Alternating-current coupling circuit for compensating direct-current offset and suitable for low-voltage application

Abstract

The invention discloses an alternating current coupling circuit which comprises an upper part and a lower part which are completely symmetrical and is used for transmitting a pair of differential signals, and devices in symmetrical positions have completely same parameters. In the upper half part circuit, one end of a coupling capacitor I serves as an input end I, and the other end serves as an output end I; the input end I is also grounded through a matched terminating characteristic impedance I; the input end I is also grounded through a resistor II and a resistor I which are connected in series; the grid electrode of the PMOS transistor I is connected between the resistor I and the resistor II, the source electrode is connected with the output end of the current source I and is also connected with the output end I through the resistor III, and the drain electrode is grounded; the common-mode feedback circuit collects the source electrode voltage of the first PMOS tube and adjusts the current of the first current source through feedback. According to the invention, the DC bias branch and the low-frequency branch are separated, so that the AC coupling circuit can work under a relatively low voltage. In addition, a low-frequency branch and a high-frequency branch are designed at the same time, and the direct-current baseline drift phenomenon is compensated.

Description

Technical Field

[0001] The present application relates to an AC couple circuit. Background Art

[0002] The AC coupling circuit is used to transmit the AC signal output by the previous circuit to the next circuit. The AC coupling circuit is mainly used to couple AC and isolate DC. In the front-end circuit of the receiving end of the high-speed data transmission system, an AC coupling circuit is usually set to reconstruct the common mode level of the receiving end to solve the common mode mismatch problem between the transmitting end and the receiving end.

[0003] The most primitive AC coupling circuit consists of only one capacitor and one resistor, and is used to transmit high-frequency signals. Since there is only a high-frequency path, when there are continuous high-level signals (binary 1) or continuous low-level signals (binary 0), the transmitted signal will have a DC offset, namely the DC baseline wander phenomenon. In order to solve the DC baseline wander phenomenon, the existing AC coupling circuit usually provides both a low-frequency path and a high-frequency path. The low-frequency path is used to allow the low-frequency signal component to pass through, so that the low-frequency component in the transmitted signal will not be completely eliminated.

[0004] See also Figure 1 , which is an existing AC coupling circuit. In high-speed data transmission systems, signals are usually transmitted in the form of differential pairs, so Figure 1 The AC coupling circuit shown has two completely symmetrical upper and lower parts, and the devices in the symmetrical positions have exactly the same parameters. inp and inm are a pair of differential input signals, and outp and outm are a pair of differential output signals.

[0005] by Figure 1 Take the upper half of the figure as an example. One end of the coupling capacitor Cac is used as the input end to receive inp; the other end of the coupling capacitor Cac is used as the output end to output outp. The input end is also grounded through a matching butt joint characteristic impedance Z. The input end is also connected to the reference voltage Vref through the series resistor R2 and resistor R1. One end of the resistor R3 is connected between the resistor R1 and the resistor R2. The DC voltage at this position is V DC_out The other end of resistor R3 is connected to the output end. The coupling capacitor Cac and resistor R3 form a high-frequency path. Resistor R1, resistor R2 and resistor R3 form a DC bias branch. The DC point V of the output voltage outp DC_out The reference voltage Vref is obtained by dividing the voltage through resistors R1 and R2, and then transmitted to the output terminal through resistor R3. The DC point of the output voltage outp is Resistor 1 R1, resistor 2 R2 and resistor 3 R3 also form a low-frequency path, providing a path for the low-frequency component of the inp signal input at the input end to reach the output end. The expression of the low-frequency gain is Since both the DC point of the output voltage outp and the low-frequency gain are related to the resistance ratio of resistor 1 R1 and resistor 2 R2, it is necessary to adjust the ratio of resistor 1 and resistor 2 to change the low-frequency gain. When the proportion of resistor 1 increases, the proportion of resistor 2 decreases, which will reduce the DC point of the output voltage, that is, the low-frequency gain and the DC point of the output voltage have a mutual influence and mutual restriction relationship. In order to ensure the low-frequency gain and set the DC bias at the same time, the voltage value of the required reference voltage Vref is relatively high. In advanced processes, it is difficult to achieve a higher reference voltage due to the limitation of the chip power supply voltage. Summary of the invention

[0006] The technical problem to be solved by the present application is how to realize an AC coupling circuit under a lower voltage condition.

