Amplifier circuit of capacitor microphone
View Patent ↗In an amplifier circuit of a capacitor microphone, when a too high input signal from the capacitor microphone is inputted, the levels of output signals of the amplifier circuit are limited. A first feedback capacitor of an operational amplifier is formed using a changeable capacitance type MOS capacitor element, and has a characteristic of increasing the capacitance value CAf 1 according to the amplitude of an input signal generated by a capacitor increases. Therefore, CAf (=CAf 1+ CAF 2 ) increases according to the amplitude of the input signal increases, and accordingly the gain of the operational amplifier decreases, thereby limiting the output signals of the operational amplifier. This realizes the appropriate limitation of the output signals of the operational amplifier, even when the amplitude of the input signal becomes too high.
1. An amplifier circuit of a capacitor microphone, comprising:
a capacitor microphone generating an input signal corresponding to a sound received by the capacitor microphone;
an operational amplifier comprising an input terminal and an output terminal, the input signal from the capacitor microphone being applied to the input terminal;
a first feedback capacitor connected between the output terminal and the input terminal of the operational amplifier and configured to increase a capacitance in response to an increase in an amplitude of the input signal; and
a feedback resistor connected between the output terminal and the input terminal of the operational amplifier,
wherein the first feedback capacitor comprises a first MOS transistor comprising a first gate, a first back gate, a first source and a first drain and a second MOS transistor comprising a second gate, a second back gate, a second source and a second drain, the first gate is connected to the second back gate, and the second gate is connected to the first back gate.
2. The amplifier circuit of claim 1 , wherein the first back gate is connected to the first source and the first drain, and the second back gate is connected to the second source and the second drain.
3. The amplifier circuit of claim 1 , further comprising an AD converter converting an output signal of the operational amplifier to a digital signal.
4. An amplifier circuit of a capacitor microphone, comprising:
a capacitor microphone generating an input signal corresponding to a sound received by the capacitor microphone;
an operational amplifier comprising an input terminal and an output terminal, the input signal from the capacitor microphone being applied to the input terminal;
a first feedback capacitor connected between the output terminal and the input terminal of the operational amplifier and configured to increase a capacitance in response to an increase in an amplitude of the input signal;
a feedback resistor connected between the output terminal and the input terminal of the operational amplifier; and
a second feedback capacitor connected between the output terminal and the input terminal of the operational amplifier, a capacitance of the second feedback capacitor being less dependent on the input signal than the capacitance of the first feedback capacitor.
5. The amplifier circuit of claim 4 , wherein
the first feedback capacitor comprises a first MOS transistor comprising a first gate, a first back gate, a first source and a first drain and a second MOS transistor comprising a second gate, a second back gate, a second source and a second drain, the first gate is connected to the second back gate, and the second gate is connected to the first back gate,
the second feedback capacitor comprises first and second polycrystalline semiconductor layers and a capacitor insulation layer disposed between the first polycrystalline semiconductor layer and the second polycrystalline semiconductor layer, and
impurity concentrations of the first and second polycrystalline semiconductor layers are higher than impurity concentrations of the first and second back gates.
6. The amplifier circuit of a capacitor microphone, comprising:
a capacitor microphone generating an input signal corresponding to a sound received by the capacitor microphone;
an operational amplifier comprising an input terminal and an output terminal, the input signal from the capacitor microphone being applied to the input terminal;
a first feedback capacitor connected between the output terminal and the input terminal of the operational amplifier and configured to increase a capacitance in response to an increase in an amplitude of the input signal; and
a feedback resistor connected between the output terminal and the input terminal of the operational amplifier,
wherein a C-V characteristic curve of the first feedback capacitor is symmetric with respect to a center voltage of the characteristic curve.