Analysis of the behaviour of thermally coupled transistors in a differential amplifier

Transistors are frequently used in pairs (e.g. in push-pull circuits, differential amplifiers, d.c. amplifiers etc.), and when so used may be mounted on a common heat sink or contained in the same encapsulation. Under certain circumstances, thermal instability can occur in circuits of this kind whereby the standing currents flowing through the two transistors are grossly unequal, despite otherwise completely balanced circuits. The effect with thermally isolated transistors has been described by the author and explained in terms of empirical operational equations describing transistor behaviour. In this paper, the explanation is extended to include the effect of thermal coupling between the transistors, and a fairly exact analysis of a simple configuration yields results which accord well with the observed effects.