[0007] In order to solve the above technical problems, the present application proposes an AC coupling circuit, including two completely symmetrical upper and lower parts, which are used to transmit a pair of differential signals, and the devices in symmetrical positions have completely the same parameters. In the upper circuit, one end of the coupling capacitor 1 is used as the input terminal 1, and the other end is used as the output terminal 1; the input terminal 1 is also grounded through the matching termination characteristic impedance 1; the input terminal 1 is also grounded through the series resistor 2 and the resistor 1; the gate of the PMOS tube 1 is connected between the resistor 1 and the resistor 2, the source is connected to the output end of the current source 1, the source is also connected to the output terminal 1 through the resistor 3, and the drain is grounded; the common mode feedback circuit collects the source voltage of the PMOS tube 1, and adjusts the current of the current source 1 through feedback. In the lower half of the circuit, one end of coupling capacitor 2 serves as input terminal 2, and the other end serves as output terminal 2; input terminal 2 is also grounded through matching termination characteristic impedance 2; input terminal 2 is also grounded through series resistor 5 and resistor 4; the gate of PMOS tube 2 is connected between resistor 4 and resistor 5, the source is connected to the output end of current source 2, the source is also connected to output terminal 2 through resistor 6, and the drain is grounded; the common-mode feedback circuit collects the source voltage of PMOS tube 2, and adjusts the current of current source 2 through feedback.

[0008] Furthermore, in the upper circuit, PMOS tube 1, resistor 3 and current source 1 constitute a DC bias branch; in the lower circuit, PMOS tube 2, resistor 6 and current source 2 constitute a DC bias branch; the DC bias branch is used to determine the DC point of the output voltage, that is, the DC bias voltage of the output signal.

[0009] Furthermore, in the DC bias branch, the DC voltage V at the source of the PMOS tube 1 is DC_out is the gate-source voltage of PMOS tube 1, and the DC voltage V at the source of PMOS tube 2DC_out ' is the gate-source voltage of PMOS tube 2, the two are equal and serve as the DC point of the output voltage.

[0010] Furthermore, in the upper circuit, resistor 1, resistor 2, PMOS tube 1, and resistor 3 constitute a low-frequency branch; in the lower circuit, resistor 4, resistor 5, PMOS tube 2, and resistor 6 constitute a low-frequency branch; the low-frequency branch is used to determine the low-frequency gain of the AC coupling circuit and provide a low-frequency path from the input signal to the output signal.

[0011] Furthermore, in the low-frequency branch of the upper circuit, resistor 2 and coupling capacitor 1 form a low-pass filter. Resistor 2 and coupling capacitor 1 produce a pole, and the low-frequency component of input signal 1 can only go backward through resistor 2 and be sent to the gate of PMOS tube 1; PMOS tube 1 is a source follower, and the low-frequency component of input signal 1 is transmitted from the gate of PMOS tube 1 to the source, and reaches output terminal 1 through resistor 3; the low-frequency gain of input signal 1 is in, is the gain of PMOS tube 1, which is approximately 1. In the low-frequency branch of the lower circuit, resistor 5 and coupling capacitor 2 form a low-pass filter. Resistor 5 and coupling capacitor 2 produce a pole. The low-frequency component of input signal 2 can only go backward through resistor 5 and be sent to the gate of PMOS tube 2; PMOS tube 2 is a source follower. The low-frequency component of input signal 2 is transmitted from the gate of PMOS tube 2 to the source, and reaches output terminal 2 through resistor 6; the low-frequency gain of input signal 2 is in, is the gain of PMOS tube 2, which is approximately 1.

