# Understanding constant current source in amplifier stage

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.

• Well, you are correct. The main purpose of using a constant current source as a load (active load) for the CE amplifier is to increases the stage voltage gain. Because in this amplifier topology almost all voltage gain is provided by a VAS stage. The gain will be high because the smallest difference in collector current between the active load (Q5) and CE stage will give a large voltage swing. A transistor which tries to make the lowest current will win the fight.
– G36
Oct 24, 2020 at 14:56
• And some interesting post 1... electronics.stackexchange.com/questions/336130/… 2... electronics.stackexchange.com/questions/503365/… 3...electronics.stackexchange.com/questions/333502/…
– G36
Oct 24, 2020 at 14:58
• What are "N.S.A circuit" abbreviations? Jun 7, 2022 at 8:07
• @Antonio51 N.S.A -- Non Switching Amplifier, in short, a biasing network that makes that the output transistor never turns off. cieri.net/Documenti/Altri%20marchi/…
– G36
Jun 7, 2022 at 15:07
• Ok. Very interesting pieces of information and circuits. Never heard of these although I have used them, or something like them, but a "little" simpler! I think these were used when amplifiers were made with "old" transistors (Ge). Jun 7, 2022 at 18:27