My definition is class A both devices conduct complementarily all the time, AB one of the devices does not pass current for a fraction of the time, B is each device conducts strictly alternatively (which is not a real possibility).
Yep, and I will admit that you're probably in the majority as far as what we call them
I'm not familiar with this. Turn on the light, please.
I suspect you’re more than familiar with the theory involved
But you want others to be on the same page too, which is fair enough and what this forum is about. So:
When we talk about classes of amplifier, we’re really talking about the conductance angle of the output transistors. Or tubes for that matter.
In a class A amplifier, both output transistors are continuously conducting throughout the full cycle of audio signal and we say that the conductance angle is 360 degrees (all of the cycle)
In the other type of amplifier we’re discussing here, each output transistor conducts for about 50% of the audio signal cycle and we say that the conductance angle is 180 degrees (half the cycle).
Our amplifier is like a relay race with the output transistors being two runners in that race, and the audio signal being the batton that’s usually passed between the runners at an appropriate point.
If both runners were running around the track together, each runner simultaneously holding one end of the batton, then this is analogous to the class A amplifier. It gets the job done, but isn’t efficient in terms of energy usage.
If we have a well trained pair of runners, each runner carrying the batton for half the time, and there is a well executed hand off of the batton between the two, then this is analogous to what I am calling a class B amplifier.
I do believe it’s an issue of semantics as to whether this should be called AB rather than B.
A quick poll of a few books on amplifier design and I see I’m in in a minority, but am in agreement wth Doug Self in calling this class B. On the other hand, Bob Cordell calls it AB but admits that semantics are involved.
Most other folks call it AB with the exeption of Barney Oliver whose paper from the 1970’s explicitly talks about this class of amplifiers as B.
On the optimum bias for our non class A amplifier:
It isn’t so much the absolute current that we are placing a value on, but the voltage drop across our emitter resistors. When this is optimum, our small signal and large signal output impedance is the same, or as near as we can get it to being the same.
We’ll only ever get close, never exactly, to this ideal - but this condition is met when Re (the resistor attached to our transistor’s emitter) and our transistor’s intrinsic resistance (re’) is the same.
This condition is met when there is an approx 26mV drop across our emitter resistor.
Edit: I just noticed that PRR touched on this last part in an old post above. However, he says 30mV (then adds "or 26mV whatever") across the emitter resistor.
Without wishing to disagree with PRR, I will add that my own experience of Iq = .026/(Re+re+(rb/hFE)) generally errs on the ideal value being between 14mV and 26mV drop across the output transistor's emitters rather than being higher than the 26mV figure. Not always hence, measure it and A.O.T.