Bias considerations in d.c. transistor amplifiers

General expressions for gain and thermal drift in a d.c. transistor amplifier are discussed. Variations of the transistor parameters due to temperature are shown, and permissible simplifications in the general expressions are introduced. A figure of merit¿termed drift/gain ratio¿is defined, which permits comparison of differentcircuits and transistors with respect to their thermal behaviour in d.c. amplifiers. It is shown that the desired condition is obtained whenthis ratio becomes a minimum, while a certain necessary gain, chosenby the designer, is maintained. Successively the common-collector, common-emitter and common-base configurations are analysed forsingle stages. Plots computed for particular transistor parametervalues aid the reader in arriving at an optimum design. Multi-stage amplifiers are discussed, using the drift/gain ratio toanalyse direct-coupled feedback amplifiers. After a detailed considerationof a 2- and 3-stage direct-coupled amplifier, experimentalverification of some of the results concludes the paper.