An adaptive biasing wideband high power amplifier

Abstract

The application discloses a kind of self-adapting bias wideband high-power amplifier, it is related to radio frequency integrated circuit technical field.The amplifier includes active self-adapting bias module, amplifier drive stage module, interstage high coupling coefficient transformer matching network module, amplifier power stage module and high-power electrostatic discharge output matching common module.The application is realized by the active self-adapting bias module, interstage high coupling coefficient transformer matching network module and high-power electrostatic discharge output matching common module of setting, the combination innovation of high-power amplifier self-adapting bias, wideband and electrostatic discharge multiplexing is realized, performance surpasses traditional gallium arsenide process on silicon-based platform, cost is the high-power amplifier of CMOS, and 5G / 6G radio frequency chip provides core technical support.

Description

Technical Field

[0001] This invention relates to the field of radio frequency integrated circuit technology, and more specifically to an adaptive bias broadband high-power amplifier. Background Technology

[0002] As 5G communication places higher demands on base station power consumption and integration, traditional gallium arsenide (GaAs) high-power amplifiers face the following challenges: high power consumption, as the power consumption of traditional GaAs high-power amplifiers cannot meet the low-energy consumption requirements of 5G base stations; integration difficulties, as GaAs chips need to be co-packaged with silicon-based control circuits, increasing area and cost; bandwidth limitations, as traditional inter-stage matching networks cannot achieve over 92% ultra-wideband impedance matching; and electrostatic discharge (ESD) protection deficiencies, as a large number of diodes need to be stacked under large signal swings, resulting in a large footprint and reduced ESD performance.

[0003] While existing high-power amplifiers based on silicon heterojunction bipolar transistors (HBTs) can reduce costs, they suffer from problems such as lack of adaptive biasing and insufficient broadband matching. Therefore, there is an urgent need for a high-power amplifier based on silicon HBTs that integrates adaptive biasing, broadband matching, and efficient electrostatic discharge. Summary of the Invention

[0004] To address the aforementioned shortcomings in the prior art, the present invention provides an adaptive bias broadband high-power amplifier.

[0005] To achieve the above-mentioned objectives, the technical solution adopted by this invention is as follows:

[0006] An adaptive bias broadband high-power amplifier includes an active adaptive bias module, an amplifier driver stage module, an interstage high coupling coefficient transformer matching network module, an amplifier power stage module, and a high-power electrostatic discharge output matching shared module.

[0007] The active adaptive bias module includes a first operating frequency band low-dropout linear regulator group, a second operating frequency band low-dropout linear regulator group, an amplifier driver stage active bias submodule, and an amplifier power stage active bias submodule. The first ports of both the first and second operating frequency band low-dropout linear regulator groups are connected to the frequency band selection signal input. The second ports of both groups are connected to the first port of the amplifier driver stage module. The third ports of both groups are connected to the first port of the amplifier power stage module. The amplifier driver stage active bias submodule is connected to the RF signal input, and the amplifier power stage active bias submodule is connected to the second port of the interstage high coupling coefficient transformer matching network module. The active adaptive bias module is used to adaptively switch the bias voltage of the output amplifier driver stage module and the amplifier power stage module according to the operating frequency band of the RF signal input.

[0008] The second port of the amplifier driver module is connected to the RF signal input terminal, and the third port of the amplifier driver module is connected to the first port of the interstage high coupling coefficient transformer matching network module.

[0009] The second port of the interstage high coupling coefficient transformer matching network module is connected to the second port of the amplifier power stage module; the interstage high coupling coefficient transformer matching network module is used for impedance matching between the amplifier driver stage module and the amplifier power stage module over a wide bandwidth;

[0010] The second port of the amplifier power stage module is connected to the second port of the interstage high coupling coefficient transformer matching network module, and the third port of the amplifier power stage module is connected to the first port of the high-power electrostatic discharge output matching common module.

[0011] The second port of the high-power electrostatic discharge output matching shared module is connected to the radio frequency signal output terminal; the high-power electrostatic discharge output matching shared module is used to multiplex electrostatic discharge protection and radio frequency choke through the transformer secondary inductance and the additional inductance.

