This microphone preamplifier incorporates automatic gain control, which keeps the output level fairly constant over a wide range of input levels. The circuit is especially suitable for driving the modulator of a radio transmitter and allows a high average modulation index to be achieved.

It may also be used in P.A. systems and intercoms to provide greater intelligibility and compensate for variations between speakers (the users of those devices).

The actual signal amplifier stage is Q2, which operates in common emitter mode, the output signal being taken from its collector. A portions of the output signal is fed through emitter follower Q3 to a peak rectifier comprising D1/D2 and C4. The voltage on C4 is used to control the base current of Q1, which forms part of  the input attenuator. At low signal levels the voltage on C4 is small and Q1 draws little current. As the input signal level increases the voltage on C4 rises and Q1 turns on mode, thus attenuating the input signal. The net result is that as the input signal increases it is subject to a greater and greater degree of attenuation and the output signal therefore remains fairly constant for a wide range of input levels. The circuit is suitable for signals with a peak input level up to 1 volt. The microphone may be  replaced by a small loud-speaker for intercom use.

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