non-inverting input terminal of the second operational amplifier is connected to the first terminal of the ninth resistor and the first terminal of the tenth resistor, respectively. The second terminal of the ninth resistor is connected to the power supply, and the second terminal of the tenth resistor is grounded. The inverting input terminal of the second operational amplifier is connected to the output terminal of the second operational amplifier via the eleventh resistor, and the inverting input terminal of the second operational amplifier is connected to the output terminal of the second operational amplifier via the fourth capacitor. The output of the second operational amplifier is grounded via the twelfth resistor and the thirteenth resistor in series.

8. The CP sampling circuit of the vehicle AC charging pile according to claim 1, characterized in that, The filtering unit includes a fifth capacitor and a sixth capacitor, wherein: The output terminal of the second signal amplification unit is grounded through the fifth capacitor, and the output terminal of the second signal amplification unit is grounded through the sixth capacitor.

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