Automatic Gain Control AGC

a system which automatically varies the amplification of a receiver of electrical frequencies with changes of signal voltage at the input. In radio receivers automatic gain control is sometimes referred to by its old name, automatic sound control, and in wire receivers by the term automatic level control. Automatic gain control is in use in radar and other pulse receivers where the peculiarities of working in a pulse mode must be taken into account.

In the majority of cases the signal voltage arriving at the input of the receiver changes appreciably. This is due to the difference in power of the transmitters and the difference in their distance from the location of the receiver, the fading of diffused signals, the abrupt change of distance, and reception conditions between a transmitter and a receiver mounted on moving objects, such as airplanes and automobiles. These changes lead to unacceptable oscillations or distortions of signals in the receiver. The automatic gain control is designed to significantly reduce variations in the output signal voltage of the receiver with respect to the input signal voltage. This is accomplished through circuits which transmit a rectified regulating voltage from the detector to the base of the transistors or to the control grids of variable-mu tubes, to amplifiers of high and medium frequency, and to the frequency converter; this decreases their amplification while increasing signal voltage at the input and vice versa. Thus compensation is made for variations in voltage of the input signals to the receiver.

In radio communications equipment three types of automatic gain control predominate—the simple gain control, the delayed gain control, and the amplified-delayed gain control. The operation of the automatic gain control may be represented graphically with respect to the amplitude characteristics of the receiver as shown in Figure 1.

Figure 1. Amplitude characteristics of radio receivers with various types of automatic gain control. The broken line indicates the voltage level of the sigral at the output at which distortions of received signals occur.

In the absence of automatic gain control the amplitude is represented by a straight line (A)—that is, the signal voltage at the output is directly proportional to the input voltage up to the level indicated by the broken line. As a result of the simple automatic gain control (C) only partial compensation for variation in voltage of the input signals takes place. The disadvantage of the simple automatic gain control is that there is a decrease in amplification of weak signals; this is eliminated by a delay in the start of operation of the automatic gain control. The delayed automatic gain control (B) acts in the same way as the simple one when the input signal voltage exceeds a certain level which is determined by the threshold voltage. The amplified-delayed automatic gain control with a direct current amplifier in the feedback circuit is used to gain much greater stability of the signal voltage at the receiver output (D). Delayed automatic gain control is most frequently used in receivers.