Automatic gain control

Automatic gain control (AGC), is a closed-loop feedback regulating circuit in an amplifier or chain of amplifiers, the purpose of which is to maintain a suitable signal amplitude at its output, despite variation of the signal amplitude at the input. The average or peak output signal level is used to dynamically adjust the gain of the amplifiers, enabling the circuit to work satisfactorily with a greater range of input signal levels. It is used in most radio receivers to equalize the average volume (loudness) of different radio stations due to differences in received signal strength, as well as variations in a single station's radio signal due to fading. Without AGC the sound emitted from an AM radio receiver would vary to an extreme extent from a weak to a strong signal; the AGC effectively reduces the volume if the signal is strong and raises it when it is weaker. In a typical receiver the AGC feedback control signal is usually taken from the detector stage and applied to control the gain of the IF or RF amplifier stages. Automatic gain control (AGC), is a closed-loop feedback regulating circuit in an amplifier or chain of amplifiers, the purpose of which is to maintain a suitable signal amplitude at its output, despite variation of the signal amplitude at the input. The average or peak output signal level is used to dynamically adjust the gain of the amplifiers, enabling the circuit to work satisfactorily with a greater range of input signal levels. It is used in most radio receivers to equalize the average volume (loudness) of different radio stations due to differences in received signal strength, as well as variations in a single station's radio signal due to fading. Without AGC the sound emitted from an AM radio receiver would vary to an extreme extent from a weak to a strong signal; the AGC effectively reduces the volume if the signal is strong and raises it when it is weaker. In a typical receiver the AGC feedback control signal is usually taken from the detector stage and applied to control the gain of the IF or RF amplifier stages. The signal to be gain controlled (the detector output in a radio) goes to a diode & capacitor, which produce a peak-following DC voltage. This is fed to the RF gain blocks to alter their bias, thus altering their gain. Traditionally all the gain-controlled stages came before the signal detection, but it is also possible to improve gain control by adding a gain-controlled stage after signal detection. In 1925, Harold Alden Wheeler invented automatic volume control (AVC) and obtained a patent. Karl Küpfmüller published an analysis of AGC systems in 1928. By the early 1930s most new commercial broadcast receivers included automatic volume control. AGC is a departure from linearity in AM radio receivers. Without AGC, an AM radio would have a linear relationship between the signal amplitude and the sound waveform – the sound amplitude, which correlates with loudness, is proportional to the radio signal amplitude, because the information content of the signal is carried by the changes of amplitude of the carrier wave. If the circuit were not fairly linear, the modulated signal could not be recovered with reasonable fidelity. However, the strength of the signal received will vary widely, depending on the power and distance of the transmitter, and signal path attenuation. The AGC circuit keeps the receiver's output level from fluctuating too much by detecting the overall strength of the signal and automatically adjusting the gain of the receiver to maintain the output level within an acceptable range. For a very weak signal, the AGC operates the receiver at maximum gain; as the signal increases, the AGC reduces the gain. It is usually disadvantageous to reduce the gain of the RF front end of the receiver on weaker signals as low gain can worsen signal-to-noise ratio and blocking; therefore, many designs reduce gain only for stronger signals. Since the AM detector diode produces a DC voltage proportional to signal strength, this voltage can be fed back to earlier stages of the receiver to reduce gain. A filter network is required so that the audio components of the signal don't appreciably influence gain; this prevents 'modulation rise' which increases the effective modulation depth of the signal, distorting the sound. Communications receivers may have more complex AVC systems, including extra amplification stages, separate AGC detector diodes, different time constants for broadcast and shortwave bands, and application of different levels of AGC voltage to different stages of the receiver to prevent distortion and cross-modulation. Design of the AVC system has a great effect on the usability of the receiver, tuning characteristics, audio fidelity, and behavior on overload and strong signals.