What is a BJT Class A Output Stage?
Actually, it is an output stage that is quite common. It is called the common collector amplifier when you are learning basic electronics. Its voltage gain is around .9 to .999 and the current gain is generally greater than 1, more like 100-1000. The current flowing through the output transistor always stays above cut off.
It can consist of a transistor with the collector connected to a power supply and the emitter connected to a resistor which is then connected to ground (or a negative supply). This would be the simplest configuration I can think of. You can get more efficiency from this circuit buy using an inductor instead of a resistor and more efficient yet by using a constant current source (CCS) instead of an inductor or resistor. All the mentioned circuits would be considered singled ended. The greatest efficiency achieved by these amps is ~25% and that would come from the amplifier with the constant current source.
The next step up in efficiency would be to modulate the constant current source or move to a push pull configuration. Push pull has an NPN transistor with its collector connected to a positive supply, then its emitter is connected to the emitter of a complimentary transistor (PNP) with its collector tied to the negative supply. The greatest efficiency achieved with this push pull output stage is ~ 50%.
Check these out:
1.
https://www.dself.dsl.pipex.com/ampins/discrete/ef.htm
Simple output stage with a resistor, one with a ccs, and one with what I would call modulated ccs and he calls push-pull
2.
https://sound.westhost.com/project3b.htm
Would be what I would call a push pull class a
3.
https://www.tcaas.btinternet.co.uk/
John Linsley Hood amp with some other class a's
4.
https://www.passdiy.com/
