Microphone interface circuit

Abstract

The utility model relates to the technical field of audio circuits, in particular to a microphone interface circuit, which comprises a field-effect transistor T2 of which the drain electrode is coupled with the positive electrode of a direct-current bias power supply; the positive electrode of the microphone is coupled with the source electrode of the field effect transistor T2, and the negative electrode of the microphone is electrically coupled with the negative electrode of the direct-current bias power supply; the input end of the time constant circuit is coupled with the two ends of the direct current bias power supply, and the output end is coupled with the grid electrode of the field effect transistor T2; in the process of the time constant circuit from access to reaching the time constant, the output voltage is gradually increased, the impedance of the field effect transistor T2 is controlled to be continuously reduced to a conducting state, and in the process, the voltage feedback is continuously changed along with the gradual reduction of the total resistance of the field effect transistor T2 and the microphone; and the change value of the voltage just meets the identification requirement of the system at a certain moment, so that the microphone is identified as the external microphone by the system.

Description

Technical Field

[0001] The utility model relates to the technical field of audio circuits, in particular to a microphone interface circuit. Background Art

[0002] The analog microphone ports on computers, tablets, and mobile phones automatically identify the type of external microphone, providing an operating voltage of approximately 1.8V and 1mA for condenser microphones. When a microphone is plugged in, the system automatically detects this voltage change and responds accordingly. However, different microphones often have different impedances. If the external microphone's impedance is mismatched, typically too low, the voltage change will be too large, causing the system to interpret it as a short circuit. This prevents the system from recognizing the external microphone and continues to use the internal microphone, rendering the external microphone unusable. Utility Model Content

[0003] The utility model aims to solve the problem that the existing microphone cannot be recognized by the system due to large voltage changes when connected to the device, and to provide a microphone interface circuit that can effectively identify low-impedance external microphones.

[0004] The technical solutions provided by this utility model are as follows:

[0005] A microphone interface circuit, comprising:

[0006] a field effect transistor T2, a drain of which is coupled to the positive electrode of a DC bias power supply for providing a bias voltage;

[0007] a microphone, the positive electrode of which is coupled to the source of the field effect transistor T2, and the negative electrode of which is coupled to the negative electrode of the DC bias power supply;

[0008] The time constant circuit has an input coupled to both ends of a DC bias power supply and an output coupled to the gate of field-effect transistor T2. From the time the DC bias power supply is connected to the time constant, the output voltage to the gate of field-effect transistor T2 gradually increases, thereby controlling the impedance of field-effect transistor T2 to gradually decrease until it reaches the on state.

[0009] When the microphone interface circuit is connected to a computer or other device, the time constant circuit receives the input voltage and outputs a gradually increasing voltage within the time constant, controlling the impedance of FET T2 to continuously decrease until it reaches the on-state. During this process, the system of the computer or other device recognizes the changing voltage. As the impedance of FET T2 continuously decreases until it reaches the on-state, the voltage feedback continuously changes as the total resistance of FET T2 and the microphone gradually decreases. At a certain moment, the voltage change value just meets the system recognition requirement, thus being recognized as an external microphone. After that, the impedance of FET T2 continues to decrease until the on-state impedance is negligible, thus having no impact on the microphone's operation.

[0010] Preferably, the time constant circuit includes a resistor R4 and a capacitor C3 connected in series, one end of the capacitor C3 is coupled to the negative electrode of the DC bias power supply, the other end is coupled to the gate of the field effect transistor T2 and one end of the resistor R4, and the other end of the resistor R4 is coupled to the positive electrode of the DC bias power supply.

[0011] The midpoint voltage of resistor R4 and capacitor C3 is supplied to the gate of field-effect transistor T2. When the microphone interface circuit is plugged into a computer or other device, capacitor C3 is charged through resistor R4, causing the midpoint voltage of resistor R4 and capacitor C3 to rise, causing field-effect transistor T2 to gradually turn on.

[0012] Preferably, the output impedance of the microphone is 800Ω to 3000Ω, the output voltage is 1-20mV@1KHz@1Pa, and the signal sensitivity is -30dB@1KHz@1Pa to -60dB@1KHz@1Pa.

[0013] Preferably, the DC bias power supply is provided by a built-in power supply of a computer, tablet or mobile phone, the internal resistance of the power supply is equivalent to 1800Ω, and the bias voltage is 1.8V@1mA.

