If you make an internet search for "Push pull BJT amplifier" you will find many different variants of the following schematics:
These three configurations are the ones I am interesting in understanding better. First, I see that this circuit is often labelled as amplifier but since the gain appears to be in the best case equal to 1 it looks to me more like a current amplifier perhaps rather than a voltage one.
Suppose that V_signal is a sinusoidal 1 Vpp (maybe even lower 100 mVpp) signal at a given frequency. My main question is as follows:
- Referring to configuration A, while I understand the role of the diodes and R1 and R2 in the biasing of the BJTs to avoid crossover distorsion, I do not understand what is the role of R3 and R4 which are in series to the BJTs. My first thought is that they should limit the current through the load but in the simulations the circuit works fine even without them so I am a bit confused on their use and how to size them.
As a sidequestion, what are the advantages/disadvantages of configurations B and/or C over A? I can see how C is more advantageos in that it does not need a double side power supply and can work straight from 12 V but I cannot understand why I should use B which is using an additional capacitor apparently for the same result as A and C.
Edit: I noted the circuit is not easy to visualize, so I've just added 3 screenshots (Vcc=12V, Vee=-12V)