Power synthesis circuit and power amplifier

Abstract

The invention relates to the technical field of wireless communication, and provides a power synthesis circuit and a power amplifier. Comprising a first signal input end, a second signal input end, an input impedance matching circuit, a first harmonic suppression circuit, a second harmonic suppression circuit, a first high-pass filter circuit, a second high-pass filter circuit, a low-pass filter circuit and a signal synthesis output end, the low-pass filter circuit comprises a first inductor, a second inductor and a first capacitor; the first end of the first inductor serves as the first input end of the low-pass filter circuit, the first end of the second inductor serves as the second input end of the low-pass filter circuit, the second end of the first inductor is connected with the first end of the first capacitor and the second end of the second inductor and serves as the output end of the low-pass filter circuit, and the second end of the first capacitor is grounded. The power synthesis circuit has the impedance conversion characteristic, and has the advantages of low cost, high isolation, high integration and the like.

Description

Technical Field

[0001] The present invention relates to the field of wireless communication technologies, and in particular, to a power combining circuit and a power amplifier. Background Art

[0002] In a wireless communication system, with the rapid development of 5G network and Internet of Things (IoT) technologies, higher requirements are imposed on the linear power of radio frequency front-end modules. However, a single-ended power amplifier is difficult to output higher power, so power combining technology needs to be adopted. In existing radio frequency front-end modules for cellular communication, differential phase combining and 90° balanced phase combining are usually used.

[0003] Currently, a differential structure usually adds large-sized baluns at both the input and output ends of a power amplifier, increasing the cost of the chip and the module. Moreover, the balance of the differential balun applied to 5G communication is generally poor, manifested as poor stability and isolation of the power amplifier. For the 90° balanced phase combining applied to the radio frequency front-end of 5G communication, a 90° combining scheme based on a transformer is often used. Similar to the aforementioned differential combining, 90° couplers based on a transformer architecture with a relatively large size are added at both the input and output of the power amplifier. At the same time, this 90° coupler does not have impedance transformation characteristics and an impedance change circuit needs to be added additionally, so it occupies a very large area of the chip and the module; meanwhile, it results in a relatively high cost. Summary of the Invention

[0004] Aiming at the deficiencies of the above prior art, the present invention proposes a power combining circuit to solve the problem of high cost of existing power combining circuits.

[0005] To solve the above technical problems, the present invention adopts the following technical solutions:

[0006] In a first aspect, an embodiment of the present invention provides a power combining circuit, including an input impedance matching circuit, a first harmonic suppression circuit, a second harmonic suppression circuit, a first high-pass filter circuit, a second high-pass filter circuit, and a low-pass filter circuit;

[0007] The first end of the input impedance matching circuit serves as the first signal input end of the power combining circuit, and the second end of the input impedance matching circuit serves as the second signal input end of the power combining circuit; the first end of the first harmonic suppression circuit is connected to the first end of the input impedance matching circuit; the second end of the first harmonic suppression circuit is grounded, the first end of the second harmonic suppression circuit is connected to the second end of the input impedance matching circuit, and the second end of the second harmonic suppression circuit is grounded; the input impedance matching circuit is used to provide power supply and impedance matching; the first harmonic suppression circuit is used to filter the second harmonic of the signal output from the first end of the input impedance matching circuit; the second harmonic suppression circuit is used to filter the second harmonic of the signal output from the second end of the input impedance matching circuit;

[0008] The input end of the first high-pass filter circuit is connected to the first end of the first harmonic suppression circuit, and the output end of the first high-pass filter circuit is connected to the first input end of the low-pass filter circuit; the input end of the second high-pass filter circuit is connected to the first end of the second harmonic suppression circuit, and the output end of the second high-pass filter circuit is connected to the second input end of the low-pass filter circuit. The output end of the low-pass filter circuit serves as the signal combining output end of the power combining circuit; the first high-pass filter circuit is used to filter the signal output from the first end of the input impedance matching circuit; the second high-pass filter circuit is used to filter the signal output from the second end of the input impedance matching circuit; the low-pass filter circuit is used to combine the signals respectively output from the first high-pass filter circuit and the second high-pass filter circuit and perform filtering processing before outputting;

[0009] The low-pass filter circuit includes a first inductor, a second inductor and a first capacitor; the first end of the first inductor serves as the first input end of the low-pass filter circuit, the first end of the second inductor serves as the second input end of the low-pass filter circuit, the second end of the first inductor is connected to the first end of the first capacitor and the second end of the second inductor and serves as the output end of the low-pass filter circuit, and the second end of the first capacitor is grounded.