[0014] Preferably, the field effect transistor T2 is an N-channel MOSFET low-resistance field effect transistor with a switching speed of nS level, a junction capacitance of pF level, and an Igs current of uA level.

[0015] Preferably, a squelch circuit is further included, which is coupled to both ends of the capacitor C3. The squelch circuit includes a mechanical switch SW1. The mechanical switch SW1 is closed to reduce or cut off the output voltage of the time constant circuit, thereby turning off the field effect transistor T2. The microphone has no operating voltage, thereby realizing the circuit squelch function.

[0016] Preferably, the noise suppression circuit further includes a resistor R5 connected in series with the mechanical switch SW1.

[0017] Preferably, the device further includes an electrostatic protection diode D2 coupled to both ends of the time constant circuit.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] After the microphone interface circuit is connected to a computer or other device, the time constant circuit receives the input voltage and outputs a gradually increasing voltage within the time constant, controlling the impedance of the field-effect transistor T2 to continuously decrease until it reaches the on-state. During this process, the system of the computer or other device recognizes the changing voltage, and the voltage feedback changes continuously as the total resistance of the field-effect transistor T2 and the microphone gradually decreases. At a certain moment, the voltage change value just meets the system recognition requirement, and the system recognizes it as an external microphone. After that, the impedance of the field-effect transistor T2 continues to decrease until the on-state impedance is negligible, thus having no effect on the working state of the microphone.

[0020] The mechanical switch SW1 is closed to reduce or cut off the output voltage of the time constant circuit, thereby turning off the field effect transistor T2. The microphone has no operating voltage, thereby realizing the circuit noise reduction function. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a circuit diagram of a microphone interface circuit according to a first embodiment of the present invention. DETAILED DESCRIPTION

[0022] The following further describes the specific embodiments of the present invention in conjunction with the accompanying drawings. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present invention and are not intended to limit the present invention.

[0023] Example 1, as Figure 1 As shown, a microphone interface circuit includes:

[0024] Field effect transistor T2, whose drain is coupled to the positive electrode of a DC bias power supply for providing a bias voltage. In this embodiment, field effect transistor T2 is an N-channel MOSFET low-resistance field effect transistor with a typical DS value of 1.6Ω@0.36A, a switching speed of the nanosecond level, a junction capacitance of the pF level, and an Igs current of the uA level;

[0025] a microphone MIC, having an anode coupled to the source of the field effect transistor T2 and a cathode electrically coupled to the negative electrode of the DC bias power supply;

[0026] The time constant circuit has an input coupled to both ends of a DC bias power supply and an output coupled to the gate of field-effect transistor T2. From the time the DC bias power supply is connected to the time constant, the output voltage to the gate of field-effect transistor T2 gradually increases, thereby controlling the impedance of field-effect transistor T2 to gradually decrease until it reaches the on state.

[0027] In this embodiment, the bias voltage provided by the computer / tablet / mobile phone to the microphone MIC is typically 1.8V, 1mA, and its power supply internal resistance is equivalent to 1800Ω. The microphone MIC is a capacitive microphone MIC, with an output impedance between 800Ω and 3000Ω, typically 2000Ω, and a typical signal sensitivity between -30dB@1kHz@1Pa and -60dB@1kHz@1Pa. The output voltage is between 1-20mV@1kHz@1Pa, which is superimposed on the microphone MIC's bias DC voltage, which is 1.8V@1mA.

[0028] When the microphone interface circuit is connected to a computer or other device, the time constant circuit receives the input voltage and outputs a gradually increasing voltage within the time constant, controlling the impedance of FET T2 to continuously decrease until it reaches the on-state. During this process, the system of the computer or other device recognizes the changing voltage. As the impedance of FET T2 continuously decreases until it reaches the on-state, the voltage feedback changes continuously as the total resistance of FET T2 and microphone MIC gradually decreases. At a certain moment, the voltage change reaches the system recognition requirement, thus being recognized as an external microphone. After that, the impedance of FET T2 continues to decrease until the on-state impedance is negligible, thus having no impact on the microphone's operation. The current driving FET T2's switch is in the uA range, meeting the micropower consumption requirement and not affecting the microphone's operating characteristics.

[0029] The time constant circuit includes a resistor R4 and a capacitor C3 connected in series. One end of the capacitor C3 is coupled to the negative terminal of the DC bias power supply, and the other end is coupled to the gate of the field effect transistor T2 and one end of the resistor R4. The other end of the resistor R4 is coupled to the positive terminal of the DC bias power supply.