[0012] Furthermore, in the upper circuit, coupling capacitor 1 and resistor 3 constitute a high-frequency branch; in the lower circuit, coupling capacitor 2 and resistor 6 constitute a high-frequency branch; the high-frequency branch is used to provide a high-frequency path from the input signal to the output signal.

[0013] Furthermore, in the high-frequency branch of the upper circuit, coupling capacitor 1 and resistor 3 form a high-pass filter, and the high-frequency component of input signal 1 goes backward through coupling capacitor 1 to reach output terminal 1. In the high-frequency branch of the lower circuit, coupling capacitor 2 and resistor 6 form a high-pass filter, and the high-frequency component of input signal 2 goes backward through coupling capacitor 2 to reach output terminal 2.

[0014] Furthermore, the common-mode feedback circuit, the reference voltage, the current source 1 and the current source 2 constitute a feedback branch; the feedback branch is used to stabilize the common-mode point of the output voltage, that is, the DC bias voltage of the output signal.

[0015] Furthermore, in the feedback branch, the common-mode feedback circuit takes out the common-mode voltage of the differential output signal and compares it with the reference voltage, and uses the comparison result to control current source one and current source two, thereby adjusting the current flowing through PMOS tube one and PMOS tube two, and then changing the DC voltage of the source of PMOS tube one and the DC voltage of the source of PMOS tube two, thereby adjusting the common-mode voltage of the differential output signal.

[0016] Further, in the feedback branch, when the common mode voltage of the differential output signal is less than the reference voltage, the common mode feedback circuit outputs a control signal to increase the current of current source 1 and current source 2, at which time the current flowing through PMOS tube 1 and PMOS tube 2 increases, the gate-source voltage of PMOS tube 1 and PMOS tube 2 increases, and the common mode voltage of the differential output signal increases. When the common mode voltage of the differential output signal is greater than the reference voltage, the common mode feedback circuit outputs a control signal to reduce the current of current source 1 and current source 2, at which time the current flowing through PMOS tube 1 and PMOS tube 2 decreases, the gate-source voltage of PMOS tube 1 and PMOS tube 2 decreases, and the common mode voltage of the differential output signal decreases.

[0017] The technical effect achieved by the present application is: a source follower structure composed of a passive device (resistor) and a PMOS tube is used to separate the DC bias branch and the low-frequency branch, so that the AC coupling circuit can work at a lower voltage, and the structure is simple and the design flexibility is high. In addition, the present application designs the low-frequency branch and the high-frequency branch at the same time to compensate for the DC baseline drift phenomenon. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 The present invention is a structural schematic diagram of an existing AC coupling circuit.

[0019] Figure 2 It is a structural schematic diagram of the AC coupling circuit proposed in this application.

[0020] Figure 3 It is a schematic diagram of the amplitude-frequency response curve of the AC coupling circuit proposed in this application.

[0021] Explanation of reference numerals in the figure: inp and inm are input signals, outp and outm are output signals, Cac and Cac' are coupling capacitors, Z and Z' are characteristic impedances, R1 to R3 and R1' to R3' are resistors, MP1 and MP1' are PMOS tubes, V DC_out and V DC_out ' is a DC voltage (common mode voltage of the output signal), and I and I' are current sources. DETAILED DESCRIPTION

[0022] See also Figure 2The AC coupling circuit proposed in this application includes two completely symmetrical upper and lower parts, which are used to transmit a pair of differential signals. The devices in the symmetrical positions have completely the same parameters. inp and inm are a pair of differential input signals, and outp and outm are a pair of differential output signals.

