Microphone module and microphone device
A microphone module includes: a microphone element outputting a microphone signal representing collected sound; an amplifier circuit outputting an amplified signal obtained by amplifying a difference between the microphone signal and a reference voltage; a high-pass filter outputting a high-band amplified signal obtained by filtering the amplified signal; a buffer circuit outputting an audio signal obtained by buffering the high-band amplified signal; a first low-pass filter outputting a low-pass bias voltage obtained by filtering the audio signal superimposed on a DC power supply, and a separation circuit outputting an internal bias voltage obtained by removing the influence of an external circuit from the low-pass bias voltage. The second low-pass filter outputs the reference voltage obtained by filtering the internal bias voltage. A circuit from the high-pass filter to the amplifier circuit is set such that an open loop gain is smaller than 0 dB.
1. A microphone module comprising:
a microphone element outputting a microphone signal representing collected sound;
an amplifier circuit having a band-pass characteristic of amplifying a frequency component of at least equal to or higher than a first frequency and equal to or lower than a second frequency higher than the first frequency, the amplifier circuit outputting an amplified signal obtained by amplifying a difference between the microphone signal and a reference voltage using an operational amplifier;
a high-pass filter having a high-pass characteristic of causing attenuation in a low band and allowing passage in a high band, the high-pass filter outputting a high-band amplified signal obtained by high-pass filtering the amplified signal;
a buffer circuit buffering the high-band amplified signal and outputting the buffered high-band amplified signal as an audio signal;
a first low-pass filter having a low-pass characteristic of causing attenuation in a high band and allowing passage in a low band, the first low-pass filter outputting a low-pass bias voltage obtained by low-pass filtering the audio signal superimposed on a DC power supply;
a separation circuit receiving the low-pass bias voltage and outputting an internal bias voltage obtained by removing an influence of an external circuit from the low-pass bias voltage; and
a second low-pass filter having a low-pass characteristic of causing attenuation in a high band and allowing passage in a low band, the second low-pass filter outputting, as the reference voltage, a voltage obtained by dividing a voltage obtained by low-pass filtering the internal bias voltage by a predetermined resistance ratio, wherein
parameters of the high-pass filter, the first low-pass filter, the second low-pass filter, and the amplifier circuit are set such that an open loop gain in a range from the first frequency to the second frequency in a circuit from an input terminal of the high-pass filter to an output terminal of the amplifier circuit via the first low-pass filter and the second low-pass filter is smaller than 0 dB.
2. The microphone module according to claim 1 , wherein
the high-pass filter includes a resistor and a capacitor.
3. The microphone module according to claim 1 , wherein
the buffer circuit is an emitter-follower circuit using a bipolar transistor, and is supplied with, as a bias, an external bias voltage output from the DC power supply.
4. The microphone module according to claim 1 , wherein
the first low-pass filter includes a resistor and a capacitor.
5. The microphone module according to claim 1 , wherein
the separation circuit is a bipolar transistor having a base connected to an output terminal of the first low-pass filter, a collector to which the DC power supply is applied, and an emitter outputting the internal bias voltage.
6. The microphone module according to claim 1 , wherein
the second low-pass filter includes a resistor and a capacitor.
7. The microphone module according to claim 1 , wherein
the operational amplifier has an inverting input terminal to which the microphone signal is input, and a non-inverting input terminal to which the reference voltage is applied.
8. The microphone module according to claim 1 , wherein
the microphone element and the operational amplifier are driven by the internal bias voltage.
9. A microphone device comprising:
the microphone module according to claim 1 having a positive-side module output terminal outputting the audio signal on a positive side, and a negative-side module output terminal outputting the audio signal on a negative side;
a positive-side capacitor connected between the positive-side module output terminal and a positive-side audio output terminal;
a negative-side capacitor connected between the negative-side module output terminal and a negative-side audio output terminal;
a positive-side resistor connected between the positive-side module output terminal and a positive-side terminal of the DC power supply; and
a negative-side resistor connected between the negative-side module output terminal and a negative-side terminal of the DC power supply.
10. A microphone device comprising:
the microphone module according to claim 1 having a positive-side module output terminal outputting the audio signal;
a positive-side capacitor connected between the positive-side module output terminal and a positive-side audio output terminal; and
a positive-side resistor connected between the positive-side module output terminal and a positive-side terminal of the DC power supply.