Automatic gain control of transistor amplifiers

Due to the dependence of transistor small signal parameters on the dc operating point, the gain of a transistor amplifier is function of the emitter current Ie and of the collector voltage Vc. An analysis of the variation of the series-parallel transistor parameters hij with the operating point shows that the gain decreases with decreasing Ie or decreasing Vc in the region of small values of Ie and Vc. The appreciable control power necessary to vary Ie or Vc (of possibly several amplifying stages) can be obtained advantageously by operating the controlled stages as dc amplifiers of the control signal and employing transistor detectors. Automatic gain control systems applicable to linear amplifiers and converters using Ie and Vc control have been developed, and their performance is described in this paper. Some problems arising in-transistor AGC systems are distortion, detuning and bandwidth variation.