Class-A and Class-AB Amplifier
This circuit developed out of my 30+ years of JLH class-A based investigations.
The original ’simple’ 1969 JLH class-A design provided excellent first cycle accuracy through mid and high frequencies (dynamic clarity) because there were no stabilisation components nor a series output choke, whilst the NFB error correction was established via the input emitter (some describe this as current feedback).
However it did generate rather a lot of heat, the damping was ’soft’ at lower frequencies and the positive slew was weaker in the presence of loudspeaker system generated back-EMF at higher audio frequencies. I was eventually able to improve upon the original JLH circuit by;-
1) adding a ‘helper’ class-AB output stage to substantially increase power and efficiency, but leave sufficient class-A bias for real-time control maintenance through phase shifted zero current class-AB crossovers;
2) adding a differential input stage for zero output voltage control, but also fit a 10nF base-emitter capacitor for differential voltage operation at audio frequencies, though with leading emitter routed feedback to maintain circuit stability and NFB control above 20kHz;
3) adding a current mirror to obviate power-up thump, but also fit a 220nF base-emitter capacitor to ensure stable ’source only’ operation above audio frequencies. [....]
