Gain Controlled Amplifier

This single-chip circuit adjusts it's audio gain according to the ambient noise picked up by the microphone. When operating in a quiet environment, the audio output is quiet, while a noisy environment results in a louder audio output. Audio to pin 13 is amplified by the variable-gain amplifier within the LM1894 IC. Audio from the microphone connected through 0.1-mF capacitor to pin 6 controls the audio gain of the variable-gain amplifier. The output appears on pin 11 and is taken off through an 0.1-mF capacitor.
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Gain Controlled Amplifier - Circuitos de Electronica