High-efficiency linear inverse class-F power amplifier for improving AM-AM distortion

Abstract

The invention relates to a high-efficiency linear inverse class F power amplifier for improving AM-AM distortion, which is characterized by comprising an SMA input connecting line, an input matching circuit, a grid bias circuit, a nonlinear factor control circuit, a power amplifier tube CG2H40010F, an output fundamental wave matching circuit, a drain bias circuit, an output harmonic suppression circuit and an SMA output connecting line, wherein one end of the SMA input connecting line is connected with a radio frequency signal source; the other end of the SMA input connecting line is connected with the input end of the input matching circuit; the output end of the input matching circuit is connected with the input end of the nonlinear factor control circuit; the output end of the nonlinear factor control circuit is connected with the grid electrode of the power amplifier tube CG2H40010F; the grid bias circuit is connected in parallel between the output end of the input matching circuit and the input end of the nonlinear factor control circuit; the source electrode of the power amplifier tube CG2H40010F is grounded; the drain electrode of the CG2H40010F is connected with the input end of the output fundamental wave matching circuit; the output end of the output fundamental wave matching circuit is connected with the input end of the output harmonic suppression circuit; the drain biasing circuit is connected in parallel between the output end of the output fundamental wave matching circuit and the input end of the output harmonic suppression circuit; the output end of the output harmonic suppression circuit is connected with one end of the SMA output connecting line; and the other end of the SMA output connecting line is a radio frequency signal output end.

Description

Technical Field

[0001] The present invention belongs to the technical field of wireless communication, and particularly relates to a high-efficiency linear inverse class-F power amplifier for improving AM-AM distortion. Background Art

[0002] As an important part of the electromagnetic spectrum, radio frequency (RF), microwave, and millimeter-wave technologies have a profound historical accumulation in the engineering field and possess extensive application potential. The frequency range of RF is between 300 kHz and 300 GHz, the microwave frequency range is from 300 MHz to 300 GHz, and the millimeter-wave frequency range is 30 GHz to 300 GHz. Microwaves belong to the RF category, and millimeter-waves belong to the microwave category. Their unique propagation characteristics enable RF to be effectively transmitted in the atmospheric environment, making it a key foundation for realizing 5G wireless communication.

[0003] Radio frequency power amplifiers (PAs) play a core role in 5G communication systems. As a key hardware module responsible for power amplification at the end of the signal chain, their performance directly affects communication quality, coverage, and system energy efficiency. In the transmitting system, the RF power amplifier undertakes the task of amplifying the small-power RF signal generated by the modulation oscillator circuit to a sufficient intensity for radiation through the antenna. Without an efficient RF power amplifier, the signal will be difficult to reach the intensity required for effective wireless transmission. In 5G wireless communication, linearity and efficiency are the most critical technical indicators of RF power amplifiers and have become a popular research direction. With the rapid development of 5G wireless communication technology, the performance requirements for PAs in base stations and terminal devices are becoming increasingly stringent. In the application scenarios of the 5G frequency band, the PA needs to meet the two challenges of high efficiency and high linearity simultaneously, which plays a decisive role in the overall performance, energy consumption cost, and deployment feasibility of the system. 5G base stations adopt large-scale MIMO architectures and dense networking technologies, and dozens or even hundreds of RF channels need to be deployed in a single base station, resulting in a significant increase in the overall system power consumption and thus posing higher requirements for the efficiency of the PA. At the same time, 5G communication systems use high-order modulation methods and are extremely sensitive to signal distortion. The nonlinearity of the PA easily causes problems such as intermodulation distortion and spectral regrowth, reducing signal quality and affecting communication rate and reliability, which poses a severe challenge to the linearity of the PA.