[0010] Preferably, the first high-pass filter circuit includes a second capacitor and a third inductor; the first end of the second capacitor serves as the input end of the first high-pass filter circuit, the second end of the second capacitor is connected to the first end of the third inductor and serves as the output end of the first high-pass filter circuit, and the second end of the third inductor is grounded;

[0011] The second high-pass filter circuit includes a third capacitor and a fourth inductor; the first end of the third capacitor serves as the input end of the second high-pass filter circuit, the second end of the third capacitor is connected to the first end of the fourth inductor and serves as the output end of the second high-pass filter circuit, and the second end of the fourth inductor is grounded.

[0012] Preferably, the power combining circuit further includes a first resistor; the first end of the first resistor is connected to the output end of the first high-pass filter circuit, and the second end of the first resistor is connected to the output end of the second high-pass filter circuit.

[0013] Preferably, the input impedance matching circuit includes a voltage source, a fifth inductor, and a sixth inductor; the negative pole of the voltage source is grounded, the positive pole of the voltage source is respectively connected to the first end of the fifth inductor and the first end of the sixth inductor, and the second end of the fifth inductor serves as the first end of the input impedance matching circuit; the second end of the sixth inductor serves as the second end of the input impedance matching circuit.

[0014] Preferably, the first harmonic suppression circuit includes a fourth capacitor and a seventh inductor; the first end of the fourth capacitor serves as the second end of the first harmonic suppression circuit, the second end of the fourth capacitor is connected to the first end of the seventh inductor, and the second end of the seventh inductor serves as the first end of the first harmonic suppression circuit;

[0015] The second harmonic suppression circuit includes a fifth capacitor and an eighth inductor; the first end of the fifth capacitor serves as the second end of the second harmonic suppression circuit, the second end of the fifth capacitor is connected to the first end of the eighth inductor, and the second end of the eighth inductor serves as the first end of the second harmonic suppression circuit.

[0016] Preferably, the first capacitor is an SMD capacitor or an HBT / SOI on-chip capacitor.

[0017] In a second aspect, an embodiment of the present invention provides a power amplifier, which includes a signal source, a first amplifier, a second amplifier, and the power combining circuit as described above;

[0018] The first end of the signal source is grounded, the second end of the signal source outputs a radio frequency signal to the input ends of the first amplifier and the second amplifier respectively, the output end of the first amplifier is connected to the first signal input end, and the output end of the second amplifier is connected to the second signal input end.

[0019] In the embodiments of the present invention, compared with the related art, by directly connecting the signal source to the first amplifier and the second amplifier, compared with differential synthesis and 90° synthesis, no power distribution network needs to be added to the input end of the amplifier, simplifying the input end and reducing the chip area and cost. At the same time, after the output ends of the first amplifier and the second amplifier pass through the in-phase power combining circuit, power combining output is achieved; in addition, the first end of the input impedance matching circuit is used as the first signal input end of the power combining circuit, and the second end of the input impedance matching circuit is used as the second signal input end of the power combining circuit; the first signal input end is sequentially connected to the first harmonic suppression circuit, the first high-pass filter circuit and the first input end of the low-pass filter circuit, and the second signal input end is sequentially connected to the second harmonic suppression circuit, the second high-pass filter circuit and the second input end of the low-pass filter circuit; the output end of the low-pass filter circuit is used as the signal combining output end of the power combining circuit; the first end of the first inductor of the low-pass filter circuit is used as the first input end of the low-pass filter circuit, the first end of the second inductor is used as the second input end of the low-pass filter circuit, the second end of the first inductor is connected to the first end of the first capacitor and the second end of the second inductor and used as the output end of the low-pass filter circuit, and the second end of the first capacitor is grounded; through corresponding impedance, filtering and resonance processing, the power amplifier has impedance transformation characteristics and at the same time has advantages such as low cost, high isolation and high integration. Description of the Drawings

