I am teaching myself electronics in my spare time and I am trying to look at circuits and understand how they work. I am currently studying amplification and transistor basics, and I have started analysing the audio amplfication circuit below.
In particular, I am trying to figure out how exactly the darlington pair Q7 and Q9 associated with Q5 work: this is all quite confusing to me at the moment. The circuit description reads: 'the predriver stage (Q7, Q9) is a Darlington connection the load of which employs a constant current source (D5, Q5), resulting in a high voltage gain.'
Am I correct with the following?
- constant current source: Q5 acts as a constant current source for the darlington pair. As such, if there were an increase in current at the collector of Q5, the emitter-base forward bias of Q5 would reduce (from the higher voltage drop across the emitter resistor of Q5), and the resulting lower Ib would reduce Ic and Ie. D5 maintains forward bias between the emitter and the base.
- Darlington pair: this darlington pair is configured as a common emitter amplifier with Q5 as its load. With an AC input signal at its base, the positive portion of the signal increases the forward bias of Q7 and Q9, which would increase the collector currents. Since current is constant, what happens here, a smaller Vce drop for Q9 as the collector of Q9 gets closer to -B2? If that is the case, assuming that with constant current the voltage drops across R31 and R29 should remain constant, is also Vc of Q5 getting closer to -B2? Am I correct to deduce that this is a phase-inverting configuration?
I would appreciate if you could help me clarify these questions. Thanks.