[0023] Figure 2 In the upper circuit, one end of the coupling capacitor Cac is used as the input terminal 1 to receive inp (input signal 1); the other end of the coupling capacitor Cac is used as the output terminal 1 to output outp (output signal 1). The input terminal 1 is also grounded through the matching termination characteristic impedance Z. The input terminal 1 is also grounded through the series resistor R2 and the resistor R1. The gate of the PMOS tube MP1 is connected between the resistor R1 and the resistor R2. The source of the PMOS tube MP1 is connected to the output end of the current source I, and the source of the PMOS tube MP1 is also connected to the output terminal 1 through the resistor R3. The DC voltage at the source position of the PMOS tube MP1 is V DC_out The drain of the PMOS tube MP1 is grounded. The common mode feedback circuit collects the DC voltage V at the source of the PMOS tube MP1. DC_out , the common-mode feedback circuit adjusts the current of the current source I through feedback.

[0024] Figure 2 In the lower half of the circuit, one end of the coupling capacitor Cac' is used as the input terminal 2 to receive inm (input signal 2); the other end of the coupling capacitor Cac' is used as the output terminal 2 to output outm (output signal 2). The input terminal 2 is also grounded through the matching termination characteristic impedance Z'. The input terminal 2 is also grounded through the series-connected resistor R2' and resistor R1'. The gate of the PMOS tube MP1' is connected between the resistor R1' and the resistor R2'. The source of the PMOS tube MP1' is connected to the output end of the current source I', and the source of the PMOS tube MP1' is also connected to the output terminal 2 through the resistor R3'. The DC voltage at the source position of the PMOS tube MP1' is V DC_out The drain of the PMOS tube MP1' is grounded. The common mode feedback circuit collects the DC voltage V at the source of the PMOS tube MP1'. DC_out ', the common-mode feedback circuit adjusts the current of the current source I' through feedback.

[0025] Figure 2 In the upper circuit, the PMOS tube MP1, the resistor R3 and the current source I form a DC bias branch. The DC bias branch is used to determine the DC point of the output voltage (i.e., the DC bias voltage of the output signal). In the DC state, the differential input signals inp and inm are zero, the gate voltage of the PMOS tube MP1 is zero, and the DC voltage V output by the source of the PMOS tube MP1 is DC_outis the gate-source voltage V of the PMOS tube MP1 gs_MP1 , and sent to the output terminal 1 through resistor 3 R3. The DC point of the output voltage 1 outp is V DC_out =V gs_mp1 .

[0026] Figure 2 In the lower half of the circuit, the PMOS tube MP1', the resistor R3' and the current source I' form a DC bias branch. The DC bias branch is used to determine the DC point of the output voltage (i.e., the DC bias voltage of the output signal). In the DC state, the differential input signals inp and inm are zero, the gate voltage of the PMOS tube MP1' is zero, and the DC voltage V output by the source of the PMOS tube MP1' is DC_out ' is the gate-source voltage V of the PMOS tube MP1' gs_MP1’ , through resistor 6 R3' to output terminal 2. The DC point of output voltage 2 outm is V DC_out′ =V gs_mp1′ .

[0027] Figure 2 In the upper part of the circuit, resistor 1 R1, resistor 2 R2, PMOS tube 1 MP1, and resistor 3 R3 form a low-frequency branch. The low-frequency branch is used to determine the low-frequency gain of the AC coupling circuit and provide a low-frequency path from the input signal to the output signal. Resistor 2 R2 and coupling capacitor 1 Cac form a low-pass filter. Resistor 2 R2 and coupling capacitor 1 Cac produce a pole, so that the low-frequency component of the input signal 1 inp can only go backward through resistor 2 R2. After the low-frequency component of the input signal 1 inp passes through resistor 2 R2, it is divided by resistor 1 R1 and resistor 2 R2 and sent to the gate of PMOS tube 1 MP1. PMOS tube 1 MP1 is a source follower (Source Follower, also known as a common drain amplifier circuit). The low-frequency component of the input signal 1 inp is transmitted from the gate of PMOS tube 1 MP1 to the source, and reaches the output terminal 1 through resistor 3 R3. The low-frequency gain of the input signal 1 inp is in, is the gain of PMOS tube 1 (source follower 1) MP1, which is approximately 1.