[0020] The present invention will be described in detail below with reference to the drawings. Through the detailed description in combination with the following drawings, the above or other aspects of the present invention will become clearer and easier to understand. In the drawings:

[0021] Figure 1 is the circuit diagram of the power combining circuit provided in Embodiment 1 of the present invention;

[0022] Figure 2 is the schematic diagram of the fundamental wave impedance matching result of the power combining circuit provided in Embodiment 1 of the present invention;

[0023] Figure 3 is the circuit diagram of the power combining circuit provided in Embodiment 2 of the present invention;

[0024] Figure 4 is the circuit structure block diagram of the power amplifier provided in Embodiment 3 of the present invention.

[0025] Among them, 100, power combining circuit; 1, input impedance matching circuit; 2, first harmonic suppression circuit; 3, second harmonic suppression circuit; 4, first high-pass filter circuit; 5, second high-pass filter circuit; 6, low-pass filter circuit; 200, power amplifier. Detailed Embodiments

[0026] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs; the terms used in the specification of this application are only for the purpose of describing specific embodiments and are not intended to limit this application; the terms "including" and "having" and any variations thereof in the specification and claims of this application and the above drawings are intended to cover non-exclusive inclusion. The terms "first", "second", etc. in the specification and claims of this application or the above drawings are used to distinguish different objects and not to describe a specific order.

[0027] Reference herein to "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of this application. The phrase appears in various places in the specification and does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment mutually exclusive with other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0028] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts belong to the scope of protection of the present invention.

[0029] Embodiment 1

[0030] Please refer to Figure 1 As shown, the embodiment of the present invention provides a power combining circuit 100, including an input impedance matching circuit 1, a first harmonic suppression circuit 2, a second harmonic suppression circuit 3, a first high-pass filter circuit 4, a second high-pass filter circuit 5, and a low-pass filter circuit 6.

[0031] The first end of the input impedance matching circuit 1 serves as the first signal input terminal Input1 of the power combining circuit 100, and the second end of the input impedance matching circuit 1 serves as the second signal input terminal Input2 of the power combining circuit 100.

[0032] Specifically, the first end of the first harmonic suppression circuit 2 is connected to the first end of the input impedance matching circuit 1, and the first end of the second harmonic suppression circuit 3 is connected to the second end of the input impedance matching circuit 1; the input impedance matching circuit 1 is used to provide power supply and impedance matching. The first harmonic suppression circuit 2 is used to filter out the second harmonic of the signal output from the first end of the input impedance matching circuit 1. The second harmonic suppression circuit 3 is used to filter out the second harmonic of the signal output from the second end of the input impedance matching circuit 1.

[0033] The input end of the first high-pass filter circuit 4 is connected to the first end of the first harmonic suppression circuit 2; the output end of the first high-pass filter circuit 4 is connected to the first input end of the low-pass filter circuit 6. The input end of the second high-pass filter circuit 5 is connected to the first end of the second harmonic suppression circuit 3; the output end of the second high-pass filter circuit 5 is connected to the second input end of the low-pass filter circuit 6. The output end of the low-pass filter circuit 6 serves as the signal synthesis output end Output of the power combining circuit 100. The first high-pass filter circuit 4 is used to filter the signal output from the first end of the input impedance matching circuit 1. The second high-pass filter circuit 5 is used to filter the signal output from the second end of the input impedance matching circuit 1. The low-pass filter circuit 6 is used to synthesize the signals respectively output from the first high-pass filter circuit 4 and the second high-pass filter circuit 5 and perform filtering processing before outputting.

[0034] The low-pass filter circuit 6 includes a first inductor L1, a second inductor L2, and a first capacitor C1. The first end of the first inductor L1 serves as the first input end of the low-pass filter circuit 6, the first end of the second inductor L2 serves as the second input end of the low-pass filter circuit 6, the second end of the first inductor L1 is respectively connected to the first end of the first capacitor C1 and the second end of the second inductor L2 and serves as the output end of the low-pass filter circuit 6, and the second end of the first capacitor C1 is grounded. This enables the two in-phase combined paths to have the characteristic of high isolation within a relatively wide frequency band. Through corresponding impedance, filtering, and resonance processing, the power combining circuit 100 has the impedance transformation characteristic, and has the effects of low cost, high isolation, and high integration, while enabling the power amplifier circuit 200 to achieve in-phase power combination.

