







Automatic Gain Control PreAmplifier Circuit Diagram

The preamp circuit uses an easily obtained 741 op amp set for an internal gain about 200.

PARTS LIST R1 100kΩ R2 470Ω R3 10kΩ R4 10kΩ R5 560kΩ R6 100kΩ R7 10kΩ R8 10kΩ PRESET R9 100kΩ C1 1µF 16V C2 1µF 16V C3 10µF 16V C4 1µF 16V C5 0.1µF (104) C6 10µF 16V C7 0.22µF (224) D1 1N4148 D2 1N4148 Q1 BF245 IC1 LM741

A portion of the op amp’s output signal is rectified by the 1N4148 diodes, then filtered and fed to the gate of the FET input shunting circuit. As the output rises, more and more input shunting takes place. That is, more of the input signal is bypassed, effectively keeping the output level constant.

This preamp circuit offers a 100:1 limiting action. The input level can change over a 100:1 ratio with little or no effect on the output level.

The output output level itself can be set from less than unity all the way up to nearly the gain of the amplifier, making the circuit usable in other applications as well.









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