Looking at another question about an AB Amplifier stage the answer was the classic diode biased push-pull stage like this… which I voted up like others..
simulate this circuit – Schematic created using CircuitLab
But then, while staring at, it I could not for the life of me figure out what those two diodes buy you over the base-emitter diodes that are there anyway.
So I simulated this circuit instead…
It appears to work just as well, if not better.
Please forgive me if I am having a senior moment, but what am I forgetting here?
Best Answer
Power.
The second circuit uses much more quiescent power because the ~40 uA of base current in each BJT produces ~4 mA of collector current, burning ~ 30 mW between the two BJTs.
In the first circuit, if the diodes are chosen as a good match for the BJTs, you keep the BJT's very near the edge of forward active region without actually getting them there (until they're needed) and much less quiescent power is used.
Another important point is that with the diode biasing, the circuit can be used DC coupled:
DC coupling is handy when this is used as buffer for an op-amp as it lets the buffer be in the feedback loop for all frequencies and keeps the op-amp output from running off to the rail.