[0035] In this embodiment, the first high-pass filter circuit 4 includes a second capacitor C2 and a third inductor L3. The first end of the second capacitor C2 serves as the input end of the first high-pass filter circuit 4, the second end of the second capacitor C2 is connected to the first end of the third inductor L3 and serves as the output end of the first high-pass filter circuit 4, and the second end of the third inductor L3 is grounded. Connecting the second capacitor C2 and the third inductor L3 is to remove the low-frequency components in the signal and allow high-frequency signals to pass through. Its working principle is based on the different frequency responses of the second capacitor C2 and the third inductor L3 to the signal. By removing signals below a certain frequency, high-frequency signals are allowed to pass through, achieving the effect of high-pass filtering. Connecting the second capacitor C2 and the third inductor L3 can also perform impedance transformation on the passing signal, improving the impedance matching performance of each part of the overall circuit.

[0036] The second high-pass filter circuit 5 includes a third capacitor C3 and a fourth inductor L4. The first end of the third capacitor C3 serves as the input end of the second high-pass filter circuit 5, the second end of the third capacitor C3 is connected to the first end of the fourth inductor L4 and serves as the output end of the second high-pass filter circuit 5, and the second end of the fourth inductor L4 is grounded. Connecting the third capacitor C3 in series and the fourth inductor L4 in parallel is to remove the low-frequency components in the signal and allow high-frequency signals to pass through. Connecting the third capacitor C3 and the fourth inductor L4 can also perform impedance transformation on the passing signal, improving the impedance matching performance of each part of the overall circuit.

[0037] In this embodiment, the input impedance matching circuit 1 includes a voltage source SRC2, a fifth inductor L5, and a sixth inductor L6. The negative pole of the voltage source SRC2 is grounded, the positive pole of the voltage source SRC2 is respectively connected to the first end of the fifth inductor L5 and the first end of the sixth inductor L6, and the second end of the fifth inductor L5 serves as the first end of the input impedance matching circuit 1. The second end of the sixth inductor L6 serves as the second end of the input impedance matching circuit 1. Specifically, the second end of the fifth inductor L5 is respectively connected to the first signal input terminal Input1 and the first end of the first harmonic suppression circuit 2. The second end of the sixth inductor L6 is respectively connected to the second signal input terminal Input2 and the first end of the second harmonic suppression circuit 3. Both the fifth inductor L5 and the sixth inductor L6 are involved in impedance matching, and at the same time, they also serve as choke inductors to achieve isolation between the DC power supply of the voltage source SRC2 and the RF signal. The overall circuit is powered by the voltage source SRC2, and the input signal is impedance-matched by the fifth inductor L5 and the sixth inductor L6 and then output.

[0038] In this embodiment, the first harmonic suppression circuit 2 includes a fourth capacitor C4 and a seventh inductor L7. The first end of the fourth capacitor C4 serves as the second end of the first harmonic suppression circuit 2 and is grounded. The second end of the fourth capacitor C4 is connected to the first end of the seventh inductor L7. The second end of the seventh inductor L7 serves as the first end of the first harmonic suppression circuit 2. The fourth capacitor C4 and the seventh inductor L7 are used to filter out the second harmonic, and at the same time, the fourth capacitor C4 and the seventh inductor L7 can also perform impedance matching. Thus, the matching signal output by the input impedance matching circuit 1 is subjected to secondary impedance matching to control the second harmonic impedance value of the amplifier load impedance, and the requirement of the power amplifier 200 for the second harmonic impedance is completed.

[0039] Specifically, the second harmonic suppression circuit 3 includes a fifth capacitor C5 and an eighth inductor L8. The first end of the fifth capacitor C5 serves as the second end of the second harmonic suppression circuit 3 and is grounded. The second end of the fifth capacitor C5 is connected to the first end of the eighth inductor L8. The second end of the eighth inductor L8 serves as the first end of the second harmonic suppression circuit 3. The fifth capacitor C5 and the eighth inductor L8 are used to filter out the second harmonic, and at the same time, the fifth capacitor C5 and the eighth inductor L8 can also perform impedance matching. Thus, the matching signal output by the input impedance matching circuit 1 is subjected to secondary impedance matching to control the second harmonic impedance value of the amplifier load impedance, and the requirement of the power amplifier 200 for the second harmonic impedance is completed.