[0028] Figure 2In the lower half of the circuit, resistor four R1', resistor five R2', PMOS tube two MP1', and resistor six R3' constitute a low-frequency branch. The low-frequency branch is used to determine the low-frequency gain of the AC coupling circuit and provide a low-frequency path from the input signal to the output signal. Resistor five R2' and coupling capacitor two Cac' form a low-pass filter. Resistor five R2' and coupling capacitor two Cac' produce a pole, so that the low-frequency component of the input signal two inm can only go backward through resistor five R2'. After the low-frequency component of the input signal two inm passes through resistor five R2', it is divided by resistor four R1' and resistor five R2' and sent to the gate of PMOS tube two MP1'. PMOS tube two MP1' is a source follower. The low-frequency component of the input signal two inm is transmitted from the gate of PMOS tube two MP1' to the source, and reaches the output terminal two through resistor six R3'. The low-frequency gain of the input signal two inm is in, is the gain of PMOS tube 2 (source follower 2) MP1', which is approximately 1.

[0029] It can be seen from the formulas in the above two paragraphs that the low-frequency gain in the AC coupling circuit proposed in this application can be adjusted by the ratio of the resistors. Figure 3 By controlling the resistor R1 and the resistor R2, different low-frequency gains A, B, and C can be obtained. In the process of high-speed data transmission, the low-frequency component is usually useless signals such as noise, so reducing the useless signal at low frequency is beneficial to the subsequent circuit to receive the transmission signal.

[0030] Figure 2 In the upper circuit, coupling capacitor Cac and resistor R3 form a high-frequency branch. The high-frequency branch is used to provide a high-frequency path from the input signal to the output signal. Coupling capacitor Cac and resistor R3 form a high-pass filter, allowing the high-frequency component of the input signal inp to pass through coupling capacitor Cac and reach the output terminal.

[0031] Figure 2 In the lower half of the circuit, coupling capacitor Cac' and resistor R3' form a high-frequency branch. The high-frequency branch is used to provide a high-frequency path from the input signal to the output signal. Coupling capacitor Cac' and resistor R3' form a high-pass filter, allowing the high-frequency component of the input signal inm to pass through coupling capacitor Cac' and reach the output terminal 2.

[0032] Figure 2 In the AC coupling circuit shown in FIG. 1 , the common-mode feedback circuit, the reference voltage Vcm, the current source I and the current source I' constitute a feedback branch. The feedback branch is used to stabilize the common-mode point of the output voltage (i.e., the DC bias voltage of the output signal). The common-mode feedback circuit converts the common-mode voltage V of the differential output signals outp and outm DC_out (VDC_out 'Equal to V DC_out ) is taken out and compared with the reference voltage Vcm, and the comparison result is used to control the current source I and the current source I', so as to adjust the current flowing through the PMOS tube MP1 and the PMOS tube MP1', thereby changing the DC voltage V at the source of the PMOS tube MP1 DC_out and the DC voltage V at the source of the PMOS tube MP1' DC_out ', thereby adjusting the common-mode voltage of the differential output signals outp and outm. Specifically, when V DC_out When it is less than Vcm, the common-mode feedback circuit outputs a control signal to increase the current of the current source I and the current source I'. At this time, the current flowing through the PMOS tube MP1 and the PMOS tube MP1' increases, and the gate-source voltage of the PMOS tube MP1 and the PMOS tube MP1' increases. DC_out and V DC_out 'Increases. When V DC_out When it is greater than Vcm, the common-mode feedback circuit outputs a control signal to reduce the current of the current source I and the current source I'. At this time, the current flowing through the PMOS tube MP1 and the PMOS tube MP1' is reduced, and the gate-source voltage of the PMOS tube MP1 and the PMOS tube MP1' is reduced. DC_out and V DC_out 'Reduce.