[0004] In the article "Inverse Class-F Power Amplifier for Power Amplifier-Rectifier Integrated Design" published by Li Haodong of Northwestern Polytechnical University in June 2024, a high-efficiency inverse Class-F power amplifier operating at 2.45 GHz was reported. The power amplifier circuit includes an input matching circuit, a stability circuit, a power amplifier transistor CG2H40010F, parasitic compensation, an output harmonic suppression circuit, and an output matching circuit. Based on the inverse Class-F theory, Li Haodong achieved power amplification by controlling the harmonics at the output end and combining conjugate matching at the input end with load line matching at the output end, and also gave the circuit schematic diagram of the power amplifier. Figure 1 , circuit structure diagram Figure 2 and large-signal HB simulation diagram Figure 3 . However, there are many deficiencies in the existing technical solutions. On the one hand, this solution only analyzes and controls the output impedance space of the power amplifier, and only conjugate matching is performed at the input end. This makes the power amplifier achieve relatively high efficiency, but has poor linearity, significant gain compression, and large AM-AM distortion. On the other hand, this structure is realized based on the inverse Class-F power amplifier theory and does not combine the continuous theory, resulting in the power amplifier achieving high efficiency only at a single frequency point, and its broadband characteristics still need to be further optimized. Summary of the Invention

[0005] In order to solve the problems existing in the background technology and break through the trade-off limitations in terms of efficiency and linearity in traditional designs, by combining the continuous inverse Class-F theory, harmonic control is not only performed at the output end, but also the second harmonic is controlled at the input end to change the waveform at the intrinsic gate node. At the same time, fundamental wave conjugate matching is performed at the input end and fundamental wave load line matching is performed at the output end, and a broadband inverse Class-F power amplifier operating at 2.0 GHz - 2.4 GHz is innovatively designed to achieve high efficiency, relatively high linearity, and broadband performance to meet the requirements of 5G wireless communication.

[0006] Based on the above-mentioned invention object, the present invention provides a high-efficiency linear inverse class-F power amplifier for improving AM-AM distortion, comprising: an SMA input connection line, an input matching circuit, a gate bias circuit, a non-linear factor control circuit, a power amplifier transistor CG2H40010F, an output fundamental matching circuit, a drain bias circuit, an output harmonic suppression circuit, and an SMA output connection line; wherein, one end of the SMA input connection line is connected to a radio frequency signal source; the other end of the SMA input connection line is connected to the input end of the input matching circuit; the output end of the input matching circuit is connected to the input end of the non-linear factor control circuit; the output end of the non-linear factor control circuit is connected to the gate of the power amplifier transistor CG2H40010F; the gate bias circuit is connected in parallel between the output end of the input matching circuit and the input end of the non-linear factor control circuit; the source of the power amplifier transistor CG2H40010F is grounded; the drain of the CG2H40010F is connected to the input end of the output fundamental matching circuit; the output end of the output fundamental matching circuit is connected to the input end of the output harmonic suppression circuit; the drain bias circuit is connected in parallel between the output end of the output fundamental matching circuit and the input end of the output harmonic suppression circuit; the output end of the output harmonic suppression circuit is connected to one end of the SMA output connection line; the other end of the SMA output connection line is the radio frequency signal output end.

[0007] The present invention has at least the following beneficial effects

[0008] The present invention combines the continuous inverse class-F theory, not only controls the harmonics at the output end, but also controls the second harmonic at the input end to change the waveform at the intrinsic gate node. The fundamental wave conjugate matching is performed at the input end, and the fundamental wave load line matching is performed at the output end. Finally, a broadband inverse class-F power amplifier operating at 2.0 GHz to 2.4 GHz is innovatively designed, achieving high efficiency, relatively high linearity, and broadband performance, and is applicable to 5G wireless communication. Description of the Drawings

[0009] Figure 1 is the circuit schematic diagram of the 2.45 GHz power amplifier;

[0010] Figure 2 is the circuit structure diagram of the 2.45 GHz power amplifier;

[0011] Figure 3 is the large-signal HB simulation diagram of the 2.45 GHz power amplifier;

[0012] Figure 4 is the overall circuit framework diagram of the power amplifier of the present invention;

[0013] Figure 5 is the overall circuit diagram of the 2.0 - 2.4 GHz power amplifier of the present invention;

[0014] Figure 6Schematic diagram of the circuit structures of the input matching circuit 2, gate bias circuit 3, and non-linear factor control circuit 4 of the present invention;

[0015] Figure 7 Schematic diagram of the circuit structures of the output matching circuit 5, drain bias circuit 6, and output harmonic suppression circuit 7 of the present invention;

[0016] Figure 8 Schematic diagram of the input impedance of the 2.0 - 2.4 GHz power amplifier of the present invention;

[0017] Figure 9 Large-signal HB simulation diagram of the 2.0 - 2.4 GHz power amplifier of the present invention. Specific implementation manners

[0018] The following uses specific examples to illustrate the implementation manners of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific implementation manners. Various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the diagrams provided in the following embodiments only schematically illustrate the basic concept of the present invention. Without conflict, the following embodiments and the features in the embodiments can be combined with each other.

