A circuit and method for improving the interference rejection capability of a microphone circuit

Abstract

The application provides a circuit and method for improving the anti-interference ability of a microphone circuit. The circuit comprises: a sound head for converting a sound signal into a single-ended electrical signal; a first reverse amplification circuit, the input end of which is coupled with the output end of the sound head, for amplifying the single-ended electrical signal and outputting a first signal, the first signal being phase-inverted by 180 degrees relative to the single-ended electrical signal; an inverter, the input end of which is coupled with the output end of the first reverse amplification circuit, for phase-inverting the first signal and outputting a second signal; and a differential output end comprising a first output end and a second output end, the first output end being coupled with the output end of the first reverse amplification circuit to output the first signal, and the second output end being coupled with the output end of the inverter to output the second signal. The application realizes natural equality of the amplitudes of differential signals from the circuit topology level through a two-stage inverting cascade structure, which helps to improve the common-mode interference suppression ability.

Description

Technical Field

[0001] This invention relates to the field of signal processing circuit technology, and specifically to a circuit and method for converting a single-ended microphone output signal into a differential signal. Background Technology

[0002] Microphones have a wide range of applications, appearing in mobile phones, computers, audio equipment, recording and broadcasting devices, and more. Different scenarios require different microphones, and the demands for sound pickup quality and clarity are constantly increasing. Whether it's a dynamic microphone, a condenser microphone, or a chip-based silicon microphone, most microphones on the market use a single-ended output.

[0003] Single-ended microphone circuits have inherent limitations in practical applications. Because the signal is transmitted via a single wire and referenced to ground, it is susceptible to power supply ripple, spatial electromagnetic radiation, and common-mode interference. When the transmission line is long or the operating environment has strong electromagnetic interference, the anti-interference capability of single-ended transmission is insufficient to meet the requirements of high-fidelity audio acquisition.

[0004] To address the vulnerability of single-ended transmission to interference, the art typically converts single-ended signals to differential signals before transmission or subsequent processing. Common single-ended to differential circuits employ one operational amplifier for inverting amplification and another for inverting amplification. However, the gain of the inverting amplifier is 1 + Rf / Ri, while the gain of the inverting amplifier is -Rf / Ri. The different gain formulas make it difficult to ensure that the amplitudes of the two output signals are perfectly equal. When the amplitude difference is too large, exceeding the linear operating range of the subsequent differential receiving circuit or disrupting signal symmetry, it can lead to a decrease in the common-mode rejection ratio and even signal distortion. Summary of the Invention

[0005] The technical problem to be solved by this invention is that in existing microphone single-ended to differential circuits, due to the different gain formulas of the in-phase amplification branch and the in-phase amplification branch, it is difficult to obtain differential signals with opposite phases and completely equal amplitudes, resulting in a decrease in common-mode rejection ratio and signal distortion.

[0006] To solve the above-mentioned technical problems, the present invention provides a circuit for improving the anti-interference capability of a microphone circuit, comprising: A microphone is used to convert a sound signal into a single-ended electrical signal. The first inverting amplifier circuit has its input terminal coupled to the output terminal of the microphone, and is used to amplify the single-ended electrical signal and output a first signal, wherein the first signal is 180 degrees out of phase with respect to the single-ended electrical signal. An inverter, whose input is coupled to the output of the first inverting amplifier circuit, is used to invert the phase of the first signal and output a second signal; The differential output terminal includes a first output terminal and a second output terminal. The first output terminal is coupled to the output terminal of the first inverting amplifier circuit to output the first signal, and the second output terminal is coupled to the output terminal of the inverter to output the second signal. The voltage gain of the inverter is -1, which makes the amplitude of the second signal substantially equal to the amplitude of the first signal. The first signal and the second signal constitute a pair of differential signals with opposite phases and equal amplitudes.

