[Craig Sawyers]

I'm not sure why transistor hFE is important.

The first stage is an emitter follower, whose gain is hfe/(1+hfe). Note that most transistor testers measure static hFE at a particular current, but the gain of an emitter follower is dependent on the incremental hfe. These are not so different, but even if the current gain is only 100, the gain of the emitter follower is 0.99. The native input resistance is quite high, around 500k, but it is actually dominated by the parallel combination of the base bias resistors to 10k.

VT2 gain is set by RC/RE to 3x

VT3 is set by RC/re because the emitter resistor is bypassed; re = 25/IE with IE in mA. IE in this case is 0.3mA, so re ~ 80 ohms so the stage gain is ~ 275.

The overall gain of the amp is ~ 275x3 = 825, independent of transistor gain.

Then the actual gain is set by feedback from the voltage across the meter flowing via C8 and C9 into the 200 ohm resistor R14, fed back into the emitter of VT2. Looking into the emitter the impedance is not the 3.3k resistor R9, but our old friend re (in parallel with R9), which for VT2 is 40 ohms. So the overall gain is (I think I have this right) 200/40 = 5x.

But whatever - the native gain and fed back gain is hardly dependent on transisor hfe at all. HFE mainly impacts the base voltage (and hence emitter current) a little bit because the base takes a bit of current from the potential divider that sets the operating current. But that is a small effect for any reasonable current gain

So I'm not certain at all why AVO specified gain bands for the three transistors.

Craig