[0019] Gate-source non-linearity refers to the non-linearity caused by the gate-source capacitance Cgs of the power amplifier transistor. It will apply a second harmonic voltage to the input ideal sine wave excitation, change the input voltage waveform reaching the intrinsic gate of the power amplifier transistor, and then shape the drain current waveform through the transconductance. This non-linearity will affect the second harmonic component in the drain current. In a class-F inverse power amplifier, due to the unique load impedance space: open circuit for the second harmonic and short circuit for the third harmonic, a slight change in the drain second harmonic current component will cause a sharp change in the drain second harmonic voltage, resulting in the class-F inverse power amplifier entering the knee region in the low input power region, leading to serious AM-AM distortion and large gain compression. For a traditional class-F inverse power amplifier, the second harmonic at its input end is treated as a short circuit, that is, the non-linear factor γ (γ = amplitude of the second harmonic voltage at the intrinsic gate node / amplitude of the fundamental voltage) is 0. However, it has been found that the larger γ is, the better the AM-AM distortion of the class-F inverse power amplifier can be improved, the degree of gain compression can be reduced, and thus the linearity can be improved. The present invention proposes an input-end circuit to control the non-linear factor γ, reduce the drain second harmonic current component at the input end, and prevent the power amplifier from entering the knee region at low power input; a output-end circuit is proposed to perform harmonic control on the continuous class-F inverse power amplifier to achieve square-wave current and half-sine-wave voltage and improve the efficiency.

[0020] Based on the research of power amplifier tubes in the present invention, it is found that the nonlinear factor γ is 0 at the traditional short - circuit point, which results in large gain compression at the saturation power and serious AM - AM distortion; while when scanning near the open - circuit point, the nonlinear factor γ can reach more than 0.2. By setting the input - side second - harmonic impedance near the open - circuit point, the AM - AM distortion characteristics of the inverse class - F power amplifier can be improved, and the linearity can be enhanced.

[0021] Since the designed frequency band is in the low - frequency microwave band and the loss of the microstrip line in this frequency band is small, the microstrip line is used to build the circuit. The invented broadband inverse class - F power amplifier includes a microstrip - line circuit, lumped elements, and a Rogers4350B dielectric substrate.

[0022] Please refer to Figure 4 and Figure 5 As shown in FIGS. and, the present invention provides a high - efficiency linear inverse class - F power amplifier for improving AM - AM distortion, including: an SMA input connection line 1, an input matching circuit 2, a gate biasing circuit 3, a nonlinear - factor control circuit 4, a power - amplifier tube CG2H40010F, an output fundamental - wave matching circuit 5, a drain biasing circuit 6, an output harmonic - suppression circuit 7, and an SMA output connection line 8; wherein, one end of the SMA input connection line is connected to a radio - frequency signal source; the other end of the SMA input connection line is connected to the input end of the input matching circuit; the output end of the input matching circuit is connected to the input end of the nonlinear - factor control circuit; the output end of the nonlinear - factor control circuit is connected to the gate of the power - amplifier tube CG2H40010F; the gate biasing circuit is connected in parallel between the output end of the input matching circuit and the input end of the nonlinear - factor control circuit; the source of the power - amplifier tube CG2H40010F is grounded; the drain of the CG2H40010F is connected to the input end of the output fundamental - wave matching circuit; the output end of the output fundamental - wave matching circuit is connected to the input end of the output harmonic - suppression circuit; the drain biasing circuit is connected in parallel between the output end of the output fundamental - wave matching circuit and the input end of the output harmonic - suppression circuit; the output end of the output harmonic - suppression circuit is connected to one end of the SMA output connection line; the other end of the SMA output connection line is the radio - frequency signal output end.