[0007] The single-ended electrical signal output from the microphone is amplified by the first inverting amplifier circuit, and the amplified first signal is split into two paths. One path is output directly, and the other path is output after phase inversion by an inverter with a gain of -1. This makes the absolute value of the amplitude of the two signals the absolute value of the gain of the first inverting amplifier circuit. From the circuit topology level, the amplitudes of the two signals are naturally equal, which helps to alleviate the amplitude mismatch problem caused by different gain formulas in the traditional in-phase / inverting structure, and can obtain a highly symmetrical differential signal pair.

[0008] Furthermore, the inverter includes a second operational amplifier, a second input resistor, and a second feedback resistor. The resistance value of the second input resistor is equal to the resistance value of the second feedback resistor to achieve a voltage gain of -1. By using an operational amplifier and input and feedback resistors with equal resistance values, the inverter can accurately achieve a voltage gain of -1 with a simple structure, ensuring high symmetry of the differential signal amplitude, while also facilitating debugging and component selection.

[0009] Furthermore, the first inverting amplifier circuit includes a first operational amplifier, a first input resistor, and a first feedback resistor. The signal is input to the inverting input terminal of the first operational amplifier, and the non-inverting input terminal of the first operational amplifier is grounded. The first inverting amplifier circuit adopts an operational amplifier structure with an inverting input and a grounded non-inverting input, resulting in constant input impedance, independent gain setting, and a simple and reliable circuit design.

[0010] Furthermore, a first coupling capacitor is connected in series between the first inverting amplifier circuit and the first output terminal, and a second coupling capacitor is connected in series between the inverter and the second output terminal; a third coupling capacitor is connected in series between the first inverting amplifier circuit and the inverter. By setting coupling capacitors between each module and at the output terminal, the DC bias of each stage can be effectively isolated, transmitting only AC audio signals, and avoiding the influence of DC offset voltage on differential symmetry and the operating point of subsequent circuits.

[0011] Furthermore, the microphone capsule can be an electret microphone capsule, a moving coil microphone capsule, or a MEMS silicon microphone capsule. The selection of an electret microphone capsule, a moving coil microphone capsule, or a MEMS silicon microphone capsule allows for adaptation to the sensitivity, frequency response, and power supply requirements of different application scenarios, thus improving the circuit's versatility and compatibility.

[0012] Furthermore, the circuit is a purely analog signal processing circuit. The first inverting amplifier circuit and the inverter are composed of analog devices and do not include an analog-to-digital converter or a digital signal processor for phase inversion. Being limited to a purely analog signal processing circuit, without an analog-to-digital converter or digital phase inversion, avoids quantization noise and delay introduced by digital processing, making it suitable for audio applications with high real-time and fidelity requirements.

[0013] Furthermore, the first inverting amplifier circuit and the inverter are implemented using operational amplifiers or amplifier circuits composed of bipolar transistors or field-effect transistors, and the differential output terminal does not include an audio transformer. The first inverting amplifier circuit and the inverter can be implemented using discrete transistor devices and do not include an audio transformer, which is beneficial for achieving circuit miniaturization, integration, and low cost, while maintaining the low latency advantage of a pure analog signal path.

[0014] The present invention also provides an audio device including the above-described circuit. By integrating the above-described circuit into the audio device, a highly symmetrical differential signal can be provided for various pickup terminal products, improving the overall anti-common-mode interference capability of the device, and the infringing party is clearly identified, facilitating rights protection.

[0015] This invention also provides a method for improving the anti-interference capability of a microphone circuit, comprising: converting a sound signal into a single-ended electrical signal using a microphone capsule; amplifying the single-ended electrical signal using a first inverting amplifier circuit to obtain a first signal with a 180-degree phase reversal; splitting the first signal into two paths, one directly outputting as the positive differential signal and the other inputting to an inverter; and reversing the phase of the first signal using the inverter with a voltage gain of -1 to obtain a second signal with substantially equal amplitude but opposite phase to the first signal, which is output as the negative differential signal. Corresponding to the product claims, by combining the steps of sound-to-electric conversion, inverting amplification, splitting, inverting, and differential output, a complete signal processing flow is provided for improving the anti-interference capability of the microphone circuit.