[0012] Furthermore, both the first operating frequency band low-dropout linear regulator group and the second operating frequency band low-dropout linear regulator group include two identical low-dropout linear regulator group substructures. The input terminals of the two identical low-dropout linear regulator group substructures serve as the first port of the low-dropout linear regulator group, and the output terminals of the two identical low-dropout linear regulator group substructures serve as the second and third ports of the low-dropout linear regulator group, respectively, and are respectively connected to the first port of the amplifier driver stage module and the first port of the amplifier power stage module.

[0013] Furthermore, the low-dropout linear regulator substructure includes an error amplifier, a first resistor, a second resistor, a first MOSFET, and a second MOSFET. The negative input terminal of the error amplifier is connected to a bandgap reference voltage. The positive input terminal of the error amplifier is simultaneously connected to one end of the first resistor and one end of the second resistor. The output terminal of the error amplifier is connected to the gate of the first MOSFET. The other end of the first resistor is grounded. The other end of the second resistor is connected to the drain of the first MOSFET. The source of the first MOSFET and the first power supply port of the error amplifier are both connected to the drain of the second MOSFET. The second power supply port of the error amplifier is grounded. The source of the second MOSFET is connected to the chip operating voltage. The gate of the second MOSFET serves as the input terminal of the low-dropout linear regulator substructure.

[0014] Furthermore, a third MOSFET operating in the linear region is connected in parallel with the second resistor in the first and second low-dropout linear regulator groups. The source and drain of the third MOSFET are connected to the two ends of the second resistor, respectively. When one of the first and second low-dropout linear regulator groups fails, a replacement bias voltage is output by adjusting the gate tuning voltage of the third MOSFET in the other group.

[0015] Furthermore, the amplifier driver stage module includes a first BJT and a second BJT; the gate of the first BJT serves as the first port of the amplifier driver stage module, the drain of the first BJT serves as the third port of the amplifier driver stage module, the source of the first BJT is connected to the drain of the second BJT, the source of the second BJT is grounded, and the gate of the second BJT serves as the second port of the amplifier driver stage module.

[0016] Furthermore, the interstage high coupling coefficient transformer matching network module includes a first inductor, a second inductor, a first capacitor, and a second capacitor; the first inductor and the second inductor adopt a double-layer metal overlapping design; one end of the first inductor serves as the first port of the interstage high coupling coefficient transformer matching network module, the other end of the first inductor is connected to one end of the first capacitor, and the other end of the first capacitor is grounded; one end of the second inductor serves as the second port of the interstage high coupling coefficient transformer matching network module, the other end of the second inductor is connected to one end of the second capacitor, and the other end of the second capacitor is grounded.

[0017] Furthermore, the amplifier power stage module includes a third BJT and a fourth BJT; the gate of the third BJT serves as the first port of the amplifier power stage module, the drain of the third BJT serves as the third port of the amplifier power stage module, the source of the third BJT is connected to the drain of the fourth BJT, the source of the fourth BJT is grounded, and the gate of the fourth BJT serves as the second port of the amplifier power stage module.

[0018] Furthermore, the high-power electrostatic discharge output matching common module includes a third inductor, a fourth inductor, a fifth inductor, a third capacitor, a fourth capacitor, and an electrostatic protector; the third and fourth inductors adopt a double-layer metal overlapping design; one end of the third inductor serves as the first port of the high-power electrostatic discharge output matching common module, and the other end of the third inductor is connected to one end of the third capacitor, the other end of the third capacitor is grounded; one end of the fourth inductor serves as the second port of the high-power electrostatic discharge output matching common module, and the other end of the fourth inductor, after passing through the fifth inductor, is simultaneously connected to one end of the electrostatic protector and one end of the fourth capacitor, and the other ends of the electrostatic protector and the fourth capacitor are both grounded.

