I'm studying AOE but i can't figure out how this circuit even works(also i'm confused if Q1 and Q2 make a sziklai pair). any help with the circuit description will be great.
I'm studying AOE but i can't figure out how this circuit even works
If the 10 volt output rises a little bit then Q1's emitter is also raised a little bit and this has the effect of turning Q1 off a little bit. As Q1 turns off a little bit, its collector voltage rises. This has the effect of slightly shutting-down Q2 and therefore the slightly risen 10 volt output falls back to 10 volts.
It's negative feedback.
Overall feedback is between R5 (10K) and R4 (1k), which yields an overall voltage gain near ten. And because feedback is to the emitter of Q1, input impedance becomes quite high. Like FakeMoustache, I've never heard of "Sziklai pair", and have "invented" this circuit for my own use. Kudo's to Sziklai for publishing first.
Q1 converts its base input voltage to current. It has less-than-one voltage gain, but a portion of its collector current change feeds into the base of Q2. In this case, the base of Q2 presents a dynamic load resistance to Q1 of about 26 ohms times its Beta of about 100 (because its emitter current is about one milliamp). So dynamic input resistance of Q2 is near 2600 ohms. That's in parallel with R3.
Open-loop gain is quite high, compared to closed-loop gain of ten. So Andy's approach is a good one. Here's a slightly different approach..consider what happens if you raise the input source a small amount (say 0.1v) from 2.6v to 2.7v. If Q1's emitter stayed at 2.0 v, Q1 would conduct very much more current, pulling its collector voltage down. But it must also pull down Q2's base voltage too. That will increase Q2's base current a great deal, which multiplies by perhaps one-hundred at Q2's collector. That increase in Q2's collector current must flow through R4 and R5, pulling up output voltage.
How high will output voltage rise? It will rise until the voltage drop across R4 nearly counteracts the initial disturbance of 0.1v. Because R5 is ten times larger (and carries the same current change as R4), output voltage must rise nearly one volt. Voltage gain is nearly ten, since a 0.1v input change causes a one volt output change (slightly less than one volt). I'm assuming that output is taken from Q2's collector.