[0016] Furthermore, the voltage gain -1 of the inverter is achieved by making the input resistance and feedback resistance of the inverter equal. Achieving a voltage gain -1 by making the input resistance and feedback resistance of the inverter equal allows for rapid establishment of differential amplitude matching during debugging, reducing debugging parameters and shortening the development cycle.

[0017] Furthermore, the steps of amplifying the single-ended electrical signal by the first inverting amplifier circuit and the step of phase inversion of the first signal by the inverter are both completed in the analog domain, excluding the step of converting the signal to a digital signal and then performing phase inversion. By limiting the amplification and inversion steps to be completed entirely in the analog domain, excluding digital domain phase inversion, differential conversion can be achieved while ensuring low signal latency and no quantization distortion, making it suitable for professional audio processing links. Beneficial Effects 1. This invention amplifies the single-ended electrical signal output from the microphone by a first inverting amplifier circuit, and splits the amplified first signal into two paths. One path is output directly, and the other path is output after phase inversion by an inverter with a gain of -1. This ensures that the absolute values ​​of the amplitudes of the two signals are the absolute values ​​of the gain of the first inverting amplifier circuit. From the circuit topology level, this achieves the natural equality of the amplitudes of the two signals, which helps to alleviate the amplitude mismatch problem caused by different gain formulas in traditional in-phase / inverting structures, and can obtain a highly symmetrical differential signal pair.

[0018] 2. In this invention, the amplitude symmetry of the differential signal depends only on the matching accuracy of the inverter input resistor and the feedback resistor, and is independent of the gain setting of the first inverting amplifier circuit. Therefore, even if the gain of the first inverting amplifier circuit changes due to temperature drift or component aging, as long as the resistor ratio in the inverter remains constant, the degree to which the amplitude symmetry of the differential signal is affected can be reduced, which helps to improve the robustness of the circuit's common-mode interference suppression capability under complex operating conditions.

[0019] 3. By designing the first inverting amplifier circuit and the inverter as a unified inverting amplifier topology, the present invention allows designers to obtain a highly symmetrical differential output simply by ensuring that the input resistance of the inverter and the feedback resistance are equal. The gain of the first inverting amplifier circuit can be set independently as needed, and the debugging parameters are simplified from two proportional relationships in the traditional scheme to a single set, which helps to reduce circuit debugging time and shorten the product development cycle. Attached Figure Description

[0020] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a block diagram illustrating the circuit principle of the present invention.

[0022] Figure 2 This is a circuit schematic diagram of Embodiment 1 of the present invention. Detailed Implementation Example 1

[0023] This embodiment provides a circuit to improve the anti-interference capability of a microphone circuit. The following explains the correspondence between the names of the functional modules: the microphone module is the microphone (MIC1), the first inverting amplifier circuit is the first inverting amplifier circuit (U1A), the inverter is the inverter (U1B), and the differential output coupling part.

[0024] Figure 1 This is a block diagram of the circuit principle in this embodiment. The circuit includes a microphone head, a first inverting amplifier circuit, an inverter, and a subsequent differential receiving circuit. The microphone head is used to pick up the sound signal and complete the sound-to-electric conversion, outputting a weak analog electrical signal. The first inverting amplifier circuit is used to amplify the voltage of the weak electrical signal. The inverter is used to invert the phase of the amplified signal by 180 degrees. The subsequent differential receiving circuit receives the differential input signal for further processing. This subsequent differential receiving circuit is not a necessary component of the circuit of this invention; it is only used to exemplify the reception scenario of differential signals. In actual applications, it can be an analog-to-digital converter, a differential amplifier, an audio processor, or other circuits with differential input interfaces. The signal flow is as follows: the signal output from the microphone head is sent to the first inverting amplifier circuit. The signal amplified by the first inverting amplifier circuit is divided into two paths: one path is directly sent to the subsequent differential receiving circuit as the differential positive terminal signal, and the other path is inverted by the inverter and sent to the subsequent differential receiving circuit as the differential negative terminal signal.

[0025] Figure 2 This is the circuit schematic diagram for this embodiment. The following is in conjunction with... Figure 2 Provide a detailed explanation of the circuit structure and its working principle.

