Parasitic noise after boosting opamp output current
If you bias the two output BJTs using diodes, for example, you will avoid the cross-over distortion. That is, when the sign of the output signal reverses, one BJT goes off, but the other is not &quot;quick&quot; enough to go on completely. In other words you have their approx. 0.7 V of Vbe to provide above the zero crossing between positive and negative half cycle.
With biasing of the BJT bases (e.g. a 0.7V each give by the voltage drop across two diodes) you will have a bias current flowing &quot;vertically&quot; along the two BJTs (Q2 collector, Q2 emitter, Q1 emitter, Q2 collector). This current can be adjusted ad controlled by putting two small resistors in series with the emitters. Such resistors (named of negative feedback or degeneration) gives you 2 benefits: linearization of the circuit and less variations with temperature.



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