A brief review of the constraints and stresses on the output stage transistors of Class-B and Class-H power amplifiers is presented. The cases considered are simple resistive loading, first-order reactive loading, and complex reactive loading by a real loudspeaker or electrical equivalent model. Several simple relations are presented to aid in the thermal design of the output stage. A way of obtaining junction temperature versus time for a complex power waveform is introduced. A computer program is introduced as a design tool and tutorial aid, providing graphical and numerical output not as easily obtainable otherwise.
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