[0026] The microphone module consists of an electret microphone (MIC1), a bias resistor (R1), a power supply current-limiting resistor (R2), a high-frequency filter capacitor (C1), and power supply decoupling capacitors (C2 and C4). The microphone (MIC1) is an electret microphone used to convert audio signals into electrical signals. Resistor R1 provides a DC bias voltage to the microphone (MIC1). Resistor R2 is connected in series at the bias power supply input to limit current. Capacitor C2 is an electrolytic capacitor used to filter low-frequency ripple from the bias power supply. Capacitor C4 is a ceramic capacitor used to filter high-frequency noise from the bias power supply. Capacitor C1 is connected in parallel at the output of the microphone (MIC1) to filter high-frequency interference signals introduced by spatial electromagnetic radiation. The bias power supply is provided by an AVCC3V3, and after being divided by resistors R2 and R1, it is supplied to the microphone (MIC1). The value of resistor R1 must be selected to balance the sensitivity and noise characteristics of the electret microphone, ensuring that the microphone operates near the recommended bias current. Capacitor C1 is a 10pF ceramic capacitor with a high self-resonant frequency. It can form a low-impedance path for common radio frequency interference bands, effectively bypassing high-frequency interference without attenuating the audio signal.

[0027] The first inverting amplifier circuit consists of a first operational amplifier U1A, an input coupling capacitor C3, a first input resistor R3, a first feedback resistor R4, a first phase compensation capacitor C5, a first output matching resistor R5, a first output filter capacitor C8, and negative power supply decoupling capacitors C6 and C7. Operational amplifier U1A is the A section of the dual operational amplifier. Capacitor C3 is connected in series between the output terminal of the microphone MIC1 and resistor R3, serving to block DC and pass AC signals, coupling audio signals. One end of resistor R3 is connected to capacitor C3, and the other end is connected to the inverting input terminal of operational amplifier U1A; the signal is sent to the inverting input terminal of operational amplifier U1A through resistor R3. The non-inverting input terminal of operational amplifier U1A is grounded. Resistor R4 is connected across the output terminal and the inverting input terminal of operational amplifier U1A, forming a feedback loop; the voltage gain of operational amplifier U1A is determined by -R4 / R3. Capacitor C5 is connected in parallel across resistor R4 for phase compensation to prevent high-frequency self-oscillation in the circuit. Resistor R5 is connected in series at the output of op-amp U1A to provide output impedance matching. Capacitor C8 is connected at one end to the output of resistor R5 and the other end to ground, used to filter out high-frequency interference in the amplified signal. Capacitors C6 and C7 are connected in parallel between the negative power supply pin of op-amp U1A and ground, respectively filtering out low-frequency ripple and high-frequency noise from the negative power supply. The ratio of resistors R3 to R4 determines the first-stage gain, amplifying the millivolt-level signal from a typical electret capsule output to a level easily processed by subsequent stages in the hundreds of millivolts, while retaining sufficient dynamic headroom. Capacitor C3 is a 22μF electrolytic capacitor; together with resistor R3, the high-pass filter cutoff frequency is far below the lower limit of the audio frequency band (20Hz), avoiding low-frequency attenuation. Capacitor C5 is a 47pF ceramic capacitor; connected in parallel with resistor R4, it reduces feedback impedance in the high-frequency range, attenuating the high-frequency loop gain and ensuring op-amp stability.

