Automatic Gain Control of Transistor Amplifiers

Since transistor small signal parameters are functions of the dc emitter current (Ie) and of the dc collector voltage (Vc), gain control can be achieved by varying either Ie or Vc. The gain decreases with decreasing 1e or Vc. Using the series-parallel representation, the parameter most sensitive to Ie-variations is h11, whereas Vc-variations affect h12 and h22 considerably. In common emitter configuration changes of h21 are also important. A study of the dependance of the hii on the dc operating point explains the nature of gain variations with Ie and Vc. Satisfactory AGC circuits have been built using either Ie- or Vc-control. The control power required is very small if Ie or Vc are controlled indirectly by varying the base current. Since Ie or Vc are decreased considerably in the presence of strong input signals, the problem of distortion must be given serious consideration. Due to the variation of transistor driving point impedances, AGC may result in changes of the bandpass characteristic of tuned amplifiers. The gain of transistor converters and oscillator-converters can be controlled by conventional or special techniques.