[0040] In this embodiment, in the present invention, the first inductor L1, the second inductor L2, the third inductor L3, the fourth inductor L4, the fifth inductor L5, the sixth inductor L6, the seventh inductor L7, and the eighth inductor L8 can all use gold wires to equivalent inductors. The second capacitor C2, the third capacitor C3, the fourth capacitor C4, and the fifth capacitor C5 can all use HBT on-chip capacitors.

[0041] In this embodiment, the first capacitor C1 is an SMD capacitor or an HBT / SOI on-chip capacitor. Therefore, it occupies very little area of the HBT chip and the radio frequency front-end module, reducing the material cost. The following Figure 2 The Smith chart shows the fundamental wave impedance matching results in the frequency band of 2.3 GHz to 2.7 GHz for mobile communication. It can be seen that the impedance bandwidth is good, and the matching is achieved from the 50-ohm port to 3 ohms.

[0042] Embodiment 2

[0043] Such as Figure 3As shown in the figure, this embodiment is basically the same as the above-mentioned Embodiment 1, except that: in this embodiment, the power combining circuit 100 further includes a first resistor R1. The first end of the first resistor R1 is connected to the output end of the first high-pass filter circuit 4, and the second end of the first resistor R1 is connected to the output end of the second high-pass filter circuit 5. By adding a parallel-connected first resistor R1 between the two paths, it is used to absorb the signals leaked between the two paths, thereby achieving a higher isolation degree. Specifically, the first end of the first resistor R1 is connected to the second end of the third inductor L3, and the second end of the first resistor R1 is connected to the second end of the sixth inductor L6.

[0044] Embodiment 3

[0045] As Figure 4 As shown in the figure, an embodiment of the present invention provides a power amplifier 200, and the power amplifier 200 includes a signal source SRC1, a first amplifier AMP1, a second amplifier AMP2, and a power combining circuit 100 according to any one of the above-mentioned Embodiments 1 to 2.

[0046] The first end of the signal source SRC1 is grounded, the second end of the signal source SRC1 outputs a radio frequency signal to the input ends of the first amplifier AMP1 and the second amplifier AMP2 respectively, the output end of the first amplifier AMP1 is connected to the first signal input end Input1, and the output end of the second amplifier AMP2 is connected to the second signal input end Input2. By directly connecting the signal source SRC1 to the first amplifier AMP1 and the second amplifier AMP2, compared with differential synthesis and 90° synthesis, no power distribution network needs to be added to the input ends of the amplifiers, simplifying the input ends and reducing the chip area and cost. At the same time, after the output ends of the first amplifier AMP1 and the second amplifier AMP2 pass through the power combining circuit 100, power combining output is achieved.

[0047] It should be noted that each of the above-described embodiments described with reference to the accompanying drawings is only used to illustrate the present invention and not to limit the scope of the present invention. Those of ordinary skill in the art should understand that any modification or equivalent replacement made to the present invention without departing from the spirit and scope of the present invention shall be covered within the scope of the present invention. In addition, unless otherwise specified in the context, words in the singular form include the plural form, and vice versa. Additionally, unless otherwise specified, all or part of any embodiment can be used in combination with all or part of any other embodiment.