[0028] The inverter consists of a second operational amplifier U1B, a third coupling capacitor C13, a second input resistor R6, a second feedback resistor R7, a second phase compensation capacitor C14, a second output matching resistor R8, a second output filter capacitor C15, and positive power supply decoupling capacitors C11 and C12. Operational amplifiers U1B and U1A are two channels of the same dual operational amplifier. Capacitor C13 is connected in series between the output terminal of resistor R5 and resistor R6, used to couple the signal output from the first inverting amplifier circuit to the inverter. One end of resistor R6 is connected to capacitor C13, and the other end is connected to the inverting input terminal of operational amplifier U1B. The non-inverting input terminal of operational amplifier U1B is grounded. Resistor R7 is connected across the output terminal and the inverting input terminal of operational amplifier U1B. Resistors R6 and R7 have equal resistance values, making the voltage gain of operational amplifier U1B -R7 / R6 = -1, meaning that only the input signal undergoes a 180-degree phase reversal without changing the signal amplitude. Capacitor C14 is connected in parallel across resistor R7 to suppress high-frequency self-oscillation of the inverter. Resistor R8 is connected in series at the output of op-amp U1B. One end of capacitor C15 is connected to the output of resistor R8, and the other end is grounded for output filtering. Capacitors C11 and C12 are connected in parallel between the positive power supply pin of op-amp U1B and ground to filter out low-frequency ripple and high-frequency noise from the positive power supply, respectively. Resistors R6 and R7 are precision resistors of the same value to ensure a voltage gain of exactly -1. The selection considerations for capacitors C13 and C14 are the same as those for capacitors C3 and C5, respectively.

[0029] The differential output coupling section consists of a first coupling capacitor C10 and a second coupling capacitor C9. Capacitor C10 is connected in series between the output of resistor R5 and the differential positive input terminal MIC_IN_P of the subsequent differential receiving circuit, coupling the signal output from the first inverting amplifier circuit as the differential positive signal to the subsequent differential receiving circuit. Capacitor C9 is connected in series between the output of resistor R8 and the differential negative input terminal MIC_IN_N of the subsequent differential receiving circuit, coupling the signal output from the inverter as the differential negative signal to the subsequent differential receiving circuit. Capacitors C9 and C10 are selected with the same capacitance value as capacitor C3 to ensure consistent low-frequency response between the two signals.

[0030] The circuit operates as follows: MIC1 converts the picked-up sound signal into a weak electrical signal. After high-frequency interference is filtered out by capacitor C1, it is coupled to the first operational amplifier U1A via capacitor C3 for inverting amplification. The amplified signal is then filtered out for high-frequency interference by capacitor C8 and output from resistor R5. This output signal is divided into two paths: one path is coupled to the differential positive input terminal MIC_IN_P of the subsequent differential receiving circuit via capacitor C10, serving as the first signal; the other path is coupled to the second operational amplifier U1B via capacitor C13. Operational amplifier U1B inverts the phase of this signal with a gain of -1, generating a second signal with the same amplitude but opposite phase to the first signal. This second signal is coupled to the differential negative input terminal MIC_IN_N of the subsequent differential receiving circuit via capacitor C9. The first and second signals constitute a pair of differential signals for subsequent processing by the subsequent differential receiving circuit.

[0031] The following provides a quantitative explanation of the amplitude symmetry of the differential signal. Let the audio signal voltage output by microphone MIC1 be Vi. The gain of the inverting amplifier circuit formed by the first operational amplifier U1A is G1 = -R4 / R3, therefore the voltage of the first signal is G1 × Vi. This signal is output directly through capacitor C10; the other path passes through an inverter formed by the second operational amplifier U1B, with a gain G2 = -R7 / R6 = -1, therefore the voltage of the second signal is G2 × G1 × Vi = -G1 × Vi. The voltage amplitudes of both the first and second signals are |G1 × Vi|, with a phase difference of 180°, forming an ideal differential pair. From the above derivation, it can be seen that the matching accuracy of the differential amplitude depends only on the consistency of R6 and R7, and is independent of the specific value of the first-stage gain G1. Even if R3 or R4 drifts due to temperature changes, as long as the resistance values ​​of R6 and R7 remain equal, the amplitude symmetry of the differential pair is unaffected.

[0032] In actual printed circuit board layout, the differential signal output traces, i.e., the paths of capacitors C10 and C9 to the subsequent differential receiving circuit, should preferably be parallel traces of equal length and spacing, provided that space allows. This ensures that the common-mode interference amplitude experienced by the two signals is as consistent as possible, thus fully leveraging the anti-interference advantages of differential transmission. Simultaneously, high-frequency filter capacitors C1, C5, C8, C14, and C15 should be placed close to the pins of their corresponding components to minimize parasitic inductance and ensure effective high-frequency filtering.

