This paper reviews a progression of circuits used for protecting bipolar power transistors in the output stages of audio power amplifiers. Design oriented methods of determining the protection locus are shown in a mathematical and graphical procedure. The circuits are then expanded from their standard configurations to allow for transient excursion beyond steady state limits, and thermally dependent protection limits, to better match the protection limits to the actual output stage capability. This allows the protection scheme to prevent output stage failure in the least restrictive way. A new method is shown for achieving a junction temperature estimation system without the use of a multiplier.
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