[0023] The broadband inverse class-F power amplifier invented in this embodiment can achieve high-efficiency, relatively high linearity, and wide-bandwidth power amplification. Its working principle is as follows: The DC voltage supplies power to the power amplifier transistor CG2H40010F through the gate bias circuit 3 and the drain bias circuit 6 to provide a suitable static operating point. The RF input signal enters the amplification circuit through the SMA input connection line 1. Since the input matching circuit 2 presents a conjugate impedance to the power amplifier tube, maximum power transfer of the RF signal is achieved. The nonlinear factor control circuit 4 reduces the second harmonic component in the drain current to prevent the power amplifier from entering the knee region at low-power input, thereby improving linearity. The output fundamental matching circuit 5 and the output harmonic suppression circuit 7 together provide the optimal fundamental impedance and harmonic impedance of the inverse class-F power amplifier for the intrinsic current source plane to shape the drain voltage. At this time, both the drain voltage and the drain current are shaped as required, and the fundamental impedance is matched to the load line to obtain optimal performance.

[0024] In this embodiment, the SMA input connection line 1 and the SMA output connection line 8 are composed of 50Ω microstrip lines to minimize the impact of actual SMA soldering on input matching and output matching.

[0025] Please refer to Figure 6 , preferably, the input matching circuit includes: microstrip lines TL1 to TL5, resistor R, and capacitors C1 to C2; one end of capacitor C1 is connected to the input end of the input matching circuit; the other end of capacitor C1 is connected to one end of microstrip line TL1; the other end of microstrip line TL1 is connected to one end of microstrip line TL2; the other end of microstrip line TL2, one end of microstrip line TL3, and one end of microstrip line TL4 are connected; microstrip line TL3 is open-circuited at the load; the other end of microstrip line TL4, one end of resistor R, and one end of capacitor C2 are connected; the other end of resistor R, the other end of capacitor C2, and one end of microstrip line TL5 are connected; the other end of microstrip line TL5 is connected to the output end of the input matching circuit. Capacitor C1 plays a role in isolating DC, resistor R and capacitor C2 play a role in stabilizing the power amplifier, and microstrip lines TL1 to TL5 play a role in matching the fundamental impedance to the conjugate impedance point of the transistor's input impedance to achieve maximum power transfer.

[0026] Please refer to Figure 6 , preferably, the gate bias circuit includes: a microstrip line TL6 with a length of λ / 4; one end of the microstrip line TL6 is connected in parallel between the output end of the input matching circuit and the input end of the nonlinear factor control circuit; the other end of the microstrip line TL6 is grounded. The length of microstrip line TL6 is λ / 4 to isolate the RF signal from the DC signal and ensure the normal transmission of the RF signal to the power amplifier tube.

[0027] Please refer to Figure 6, preferably, the non-linear factor control circuit includes: microstrip lines TL7 to TL9; one end of the microstrip line TL7 is connected to the input end of the non-linear factor control circuit; the other end of the microstrip line TL7, one end of the microstrip line TL8, and one end of the microstrip line TL9 are connected; the microstrip line TL8 is open-circuited at the load; the other end of the microstrip line TL9 is connected to the output end of the non-linear factor control circuit. The microstrip lines TL7 and TL8 are used to tune the second harmonic impedance at the input end to the short-circuit point, so that the input matching circuit 2 will not affect the harmonic impedance. The microstrip line TL9 further tunes the second harmonic impedance to near the open-circuit point to obtain a larger γ. Figure 6 is the circuit diagram of the input matching circuit 2, the gate bias circuit 3, and the non-linear factor control circuit 4. Figure 6 The circuit of Figure 8 tunes the input impedance of the intrinsic surface to the desired position. As

[0028] Please refer to Figure 7 , preferably, the output matching circuit includes: microstrip lines TL10 to TL12; one end of the microstrip line TL10 is connected to the input end of the output matching circuit; the other end of the microstrip line TL10 is connected to one end of the microstrip line TL11; the other end of the microstrip line TL11 is connected to one end of the microstrip line TL12; the other end of the microstrip line TL12 is connected to the output end of the output matching circuit. The microstrip lines TL10 to TL12 perform load-line matching on the fundamental wave impedance within the designed frequency band to obtain better efficiency and output power.