[0033] Under normal operating conditions, observing the output terminals of capacitors C10 and C9 with a dual-channel oscilloscope reveals that the two waveforms have the same shape, equal amplitude, and opposite phase. When a 1kHz sine wave is input, the two signals cross symmetrically at the zero-crossing point, with identical peak amplitudes. If a differential operation is performed on the two signals, the amplitude of the resulting differential signal is twice that of a single signal.

[0034] In this embodiment, the first operational amplifier U1A and the second operational amplifier U1B are implemented using a single dual operational amplifier integrated circuit. Typical parameters for each resistor and capacitor are as follows: R1=2.2kΩ, R2=100Ω, R3=2.2kΩ, R4=10kΩ, R5=0Ω, R6=10kΩ, R7=10kΩ, R8=0Ω; C1=10pF, C2=22μF, C3=22μF, C4=100nF, C5=47pF, C6=22μF, C7=100nF, C8=1nF, C9=22μF, C10=22μF, C11=22μF, C12=100nF, C13=22μF, C14=47pF, C15=1nF. In this example, resistors R5 and R8 are output matching resistors or output isolation resistors, set to 0Ω in the illustrated example. However, in other embodiments, the values ​​of resistors R5 and R8 can be selected from 0Ω to 200Ω, such as 47Ω or 100Ω, to enhance the driving stability of the capacitive load in the subsequent stage and suppress high-frequency oscillations. It is understood that the above RC parameters are merely examples, and those skilled in the art can adjust them according to actual needs. For example, when the supply voltage changes or different sensitivity capsules are selected, the value of the bias resistor R1 can be adjusted accordingly; when different gains are required, the ratio of resistors R3 and R4 can be adjusted; when the input impedance of the subsequent stage is different, the capacitance of the coupling capacitor can be adjusted to ensure low-frequency response; and the specific values ​​of resistors R5 and R8 can be selected according to stability requirements.

[0035] In another embodiment, the bias resistor R1 can be replaced by a voltage divider network consisting of two resistors, with the middle node connected to the output of the microphone MIC1 via a resistor, to provide a more stable DC bias and reduce power supply noise coupling. The upper resistor of this voltage divider network is connected to the bias power supply, and the lower resistor is grounded; its equivalent Thevenin resistance is comparable to that of resistor R1. This alternative can be used in applications where power supply noise is more critical. Example 2

[0036] The difference between this embodiment and Embodiment 1 lies in the implementation method of the inverter.

[0037] In Example 1, the inverter uses an operational amplifier and input and feedback resistors of equal resistance to achieve a gain of -1. This example provides an alternative implementation: the inverter is constructed using a bipolar junction transistor (BJT) common-emitter amplifier circuit. Specifically, an NPN transistor Q1 is used, with its base connected to the positive power supply via bias resistor Rb1 and to ground via bias resistor Rb2 to establish a stable quiescent operating point; the base is also connected to the output of the first inverting amplifier circuit via a third coupling capacitor C13; the collector is connected to the positive power supply via collector resistor Rc, and the emitter is connected to ground via emitter resistor Re. The input signal is coupled to the base of Q1 via capacitor C13, and the amplified inverted signal is output from the collector. The voltage gain of this common-emitter amplifier circuit is determined by -Rc / Re. By setting the resistance values ​​of Rc and Re to be equal, the voltage gain can be stabilized at -1, and the gain is unaffected by changes in transistor parameters and temperature drift.

[0038] Alternatively, the inverter can also be constructed using a common-source amplifier circuit with a field-effect transistor. An N-channel enhancement-type MOSFET Q2 is used, with its gate connected to the positive power supply via bias resistor Rg1 and to ground via bias resistor Rg2. The gate is also connected to the output of the first inverting amplifier circuit via a third coupling capacitor C13. The drain is connected to the positive power supply via drain resistor Rd, and the source is connected to ground via source resistor Rs. The voltage gain of this common-source amplifier circuit is determined by -Rd / Rs. By setting Rd and Rs to be equal, a gain of -1 can be achieved.