[0033] The AC coupling circuit proposed in the present application forms a DC bias branch, a low-frequency branch, a high-frequency branch, and a feedback branch. In the DC bias branch, the DC bias voltage of the output signal is determined only by the gate-source voltage of the PMOS tube and has nothing to do with the resistance. In the low-frequency branch, the low-frequency gain is determined only by the resistance value of the two resistors. This is to design the DC bias voltage and low-frequency gain of the output signal independently, and the design flexibility is high. At the same time, because there is no restriction on the resistor voltage divider, the AC coupling circuit proposed in the present application can work under lower voltage conditions and is suitable for low-voltage applications of advanced process chips. Since there are low-frequency branches and high-frequency branches from the input end to the output end at the same time, when transmitting a continuous high-level signal (binary number 1) for a long time or a continuous low-level signal (binary number 0) for a long time, the low-frequency component of the signal will not be completely reduced, and the phenomenon of DC baseline drift is compensated.

[0034] The AC coupling circuit proposed in this application is suitable for the receiving end of a high-speed data transmission system. In this application scenario, it is necessary to achieve three functions: impedance matching, signal transmission, and providing a stable output common-mode voltage. First, the AC coupling circuit of this application can terminate any characteristic impedance, such as Figure 2As shown in the termination characteristic impedance 1 Z and characteristic impedance 2 Z'. Secondly, the AC coupling circuit of the present application provides a low-frequency branch and a high-frequency branch for signal transmission at the same time. Thirdly, the AC coupling circuit of the present application provides a feedback branch, and the stability of the output common-mode voltage can be simply and effectively achieved through the adjustment of the feedback branch. Finally, the AC coupling circuit of the present application separates the DC bias branch and the low-frequency branch, and has high design flexibility. One end of the termination resistor Z and the voltage divider resistor R1 are both grounded, and the power supply voltage requirement is not high, meeting the needs of low-voltage applications.

[0035] The above are only preferred embodiments of the present application and are not intended to limit the present application. For those skilled in the art, the present application may have various changes and variations. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present application should all be included in the protection scope of the present application.

Claims

1. An AC coupling circuit, characterized in that: It consists of two completely symmetrical upper and lower parts, which are used to transmit a pair of differential signals. The devices in symmetrical positions have exactly the same parameters. In the upper circuit, one end of the coupling capacitor 1 is used as input terminal 1, and the other end is used as output terminal 1; The input terminal 1 is also grounded through a matching termination characteristic impedance 1; the input terminal 1 is also grounded through a series resistor 2 and a resistor 1; the gate of the PMOS tube 1 is connected between the resistor 1 and the resistor 2, the source is connected to the output terminal of the current source 1, the source is also connected to the output terminal 1 through the resistor 3, and the drain is grounded; the common-mode feedback circuit collects the source voltage of the PMOS tube 1 and adjusts the current of the current source 1 through feedback. In the lower half of the circuit, one end of coupling capacitor 2 serves as input terminal 2, and the other end serves as output terminal 2; input terminal 2 is also grounded through matching termination characteristic impedance 2; input terminal 2 is also grounded through series resistor 5 and resistor 4; the gate of PMOS tube 2 is connected between resistor 4 and resistor 5, the source is connected to the output end of current source 2, the source is also connected to output terminal 2 through resistor 6, and the drain is grounded; the common-mode feedback circuit collects the source voltage of PMOS tube 2, and adjusts the current of current source 2 through feedback.

2. The AC coupling circuit according to claim 1, characterized in that: In the upper circuit, PMOS tube 1, resistor 3 and current source 1 constitute a DC bias branch; in the lower circuit, PMOS tube 2, resistor 6 and current source 2 constitute a DC bias branch; the DC bias branch is used to determine the DC point of the output voltage, that is, the DC bias voltage of the output signal.

3. The AC coupling circuit according to claim 2, characterized in that: In the DC bias branch, the DC voltage V at the source of the PMOS tube 1 is DC_out is the gate-source voltage of PMOS tube 1, and the DC voltage V at the source of PMOS tube 2 DC_out ' is the gate-source voltage of PMOS tube 2, the two are equal and serve as the DC point of the output voltage.