[0019] The present invention has the following beneficial effects:

[0020] (1) By adopting a set active adaptive bias module, the present invention reduces the debugging cost caused by flip chips, and at the same time realizes adaptive adjustment of performance at different operating frequencies;

[0021] (2) By adopting a transformer matching network module with a high interstage coupling coefficient and a high-power electrostatic discharge output matching module, this invention takes into account both impedance matching and anti-static performance during high-power operation, and has the advantages of high efficiency, high output power, small area and low power consumption. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of an adaptive bias broadband high-power amplifier structure.

[0023] Figure 2 Schematic diagram of the working principle of frequency band selection signal;

[0024] Figure 3 This is a schematic diagram of a double-layer metal overlap design for the first and second inductors. Detailed Implementation

[0025] The specific embodiments of the present invention are described below to enable those skilled in the art to understand the present invention. However, it should be understood that the present invention is not limited to the scope of the specific embodiments. For those skilled in the art, various changes are obvious as long as they are within the spirit and scope of the present invention as defined and determined by the appended claims. All inventions utilizing the concept of the present invention are protected.

[0026] like Figure 1 As shown, an adaptive bias broadband high-power amplifier includes an active adaptive bias module, an amplifier driver stage module, an interstage high coupling coefficient transformer matching network module, an amplifier power stage module, and a high-power electrostatic discharge output matching shared module.

[0027] In an optional embodiment of the present invention, the active adaptive bias module includes a first operating frequency band low-dropout linear regulator group, a second operating frequency band low-dropout linear regulator group, an amplifier driver stage active bias submodule, and an amplifier power stage active bias submodule. The first port of the first operating frequency band low-dropout linear regulator group and the first port of the second operating frequency band low-dropout linear regulator group are both connected to the frequency band selection signal input terminal. The second ports of the first and second operating frequency band low-dropout linear regulator groups are both connected to the first port of the amplifier driver stage module. The third ports of the first and second operating frequency band low-dropout linear regulator groups are both connected to the first port of the amplifier power stage module. The amplifier driver stage active bias submodule is connected to the radio frequency signal input terminal, and the amplifier power stage active bias submodule is connected to the second port of the interstage high coupling coefficient transformer matching network module. The active adaptive bias module is used to adaptively switch the bias voltage of the output amplifier driver stage module and the amplifier power stage module according to the operating frequency band of the radio frequency signal input terminal.

[0028] Specifically, in the active adaptive bias module of the present invention, the bias voltages of the output amplifier driver stage module and the amplifier power stage module are adaptively switched according to the operating frequency band of the RF signal input terminal. Specifically, the input frequency band selection signal is adaptively switched at the frequency band selection signal input terminal according to the operating frequency band of the RF signal input terminal. The frequency band selection signal generates a frequency band enable signal and a frequency band enable insufficiency signal. The frequency band enable signal acts on the first operating frequency band low dropout linear regulator group, and the frequency band enable insufficiency signal acts on the second operating frequency band low dropout linear regulator group.

[0029] like Figure 2 As shown, when the frequency band selection signal is 1 (high), the second MOSFET of the first operating frequency band low dropout linear regulator group is turned on, the first operating frequency band low dropout linear regulator group works normally, and outputs the first operating frequency band bias voltage to the amplifier driver stage module and the amplifier power stage module; when the frequency band selection signal is 0 (low), the second MOSFET of the second operating frequency band low dropout linear regulator group is turned on, the second operating frequency band low dropout linear regulator group works normally, and outputs the second operating frequency band bias voltage to the amplifier driver stage module and the amplifier power stage module.

[0030] Both the first and second operating frequency band low-dropout linear regulator groups include two identical low-dropout linear regulator substructures. The input terminals of the two identical low-dropout linear regulator substructures serve as the first port of the low-dropout linear regulator group, and the output terminals of the two identical low-dropout linear regulator substructures serve as the second and third ports of the low-dropout linear regulator group, respectively, and are connected to the first port of the amplifier driver stage module and the first port of the amplifier power stage module, respectively.