[0039] The inverter implemented with the aforementioned discrete components employs a negative feedback structure, and its gain is determined by the resistor ratio. It can stably maintain a gain of -1 over a wide temperature range and under discrete device parameters. The first inverting amplifier circuit can also be implemented using a common-emitter or common-source amplifier structure with negative feedback. When using discrete transistor components to construct the inverter, this circuit is a purely analog signal processing circuit. Both the first inverting amplifier circuit U1A and the inverter are composed of analog components, without an analog-to-digital converter or a digital signal processor for phase inversion. It can directly perform single-ended to differential conversion in the analog domain, offering the advantage of low latency. Furthermore, the entire differential signal generation path does not include an audio transformer, which is beneficial for circuit miniaturization and integration. Example 3

[0040] This embodiment provides an alternative implementation of the microphone MIC1.

[0041] In Example 1, the microphone MIC1 uses an electret microphone. Depending on the actual application scenario, the microphone MIC1 can also be replaced with a moving microphone. The moving microphone operates based on the principle of electromagnetic induction and does not require an external bias power supply. In this case, the bias resistor R1, the power supply current-limiting resistor R2, and the decoupling capacitors C2 and C4 in Example 1 can be omitted, or this part of the circuit can be replaced with a buffer circuit that matches the output impedance of the moving microphone.

[0042] The microphone capsule MIC1 can also be replaced with a MEMS silicon microphone capsule. The MEMS silicon microphone capsule integrates a charge pump and a buffer amplifier. Its output can be directly connected to the input of the first inverting amplifier circuit U1A via a coupling capacitor. The bias circuit should be adjusted according to the configuration recommended in the MEMS silicon microphone capsule datasheet. Replacing the microphone capsule with a different type does not affect the core scheme of the subsequent two-stage inverting structure for differential conversion; this circuit has good front-end compatibility.

[0043] Furthermore, the microphone MIC1 can be replaced with a piezoelectric microphone. A piezoelectric microphone utilizes the piezoelectric effect of piezoelectric materials to directly convert sound wave vibrations into a voltage signal output, eliminating the need for an external bias power supply. The output of the piezoelectric microphone can be directly connected to the input of the first inverting amplifier circuit U1A via a coupling capacitor. Since piezoelectric microphones typically have a high output impedance, a voltage follower stage can be added before the first inverting amplifier circuit U1A as an impedance transformation buffer stage. Piezoelectric microphones are robust, resistant to high temperatures and humidity, and suitable for demanding applications such as outdoor recording equipment or industrial acoustic monitoring. They also eliminate the need for a bias circuit, simplifying the circuit board layout. Example 4

[0044] This embodiment provides a method for improving the anti-interference capability of a microphone circuit, applied to the circuit described in any one of embodiments 1 to 3. The method includes the following steps: Step S1: Convert the sound signal into a single-ended electrical signal using a microphone. The microphone can be an electret microphone, a moving-coil microphone, a MEMS silicon microphone, or a piezoelectric microphone.

[0045] Step S2: The single-ended electrical signal is amplified by the first inverting amplifier circuit to obtain a first signal with a 180-degree phase reversal. The voltage gain of the first inverting amplifier circuit is set according to the actual audio signal amplitude requirements.

[0046] Step S3: Divide the first signal into two paths, one of which is directly output as the positive differential signal, and the other is input to the inverter.

[0047] Step S4: The first signal is phase-inverted using an inverter with a voltage gain of -1, resulting in a second signal with the same amplitude but opposite phase to the first signal, which is output as the negative differential signal. The voltage gain of -1 is achieved by making the input resistance of the inverter equal to the resistance of the feedback resistor.

[0048] Step S5: The first signal and the second signal are used as a differential signal pair and output to the differential positive input terminal MIC_IN_P and differential negative input terminal MIC_IN_N of the subsequent differential receiving circuit through the first coupling capacitor C10 and the second coupling capacitor C9, respectively.

