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One thing I'll tell you the diode placed next to the transistors is not a varactor.

A varactor is used for frequency control. by the time the signal get to the final transistors that would be too late.


Usually a semiconductor placed on or very near a power transistor with or without heat sink grease is being used for temperature compensation.

Bipolar transistors have a negative temperature coefficient. When they get hot they conduct more current and then get even hotter and then more current. Thermal Runaway.

 

Increasing temperature in Figure below (a) will decrease VBE from the nominal 0.7V for silicon transistors. Decreasing VBE increases collector current in a common-emitter amplifier, further shifting the bias point. The cure for shifting VBE is a pair of transistors configured as a differential amplifier. If both transistors in Figurebelow (b) are at the same temperature, the VBE will track with changing temperature and cancel.


https://www.allaboutcircuits.com/textbook/semiconductors/chpt-4/bjt-quirks/


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