15. AGC-AUTOMATIC GAIN CONTROL

1. Purpose

This is the time when we can easily understand a special circuit which was not mentioned up to here. Its purpose is to cancel effects of the environment of a radio-receiver which can cause very intensive changes in reception of the wanted signal. Such a change of reception would cause a very strong change of the output-signal which means: a considerable change of sound volume. If there is not done anything about it, the receiver gives a reasonable sound at one moment a moment later the sound can be very weak or get unacceptable loud as a consequence of change of reception.

2. Function of the AGC-circuit

In order to stabilize the output signal we need an electric signal which presents the strength of the reception in order to use it for an automatical stabilization of the output-signal. The needed signal can be derived from the detector input signal. If we have a closer look to that signal we find, that this input signal has two components:

- the first part is the AF-component which normally does not change its amplitude during changes of reception strength.

- the second part is the dc-component - the distance of the AF-sinewave from the noughtline. This second part is the one which gives us an information about the reception strength (as represented in fig. 188).

fig. 188

The wanted dc-signal can be produced by means of an RC-combination across the volume-control-potentiometer.

The voltage desired can be measured across the capacitor of that RC-combination, it is called Automatic-Gain-Control-voltage (AGC). It will be fed back to the biasing of the first IF-stage. If the AGC-voltage is growing it will decrease the amplification in that 1. IF-stage.

fig. 189