4. The AC coupling circuit according to claim 1, characterized in that: In the upper circuit, resistor 1, resistor 2, PMOS tube 1, and resistor 3 constitute a low-frequency branch; in the lower circuit, resistor 4, resistor 5, PMOS tube 2, and resistor 6 constitute a low-frequency branch; the low-frequency branch is used to determine the low-frequency gain of the AC coupling circuit and provide a low-frequency path from the input signal to the output signal.

5. The AC coupling circuit according to claim 4, characterized in that: In the low-frequency branch of the upper circuit, resistor 2 and coupling capacitor 1 form a low-pass filter. Resistor 2 and coupling capacitor 1 produce a pole. The low-frequency component of input signal 1 can only go backward through resistor 2 and be sent to the gate of PMOS tube 1. PMOS tube 1 is a source follower. The low-frequency component of input signal 1 is transmitted from the gate of PMOS tube 1 to the source, and reaches output terminal 1 through resistor 3. The low-frequency gain of input signal 1 is in, is the gain of PMOS tube 1, which is approximately 1; In the low-frequency branch of the lower circuit, resistor 5 and coupling capacitor 2 form a low-pass filter. Resistor 5 and coupling capacitor 2 produce a pole. The low-frequency component of input signal 2 can only go backward through resistor 5 and be sent to the gate of PMOS tube 2. PMOS tube 2 is a source follower. The low-frequency component of input signal 2 is transmitted from the gate of PMOS tube 2 to the source and reaches output terminal 2 through resistor 6. The low-frequency gain of input signal 2 is in, is the gain of PMOS tube 2, which is approximately 1.

6. The AC coupling circuit according to claim 1, characterized in that: In the upper circuit, coupling capacitor 1 and resistor 3 form a high-frequency branch; in the lower circuit, coupling capacitor 2 and resistor 6 form a high-frequency branch; the high-frequency branch is used to provide a high-frequency path from the input signal to the output signal.

7. The AC coupling circuit according to claim 6, characterized in that: In the high-frequency branch of the upper circuit, coupling capacitor 1 and resistor 3 form a high-pass filter, and the high-frequency component of input signal 1 goes backward through coupling capacitor 1 and reaches output terminal 1; In the high-frequency branch of the lower circuit, coupling capacitor 2 and resistor 6 form a high-pass filter, and the high-frequency component of input signal 2 goes backward through coupling capacitor 2 and reaches output terminal 2.

8. The AC coupling circuit according to claim 1, characterized in that: The common-mode feedback circuit, the reference voltage, the current source 1 and the current source 2 constitute a feedback branch; the feedback branch is used to stabilize the common-mode point of the output voltage, that is, the DC bias voltage of the output signal.

9. The AC coupling circuit according to claim 8, characterized in that: In the feedback branch, the common-mode feedback circuit takes out the common-mode voltage of the differential output signal and compares it with the reference voltage, and uses the comparison result to control the current source 1 and the current source 2, so as to adjust the current flowing through the PMOS tube 1 and the PMOS tube 2, thereby changing the DC voltage of the source of the PMOS tube 1 and the DC voltage of the source of the PMOS tube 2, thereby adjusting the common-mode voltage of the differential output signal.

10. The AC coupling circuit according to claim 9, characterized in that: In the feedback branch, when the common-mode voltage of the differential output signal is less than the reference voltage, the common-mode feedback circuit outputs a control signal to increase the current of the current source 1 and the current source 2. At this time, the current flowing through the PMOS tube 1 and the PMOS tube 2 increases, the gate-source voltage of the PMOS tube 1 and the PMOS tube 2 increases, and the common-mode voltage of the differential output signal increases; When the common-mode voltage of the differential output signal is greater than the reference voltage, the common-mode feedback circuit outputs a control signal to reduce the current of current source 1 and current source 2. At this time, the current flowing through PMOS tube 1 and PMOS tube 2 decreases, the gate-source voltage of PMOS tube 1 and PMOS tube 2 decreases, and the common-mode voltage of the differential output signal decreases.

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