[0031] The low-dropout linear regulator substructure includes an error amplifier, a first resistor, a second resistor, a first MOSFET, and a second MOSFET. The negative input terminal of the error amplifier is connected to a bandgap reference voltage. The positive input terminal of the error amplifier is connected to one end of the first resistor and one end of the second resistor. The output terminal of the error amplifier is connected to the gate of the first MOSFET. The other end of the first resistor is grounded. The other end of the second resistor is connected to the drain of the first MOSFET. The source of the first MOSFET and the first power supply port of the error amplifier are both connected to the drain of the second MOSFET. The second power supply port of the error amplifier is grounded. The source of the second MOSFET is connected to the chip operating voltage. The gate of the second MOSFET serves as the input terminal of the low-dropout linear regulator substructure.

[0032] Specifically, in the low-dropout linear regulator group substructure of the first operating frequency band low-dropout linear regulator group, the source of the second MOSFET is connected to the chip operating voltage VDD1; in the low-dropout linear regulator group substructure of the second operating frequency band low-dropout linear regulator group, the source of the second MOSFET is connected to the chip operating voltage VDD2.

[0033] In this invention, a third MOS transistor operating in the linear region is connected in parallel with the second resistor in the first and second low-dropout linear regulator groups. The source and drain of the third MOS transistor are respectively connected to the two ends of the second resistor. When one of the first and second low-dropout linear regulator groups fails, a replacement bias voltage is output by adjusting the gate tuning voltage of the third MOS transistor in the other group.

[0034] Specifically, the active bias submodule of the amplifier driver stage and the active bias submodule of the amplifier power stage have the same structure, both including 3 BJT transistors, 2 resistors and 1 capacitor. The specific circuit structure of the active bias submodule of the amplifier driver stage and the active bias submodule of the amplifier power stage is as follows: Figure 1 As shown.

[0035] In an optional embodiment of the present invention, the second port of the amplifier driver stage module is connected to the radio frequency signal input terminal, and the third port of the amplifier driver stage module is connected to the first port of the interstage high coupling coefficient transformer matching network module. The amplifier driver stage module is used to amplify the input radio frequency signal, provide sufficient driving capability for the amplifier power stage module, and convert the high output impedance of the front-end circuit to a low output impedance to drive the high input impedance of the amplifier power stage module.

[0036] The amplifier driver stage module includes a first BJT and a second BJT; the gate of the first BJT serves as the first port of the amplifier driver stage module, the drain of the first BJT serves as the third port of the amplifier driver stage module, the source of the first BJT is connected to the drain of the second BJT, the source of the second BJT is grounded, and the gate of the second BJT serves as the second port of the amplifier driver stage module.

[0037] In an optional embodiment of the present invention, the second port of the interstage high coupling coefficient transformer matching network module is connected to the second port of the amplifier power stage module; the interstage high coupling coefficient transformer matching network module is used to perform impedance matching between the amplifier driver stage module and the amplifier power stage module over a wide bandwidth.

[0038] The interstage high coupling coefficient transformer matching network module includes a first inductor, a second inductor, a first capacitor, and a second capacitor. The first inductor and the second inductor adopt a double-layer metal overlapping design. One end of the first inductor serves as the first port of the interstage high coupling coefficient transformer matching network module, and the other end of the first inductor is connected to one end of the first capacitor, which is grounded. One end of the second inductor serves as the second port of the interstage high coupling coefficient transformer matching network module, and the other end of the second inductor is connected to one end of the second capacitor, which is grounded.

[0039] like Figure 3 As shown, this invention provides a schematic diagram of a double-layer metal overlap design for a first inductor and a second inductor. The first and second inductors constitute the mutual inductance structure of a transformer. Because the first and second inductors are placed in two overlapping layers, the upper metal portion projects onto the lower layer, achieving a very high coupling coefficient. Furthermore, this invention solves the impedance mismatch introduced by asymmetrical inductors by adding a first capacitor and a second capacitor to the transformer matching network module with a high interstage coupling coefficient.

[0040] In an optional embodiment of the present invention, the second port of the amplifier power stage module is connected to the second port of the interstage high coupling coefficient transformer matching network module, and the third port of the amplifier power stage module is connected to the first port of the high-power electrostatic discharge output matching shared module. The amplifier power stage module is used to amplify the radio frequency signal output by the amplifier driver stage module to meet the base station transmission requirements.

