Abstract: The invention provides a microphone amplifier circuit which enhances the SNR (signal noise ratio) and expands the dynamic range by reducing the noise level. A microphone amplifier circuit includes a preamplifier which amplifies an audio signal from a capacitor microphone, a level detection circuit which outputs a level detection signal when the level of the audio signal is in the vicinity of the noise level of the microphone amplifier circuit, and an attenuator which attenuates the level of the audio signal outputted from the preamplifier in response to the level detection signal. The preamplifier includes an operational amplifier, a feedback capacitor and a feedback resistor.

Abstract: The invention provides a microphone amplifier circuit which enhances the SNR (signal noise ratio) and expands the dynamic range by reducing the noise level. A microphone amplifier circuit includes a preamplifier which amplifies an audio signal from a capacitor microphone, a level detection circuit which outputs a level detection signal when the level of the audio signal is in the vicinity of the noise level of the microphone amplifier circuit, and an attenuator which attenuates the level of the audio signal outputted from the preamplifier in response to the level detection signal. The preamplifier includes an operational amplifier, a feedback capacitor and a feedback resistor.

Abstract: The invention provides an amplifier circuit of a capacitor microphone of which the noise resistance against noise of a supply voltage is enhanced. In an amplifier circuit of a capacitor microphone of the invention, while a noise component of a supply voltage is applied to one inversion input terminal of an operational amplifier of an amplification portion through a parasitic capacitor existing between an external power supply wiring and an external wiring that are adjacent to each other, the problem noise component of the supply voltage is applied to the other non-inversion input terminal by capacitive coupling to an internal power supply wiring. Therefore, the noise component is cancelled at the operational amplifier.

Abstract: To efficiently obtain two outputs including one at a normal level and the other at an excessive level. An input signal input to the negative input terminal of an operational amplifier (14) having a negative feedback path is amplified to output an output signal. Signal combining units (18, 20, 22) are provided for adding in a weighted manner a negative input terminal side signal obtained by combining the input signal input to the negative input terminal of the operational amplifier (14) and a feedback signal from the negative feedback path and the output signal from the operational amplifier to output a combined signal, so that two signals, namely the output signal from the operational amplifier (14) and the combined signal, are obtained.

Abstract: To efficiently obtain two outputs including one at a normal level and the other at an excessive level. An input signal input to the negative input terminal of an operational amplifier (14) having a negative feedback path is amplified to output an output signal. Signal combining units (18, 20, 22) are provided for adding in a weighted manner a negative input terminal side signal obtained by combining the input signal input to the negative input terminal of the operational amplifier (14) and a feedback signal from the negative feedback path and the output signal from the operational amplifier to output a combined signal, so that two signals, namely the output signal from the operational amplifier (14) and the combined signal, are obtained.

Abstract: The invention provides an amplifier circuit of a capacitor microphone of which the noise resistance against noise of a supply voltage is enhanced. In an amplifier circuit of a capacitor microphone of the invention, while a noise component of a supply voltage is applied to one inversion input terminal of an operational amplifier of an amplification portion through a parasitic capacitor existing between an external power supply wiring and an external wiring that are adjacent to each other, the problem noise component of the supply voltage is applied to the other non-inversion input terminal by capacitive coupling to an internal power supply wiring. Therefore, the noise component is cancelled at the operational amplifier.

Abstract: A gradient detecting unit detects the gradient in the output of a driven circuit. An offset voltage generating unit generates an offset voltage in response to an output of a driven circuit as well as the gradient detected by the gradient detecting unit. The gradient in the output of the driven circuit is increased as the change thereof is more abrupt and decreased as the change thereof is more gentle. For example, if the detected gradient is added to the normal offset voltage to form an offset voltage, the offset voltage can follow the changes in the output of the driven circuit to supply a proper supply voltage to the driven circuit.