The Periodic Variation of Junction Temperature in a Transistor

The instantaneous junction temperature of a transistor used as a class B amplifier is considered from very low frequencies, when the temperature is shown to follow a law similar to that of the instantaneous dissipation at the junction, to high frequencies, when the junction temperature attains a mean value which is constant throughout the cycle. An expression is derived for the instantaneous junction temperature of such a transistor, computed results illustrating the conditions of maximum dissipation in the transistor, and of maximum drive. It is shown that under conditions of maximum dissipation the peak temperature attained by the junction above that of its mount is, at low frequencies, 2.48 times that at high frequencies, while under conditions of maximum drive the ratio reaches 2.97 for a practical transistor; this higher value is due to the lower mean dissipation under these conditions. The instantaneous junction temperature of a transistor type OC22 is measured when operated under conditions of maximum drive, close agreement being obtained with the predicted results.