[0041] The amplifier power stage module includes a third BJT and a fourth BJT; the gate of the third BJT serves as the first port of the amplifier power stage module, the drain of the third BJT serves as the third port of the amplifier power stage module, the source of the third BJT is connected to the drain of the fourth BJT, the source of the fourth BJT is grounded, and the gate of the fourth BJT serves as the second port of the amplifier power stage module.

[0042] In an optional embodiment of the present invention, the second port of the high-power electrostatic discharge output matching shared module is connected to the radio frequency signal output terminal; the high-power electrostatic discharge output matching shared module is used to achieve load matching and maximize power transmission efficiency by multiplexing electrostatic discharge protection and radio frequency choke through the transformer secondary inductance and the additional inductance.

[0043] The high-power electrostatic discharge output matching common module includes a third inductor, a fourth inductor, a fifth inductor, a third capacitor, a fourth capacitor, and an electrostatic protector. The third and fourth inductors adopt a double-layer metal overlapping design. One end of the third inductor serves as the first port of the high-power electrostatic discharge output matching common module, and the other end of the third inductor is connected to one end of the third capacitor, which is grounded. One end of the fourth inductor serves as the second port of the high-power electrostatic discharge output matching common module, and the other end of the fourth inductor, after passing through the fifth inductor, is simultaneously connected to one end of the electrostatic protector and one end of the fourth capacitor. The other ends of the electrostatic protector and the fourth capacitor are both grounded.

[0044] This invention is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart illustrations and / or block diagrams. Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0045] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0046] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0047] Specific embodiments have been used to illustrate the principles and implementation methods of this invention. The descriptions of the embodiments above are only for the purpose of helping to understand the method and core ideas of this invention. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of this invention. Therefore, the content of this specification should not be construed as a limitation of this invention.

[0048] Those skilled in the art will recognize that the embodiments described herein are intended to help the reader understand the principles of the invention, and should be understood that the scope of protection of the invention is not limited to such specific statements and embodiments. Those skilled in the art can make various other specific modifications and combinations based on the technical teachings disclosed in this invention without departing from the spirit of the invention, and these modifications and combinations are still within the scope of protection of this invention.

Claims

1. A broadband high-power amplifier with adaptive bias, characterized in that, This includes an active adaptive bias module, an amplifier driver stage module, an interstage high coupling coefficient transformer matching network module, an amplifier power stage module, and a high-power electrostatic discharge output matching shared module; The active adaptive bias module includes a first operating frequency band low-dropout linear regulator group, a second operating frequency band low-dropout linear regulator group, an amplifier driver stage active bias submodule, and an amplifier power stage active bias submodule. The first ports of both the first and second operating frequency band low-dropout linear regulator groups are connected to the frequency band selection signal input. The second ports of both groups are connected to the first port of the amplifier driver stage module. The third ports of both groups are connected to the first port of the amplifier power stage module. The amplifier driver stage active bias submodule is connected to the RF signal input. The amplifier power stage active bias submodule is connected to the second port of the interstage high coupling coefficient transformer matching network module; the active adaptive bias module is used to adaptively switch the bias voltage of the output amplifier driver stage module and the amplifier power stage module according to the operating frequency band of the RF signal input; the first operating frequency band low dropout linear regulator group and the second operating frequency band low dropout linear regulator group both include two identical low dropout linear regulator group substructures, the input terminals of the two identical low dropout linear regulator group substructures are both used as the first port of the low dropout linear regulator group, and the output terminals of the two identical low dropout linear regulator group substructures are used as the second port and the third port of the low dropout linear regulator group, respectively, and are respectively connected to the first port of the amplifier driver stage module and the first port of the amplifier power stage module. The second port of the amplifier driver module is connected to the RF signal input terminal, and the third port of the amplifier driver module is connected to the first port of the interstage high coupling coefficient transformer matching network module. The second port of the interstage high coupling coefficient transformer matching network module is connected to the second port of the amplifier power stage module; the interstage high coupling coefficient transformer matching network module is used for impedance matching between the amplifier driver stage module and the amplifier power stage module over a wide bandwidth; The second port of the amplifier power stage module is connected to the second port of the interstage high coupling coefficient transformer matching network module, and the third port of the amplifier power stage module is connected to the first port of the high-power electrostatic discharge output matching common module. The second port of the high-power electrostatic discharge output matching shared module is connected to the radio frequency signal output terminal; the high-power electrostatic discharge output matching shared module is used to multiplex electrostatic discharge protection and radio frequency choke through the transformer secondary inductance and the additional inductance.