[0029] Please refer to Figure 7 , preferably, the drain bias circuit includes a microstrip line TL13 with a length of λ / 4; one end of the microstrip line TL13 is connected in parallel between the output end of the output matching circuit and the input end of the output harmonic suppression circuit; the other end of the microstrip line TL13 is grounded. The length of the microstrip line TL13 is λ / 4 to isolate the RF signal from the DC signal and ensure the normal transmission of the RF signal.

[0030] Please refer to Figure 7, preferably, the output harmonic suppression circuit includes: microstrip lines TL14 to TL18 and a capacitor C3; one end of the microstrip line TL14 is connected to the input end of the output harmonic suppression circuit; the other end of the microstrip line TL14 is connected to one end of the microstrip line TL15; the other end of the microstrip line TL15, one end of the microstrip line TL16 and one end of the microstrip line TL17 are connected; the microstrip line TL16 is open-circuited at the load; the other end of the microstrip line TL17 is connected to one end of the microstrip line TL18; the other end of the microstrip line TL18 is connected to one end of the capacitor C3; the other end of the capacitor C3 is connected to the output end of the output harmonic suppression circuit. The microstrip lines TL14 to TL18 suppress the second harmonic impedance within the designed frequency band to near the short-circuit point and suppress the third harmonic impedance to near the open-circuit point to shape the square-wave current and half-sine-wave voltage of the inverse class-F power amplifier, reduce the degree of overlap to improve the overall efficiency, and the capacitor C3 plays a role in isolating the direct current.

[0031] The present invention proposes a general method for obtaining a suitable second harmonic impedance space at the input end, which can be applied to all GaN power amplifier transistors and different gate bias conditions; in addition, the present invention proposes a broadband high-efficiency linear inverse class-F power amplifier for improving AM-AM distortion. Combining a new type of non-linear factor control circuit and an output harmonic suppression circuit, high efficiency and relatively high linearity are achieved on the premise of a certain bandwidth. The large-signal electromagnetic simulation effect diagram at 2.0 GHz - 2.4 GHz is as Figure 9 shown. It can be seen that within the absolute bandwidth of 0.4 GHz, its gain characteristic is relatively flat within a large output power range without sudden changes, indicating that the AM-AM distortion characteristic is weak. At the 3 dB gain compression point, the drain efficiency DE is greater than 71.5%, the output power Pout is greater than 40.9 dBm, and the large-signal gain Gain is greater than 11.6 dB. On the premise of a certain bandwidth, high linearity and high-efficiency performance are achieved, which is very meaningful. Comparing with the inverse class-F power amplifier literature "Inverse Class-F Power Amplifier for Power Amplifier - Rectifier Integrated Design" in the same microwave low-frequency band, it also designs an inverse class-F power amplifier. Compared with this literature, the absolute bandwidth of the present invention is 0.4 GHz, while the literature is at a single frequency point; and the AM-AM characteristic of the literature is worse, and the gain characteristic changes significantly with the output power. On the basis of achieving the same output power, its gain compression reaches about 6 dB, indicating poor linearity.

[0032] In summary, the present invention designs a novel non-linear factor control circuit, which can isolate the influence of the input matching circuit, accurately tune the second harmonic impedance at the input end to near the open circuit point, so as to improve the AM-AM distortion and gain compression characteristics and enhance the linearity. The output harmonic suppression circuit of the present invention suppresses the second harmonic and third harmonic impedances to near the open circuit and short circuit points respectively. In cooperation with the non-linear factor control circuit, high efficiency is achieved within a certain bandwidth and at the 3dB compression point.

[0033] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the spirit and scope of the present technical solution, and they should all be covered within the scope of the claims of the present invention.

Claims

1. A high-efficiency linear inverse class F power amplifier for improving AM-AM distortion, characterized in that: include: SMA input connecting wire, input matching circuit, gate bias circuit, nonlinear factor control circuit, power amplifier tube CG2H40010F, output fundamental wave matching circuit, drain bias circuit, output harmonic suppression circuit and SMA output connecting wire; wherein, one end of the SMA input connecting wire is connected to the radio frequency signal source; the other end of the SMA input connecting wire is connected to the input end of the input matching circuit; the output end of the input matching circuit is connected to the input end of the nonlinear factor control circuit; the output end of the nonlinear factor control circuit is connected to the gate of the power amplifier tube CG2H40010F; the gate bias circuit The circuit is connected in parallel between the output end of the input matching circuit and the input end of the nonlinear factor control circuit; the source of the power amplifier tube CG2H40010F is grounded; the drain of CG2H40010F is connected to the input end of the output fundamental matching circuit; the output end of the output fundamental matching circuit is connected to the input end of the output harmonic suppression circuit; the drain bias circuit is connected in parallel between the output end of the output fundamental matching circuit and the input end of the output harmonic suppression circuit; the output end of the output harmonic suppression circuit is connected to one end of the SMA output connecting line; the other end of the SMA output connecting line is the RF signal output end.

