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Q1 and Q2 form a differential pair, with common-emitter amplifier Q3 amplifying its output. R6 is Q3’s collector load resistor, and push–pull pair Q4 and Q5 form the output emitter follower. The output voltage is sampled by the feedback network consisting of voltage divider R4 and R5, with C2 included to reduce the gain to unity at dc for stable biasing. R3 sets the quiescent current in the differential pair, and since overall feedback guarantees that the quiescent output voltage is at ground, Q3’s quiescent current is easily seen to be 10mA (VEE across R6, approximately). The diodes bias the push–pull pair into conduction, leaving one diode drop across the series pair R7 and R8, i.e., 60mA quiescent current. That’s class AB operation, good for minimizing crossover distortion, at the cost of 1 watt standby dissipation in each output transistor.
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