[0030] The midpoint voltage between resistor R4 and capacitor C3 is supplied to the gate of field-effect transistor T2. When the microphone interface circuit is plugged into a computer or other device, capacitor C3 is charged via resistor R4, causing the midpoint voltage between resistors R4 and C3 to rise, gradually turning on field-effect transistor T2. In this embodiment, the time constant is 510mS. The charging time constants of R4 and C3 should be selected to match the switching characteristics of T2.

[0031] The conduction ramp-up time after power-on is determined based on the conduction-off curve of field-effect transistor T2. The time from field-effect transistor T2's cut-off to conduction to stabilization is greater than the hardware recognition time requirement of the external microphone interface of a computer or other device. This ensures that the voltage variation range that can be recognized is reached after the low-impedance microphone MIC is connected and before the field-effect transistor T2 stabilizes, ensuring that the low-impedance microphone MIC is effectively recognized and switched to the external microphone MIC device.

[0032] This embodiment also includes a noise reduction circuit, which is coupled to both ends of the capacitor C3. The noise reduction circuit includes a mechanical switch SW1 and a resistor R5 connected in series with the mechanical switch SW1. The resistance values of R4 and R5 are set according to calculation requirements. In this embodiment, the resistance value of R5 is 100KΩ, and the resistance value of R4 is 510KΩ. By closing the mechanical switch SW1, the output voltage of the time constant circuit is reduced, thereby turning off the field effect transistor T2. The microphone MIC has no operating voltage, thereby realizing the circuit noise reduction function.

[0033] In addition, the noise suppression circuit may also not be provided with the resistor R5 to cut off the output voltage of the time constant circuit as needed.

[0034] This embodiment further includes an electrostatic protection diode D2 coupled to both ends of the time constant circuit.

[0035] The preferred embodiments of the present invention are described in detail above in conjunction with the accompanying drawings. However, the present invention is not limited to the specific details of the above embodiments. Within the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the scope of protection of the present invention.

Claims

1. A microphone interface circuit, characterized in that: include: a field effect transistor T2, a drain of which is coupled to the positive electrode of a DC bias power supply for providing a bias voltage; a microphone, the positive electrode of which is coupled to the source of the field effect transistor T2, and the negative electrode of which is coupled to the negative electrode of the DC bias power supply; The time constant circuit has an input coupled to both ends of a DC bias power supply and an output coupled to the gate of field-effect transistor T2. From the time the DC bias power supply is connected to the time constant, the output voltage to the gate of field-effect transistor T2 gradually increases, thereby controlling the impedance of field-effect transistor T2 to gradually decrease until it reaches the on state.

2. The microphone interface circuit according to claim 1, wherein: The time constant circuit includes a resistor R4 and a capacitor C3 connected in series. One end of the capacitor C3 is coupled to the negative electrode of the DC bias power supply, and the other end is coupled to the gate of the field effect transistor T2 and one end of the resistor R4. The other end of the resistor R4 is coupled to the positive electrode of the DC bias power supply.

3. The microphone interface circuit according to claim 1, wherein: The output impedance of the microphone is 800Ω to 3000Ω, the output voltage is 1-20mV@1KHz@1Pa, and the signal sensitivity is -30dB@1KHz@1Pa to -60dB@1KHz@1Pa.

4. The microphone interface circuit according to any one of claims 1 to 3, wherein: The DC bias power supply is provided by a built-in power supply of a computer, tablet or mobile phone, the internal resistance of the power supply is equivalent to 1800Ω, and the bias voltage is 1.8V@1mA.

5. The microphone interface circuit according to any one of claims 1 to 3, characterized in that: The field effect tube T2 is an N-channel MOSFET low-resistance field effect tube with a switching speed of nS level, a junction capacitance of pF level, and an Igs current of uA level.

6. The microphone interface circuit according to claim 2, wherein: The device further includes a mute circuit coupled to both ends of the capacitor C3. The mute circuit includes a mechanical switch SW1. The output voltage of the time constant circuit is reduced or cut off by closing the mechanical switch SW1.

7. The microphone interface circuit according to claim 6, wherein: The noise suppression circuit further includes a resistor R5 connected in series with the mechanical switch SW1 .

8. The microphone interface circuit according to any one of claims 1 to 3, characterized in that: The device further includes an electrostatic protection diode D2 coupled to both ends of the time constant circuit.

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