2. A high-efficiency linear inverse class F power amplifier for improving AM-AM distortion according to claim 1, characterized in that: The SMA input connection line and the SMA output connection line are formed by 50Ω microstrip lines to minimize the influence of the SMA on the input matching and the output matching during actual welding.

3. A high-efficiency linear inverse class F power amplifier for improving AM-AM distortion according to claim 1, characterized in that: The input matching circuit includes: microstrip lines TL1~TL5, a resistor R and capacitors C1~C2; one end of the capacitor C1 is connected to the input end of the input matching circuit; the other end of the capacitor C1 is connected to one end of the microstrip line TL1; the other end of the microstrip line TL1 is connected to one end of the microstrip line TL2; the other end of the microstrip line TL2, one end of the microstrip line TL3 and one end of the microstrip line TL4 are connected; the load of the microstrip line TL3 is open; the other end of the microstrip line TL4, one end of the resistor R and one end of the capacitor C2 are connected; the other end of the resistor R, the other end of the capacitor C2 and one end of the microstrip line TL5 are connected; the other end of the microstrip line TL5 is connected to the output end of the input matching circuit.

4. A high-efficiency linear inverse class F power amplifier for improving AM-AM distortion according to claim 1, characterized in that: The gate bias circuit comprises: a microstrip line TL6 with a length of λ / 4; one end of the microstrip line TL6 is connected in parallel between the output end of the input matching circuit and the input end of the nonlinear factor control circuit; and the other end of the microstrip line TL6 is grounded.

5. A high-efficiency linear inverse class F power amplifier for improving AM-AM distortion according to claim 1, characterized in that: The nonlinear factor control circuit includes: microstrip lines TL7 to TL9; one end of the microstrip line TL7 is connected to the input end of the nonlinear factor control circuit; the other end of the microstrip line TL7, one end of the microstrip line TL8 and one end of the microstrip line TL9 are connected; the load of the microstrip line TL8 is open; and the other end of the microstrip line TL9 is connected to the output end of the nonlinear factor control circuit.

6. A high-efficiency linear inverse class F power amplifier for improving AM-AM distortion according to claim 1, characterized in that: The output matching circuit includes: microstrip lines TL10~TL12; one end of the microstrip line TL10 is connected to the input end of the output matching circuit; the other end of the microstrip line TL10 is connected to one end of the microstrip line TL11; the other end of the microstrip line TL11 is connected to one end of the microstrip line TL12; the other end of the microstrip line TL12 is connected to the output end of the output matching circuit.

7. A high-efficiency linear inverse class F power amplifier for improving AM-AM distortion according to claim 1, characterized in that: The drain bias circuit includes a microstrip line TL13 with a length of λ / 4; one end of the microstrip line TL13 is connected in parallel between the output end of the output matching circuit and the input end of the output harmonic suppression circuit; and the other end of the microstrip line TL13 is grounded.

8. The high-efficiency linear inverse class F power amplifier for improving AM-AM distortion according to claim 1, characterized in that: The output harmonic suppression circuit includes: microstrip lines TL14~TL18 and capacitor C3; one end of the microstrip line TL14 is connected to the input end of the output harmonic suppression circuit; the other end of the microstrip line TL14 is connected to one end of the microstrip line TL15; the other end of the microstrip line TL15, one end of the microstrip line TL16 and one end of the microstrip line TL17 are connected; the microstrip line TL16 load is open; the other end of the microstrip line TL17 is connected to one end of the microstrip line TL18; the other end of the microstrip line TL18 is connected to one end of the capacitor C3; the other end of the capacitor C3 is connected to the output end of the output harmonic suppression circuit.

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