2. The adaptive bias broadband high-power amplifier according to claim 1, characterized in that, The low-dropout linear regulator substructure includes an error amplifier, a first resistor, a second resistor, a first MOSFET, and a second MOSFET. The negative input terminal of the error amplifier is connected to a bandgap reference voltage. The positive input terminal of the error amplifier is connected to one end of the first resistor and one end of the second resistor. The output terminal of the error amplifier is connected to the gate of the first MOSFET. The other end of the first resistor is grounded. The other end of the second resistor is connected to the drain of the first MOSFET. The source of the first MOSFET and the first power supply port of the error amplifier are both connected to the drain of the second MOSFET. The second power supply port of the error amplifier is grounded. The source of the second MOSFET is connected to the chip operating voltage. The gate of the second MOSFET serves as the input terminal of the low-dropout linear regulator substructure.

3. The adaptive bias broadband high-power amplifier according to claim 2, characterized in that, A third MOSFET operating in the linear region is connected in parallel with the second resistor in the first and second low-dropout linear regulator groups. The source and drain of the third MOSFET are connected to the two ends of the second resistor, respectively. When one of the first and second low-dropout linear regulator groups fails, a replacement bias voltage is output by adjusting the gate tuning voltage of the third MOSFET in the other group.

4. The adaptive bias broadband high-power amplifier according to claim 1, characterized in that, The amplifier driver stage module includes a first BJT and a second BJT; the gate of the first BJT serves as the first port of the amplifier driver stage module, the drain of the first BJT serves as the third port of the amplifier driver stage module, the source of the first BJT is connected to the drain of the second BJT, the source of the second BJT is grounded, and the gate of the second BJT serves as the second port of the amplifier driver stage module.

5. The adaptive bias broadband high-power amplifier according to claim 1, characterized in that, The interstage high coupling coefficient transformer matching network module includes a first inductor, a second inductor, a first capacitor, and a second capacitor. The first inductor and the second inductor adopt a double-layer metal overlapping design. One end of the first inductor serves as the first port of the interstage high coupling coefficient transformer matching network module, and the other end of the first inductor is connected to one end of the first capacitor, which is grounded. One end of the second inductor serves as the second port of the interstage high coupling coefficient transformer matching network module, and the other end of the second inductor is connected to one end of the second capacitor, which is grounded.

6. The adaptive bias broadband high-power amplifier according to claim 1, characterized in that, The amplifier power stage module includes a third BJT and a fourth BJT; the gate of the third BJT serves as the first port of the amplifier power stage module, the drain of the third BJT serves as the third port of the amplifier power stage module, the source of the third BJT is connected to the drain of the fourth BJT, the source of the fourth BJT is grounded, and the gate of the fourth BJT serves as the second port of the amplifier power stage module.

7. The adaptive bias broadband high-power amplifier according to claim 1, characterized in that, The high-power electrostatic discharge output matching common module includes a third inductor, a fourth inductor, a fifth inductor, a third capacitor, a fourth capacitor, and an electrostatic protector. The third and fourth inductors adopt a double-layer metal overlapping design. One end of the third inductor serves as the first port of the high-power electrostatic discharge output matching common module, and the other end of the third inductor is connected to one end of the third capacitor, which is grounded. One end of the fourth inductor serves as the second port of the high-power electrostatic discharge output matching common module, and the other end of the fourth inductor, after passing through the fifth inductor, is simultaneously connected to one end of the electrostatic protector and one end of the fourth capacitor. The other ends of the electrostatic protector and the fourth capacitor are both grounded.

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