[0049] This method simplifies the complex debugging process of traditional single-ended to differential converters, which requires simultaneous adjustment of the gains of both the in-phase and out-of-phase branches, to simply ensuring that the input resistor and feedback resistor of the inverter are equal. This reduction in debugging parameters helps shorten the product development cycle. In the steps described above, the amplification of the single-ended electrical signal by the first inverting amplifier circuit and the phase inversion of the first signal by the inverter are both completed in the analog domain, excluding the step of converting the signal to a digital signal and then performing phase inversion.

[0050] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Those skilled in the art can make various modifications and equivalent substitutions to the present invention without departing from its spirit and scope. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A circuit for improving the anti-interference capability of a microphone circuit, characterized in that, include: The microphone (MIC1) is used to convert sound signals into single-ended electrical signals. The first inverting amplifier circuit (U1A) has its input terminal coupled to the output terminal of the microphone (MIC1) and is used to amplify the single-ended electrical signal and output a first signal, wherein the first signal is 180 degrees out of phase with respect to the single-ended electrical signal. An inverter (U1B) has its input coupled to the output of the first inverting amplifier circuit (U1A) and is used to invert the phase of the first signal and output a second signal. The differential output terminal includes a first output terminal and a second output terminal. The first output terminal is coupled to the output terminal of the first inverting amplifier circuit (U1A) to output the first signal, and the second output terminal is coupled to the output terminal of the inverter (U1B) to output the second signal. The voltage gain of the inverter (U1B) is -1, which makes the amplitude of the second signal substantially equal to the amplitude of the first signal. The first signal and the second signal constitute a pair of differential signals with opposite phases and equal amplitudes.

2. The circuit according to claim 1, characterized in that, The inverter (U1B) includes a second operational amplifier, a second input resistor (R6), and a second feedback resistor (R7). The resistance value of the second input resistor (R6) is equal to the resistance value of the second feedback resistor (R7) to achieve a voltage gain of -1.

3. The circuit according to claim 1, characterized in that, The first inverting amplifier circuit (U1A) includes a first operational amplifier, a first input resistor (R3) and a first feedback resistor (R4). The signal is input to the inverting input terminal of the first operational amplifier, and the non-inverting input terminal of the first operational amplifier is grounded.

4. The circuit according to claim 1, characterized in that, A first coupling capacitor (C10) is connected in series between the first inverting amplifier circuit (U1A) and the first output terminal; a second coupling capacitor (C9) is connected in series between the inverter (U1B) and the second output terminal; and a third coupling capacitor (C13) is connected in series between the first inverting amplifier circuit (U1A) and the inverter (U1B).

5. The circuit according to claim 1, characterized in that, The microphone (MIC1) is an electret microphone, a dynamic microphone, or a MEMS silicon microphone.

6. The circuit according to claim 1, characterized in that, The circuit is a purely analog signal processing circuit. The first inverting amplifier circuit (U1A) and the inverter (U1B) are composed of analog devices and do not include an analog-to-digital converter or a digital signal processor for phase inversion.

7. The circuit according to claim 1, characterized in that, The first inverting amplifier circuit (U1A) and the inverter (U1B) are implemented by an operational amplifier or an amplifier circuit composed of bipolar transistors or field-effect transistors, and the differential output terminal does not include an audio transformer.

8. An audio device, characterized in that, Includes the circuit as described in any one of claims 1 to 7.

9. A method for improving the anti-interference capability of a microphone circuit, applied to the circuit according to any one of claims 1 to 7, characterized in that, include: The sound signal is converted into a single-ended electrical signal through the microphone (MIC1); The single-ended electrical signal is amplified by the first inverting amplifier circuit (U1A) to obtain a first signal with a 180-degree phase reversal. The first signal is split into two paths: one path is directly output as the positive differential signal, and the other path is input to the inverter (U1B). The inverter (U1B) reverses the phase of the first signal with a voltage gain of -1 to obtain a second signal with an amplitude that is substantially equal to that of the first signal but opposite in phase, which is then output as the differential negative terminal signal.

10. The method according to claim 9, characterized in that, The voltage gain -1 of the inverter (U1B) is achieved by making the input resistance of the inverter (U1B) equal to the resistance of the feedback resistance.

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