Audio AGC amplifier circuit schematic
As shown in the figure is an audio AGC amplifier circuit. The dynamic range of this amplifier is greater than 50dB. The distortion of the output waveform is very small, and it has the advantages of fast startup and slow attenuation. Circuit input 40 ~ 20mV and output 0 ~ 1.2V adjustable level, single power supply, power consumption current is less than 1mA (5V voltage). VT2 and VT4 are P-channel JEFT. It forms an equivalent resistance voltage divider with R2, R3 and R4 (R10, R11 and R12), which is connected to the input circuit. When the input level is lower than 40mV (peak-to-peak value), the input is equally distributed between R2 and R3, R4, and the output amplitude of A1 is not large enough to turn on the positive peak detector VT2. Pull the gate of the junction field effect transistor to + 5V to pinch off its channel and produce a very high resistance from the drain to the source. When the peak-to-peak value of the input voltage is higher than 40mV, VT1 turns on at the positive peak of A1 output, reducing the gate-to-source voltage of the junction field effect transistor. The channel resistance is used to reduce and attenuate the input signal to keep the peak-to-peak value of the Al output voltage at about 1.2V. 
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