I'm studying electrical engineering and we just finished a project where we had to build a power amplifier. But I think I can do better than what we did.
I read Distortion In Power Amplifiers where Douglas Self discusses several VAS stages and their benefits over the basic CEC stage.
In his results, circuit C and D perform best, apparently by increasing local negative feedback through Cdom.
As I understand it, Cdom is very small (pF) and serves to make the circuit dominantly first order by filtering very high frequencies. Isn't it essentially an open circuit for audio frequencies? The only advantage of C and D seems to be a possibly improved input impedance.
I tried simulating his circuits in LTspice (though all of the biasing circuitry is not included in his schematics), but both the beta-enhanced and cascode don't work very well.
Can anyone explain how these circuits work and what I'm doing wrong?