Published:2012/9/10 21:06:00 Author:Ecco | Keyword: Auto gain control, op-amp | From:SeekIC
An audio signal applied to VI is passed through the operational amplifier 741, U2. After being amplified, the output signal V2 is sampled and applied to a negative voltage doubler / rectifier circuit composed of diodes CRI and CR2, with the capacitor C1. The resulting negative voltage is used as a control voltage which is applied to the door] 2N5485 FET Q1. Capacitor C2 and resistor R2 form a filter for smoothing the voltage rectified audio control. The lFET is connected between pin 2 of the MC3340P grounded by a resistor of 1 kohm.
Source: discovercircuits
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