Claims

1. A power synthesis circuit, characterized in that: It includes an input impedance matching circuit, a first harmonic suppression circuit, a second harmonic suppression circuit, a first high-pass filter circuit, a second high-pass filter circuit and a low-pass filter circuit; The first end of the input impedance matching circuit serves as the first signal input end of the power synthesis circuit, and the second end of the input impedance matching circuit serves as the second signal input end of the power synthesis circuit; the first end of the first harmonic suppression circuit is connected to the first end of the input impedance matching circuit; the second end of the first harmonic suppression circuit is grounded, the first end of the second harmonic suppression circuit is connected to the second end of the input impedance matching circuit, and the second end of the second harmonic suppression circuit is grounded; the input impedance matching circuit is used to provide power supply and impedance matching; the first harmonic suppression circuit is used to filter out the second harmonic of the signal output from the first end of the input impedance matching circuit; the second harmonic suppression circuit is used to filter out the second harmonic of the signal output from the second end of the input impedance matching circuit; The input end of the first high-pass filter circuit is connected to the first end of the first harmonic suppression circuit, and the output end of the first high-pass filter circuit is connected to the first input end of the low-pass filter circuit; the input end of the second high-pass filter circuit is connected to the first end of the second harmonic suppression circuit, and the output end of the second high-pass filter circuit is connected to the second input end of the low-pass filter circuit, and the output end of the low-pass filter circuit serves as the signal synthesis output end of the power synthesis circuit; the first high-pass filter circuit is used to filter the signal output from the first end of the input impedance matching circuit; The second high-pass filter circuit is used to filter the signal output from the second end of the input impedance matching circuit; the low-pass filter circuit is used to synthesize the signals respectively output from the first high-pass filter circuit and the second high-pass filter circuit and output them after filtering; The low-pass filter circuit includes a first inductor, a second inductor and a first capacitor; the first end of the first inductor serves as the first input end of the low-pass filter circuit, the first end of the second inductor serves as the second input end of the low-pass filter circuit, the second end of the first inductor is respectively connected to the first end of the first capacitor and the second end of the second inductor and serves as the output end of the low-pass filter circuit, and the second end of the first capacitor is grounded.

2. The power synthesis circuit according to claim 1, characterized in that: The first high-pass filter circuit includes a second capacitor and a third inductor; the first end of the second capacitor serves as an input end of the first high-pass filter circuit, the second end of the second capacitor is connected to the first end of the third inductor and serves as an output end of the first high-pass filter circuit, and the second end of the third inductor is grounded; The second high-pass filter circuit includes a third capacitor and a fourth inductor; the first end of the third capacitor serves as the input end of the second high-pass filter circuit, the second end of the third capacitor is connected to the first end of the fourth inductor and serves as the output end of the second high-pass filter circuit, and the second end of the fourth inductor is grounded.

3. The power synthesis circuit according to claim 1 or 2, characterized in that: The power synthesis circuit also includes a first resistor; a first end of the first resistor is connected to the output end of the first high-pass filter circuit, and a second end of the first resistor is connected to the output end of the second high-pass filter circuit.

4. The power synthesis circuit according to claim 1, characterized in that: The input impedance matching circuit includes a voltage source, a fifth inductor and a sixth inductor; the negative electrode of the voltage source is grounded, the positive electrode of the voltage source is respectively connected to the first end of the fifth inductor and the first end of the sixth inductor, the second end of the fifth inductor serves as the first end of the input impedance matching circuit; the second end of the sixth inductor serves as the second end of the input impedance matching circuit.

5. The power synthesis circuit according to claim 4, characterized in that: The first harmonic suppression circuit includes a fourth capacitor and a seventh inductor; the first end of the fourth capacitor serves as the second end of the first harmonic suppression circuit, the second end of the fourth capacitor is connected to the first end of the seventh inductor, and the second end of the seventh inductor serves as the first end of the first harmonic suppression circuit; The second harmonic suppression circuit includes a fifth capacitor and an eighth inductor; the first end of the fifth capacitor serves as the second end of the second harmonic suppression circuit, the second end of the fifth capacitor is connected to the first end of the eighth inductor, and the second end of the eighth inductor serves as the first end of the second harmonic suppression circuit.

6. The power synthesis circuit according to claim 1, characterized in that: The first capacitor is an SMD capacitor or an HBT / SOI on-chip capacitor.

7. A power amplifier, characterized in that: The power amplifier comprises a signal source, a first amplifier, a second amplifier and a power synthesis circuit as claimed in any one of claims 1 to 6; The first end of the signal source is grounded, and the second end of the signal source outputs a radio frequency signal to the input end of the first amplifier and the input end of the second amplifier respectively. The output end of the first amplifier is connected to the first signal input end, and the output end of the second amplifier is connected to the second signal input end.

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