how can i differentiate a classB and a classAB amplifier in just looking in the ckt??

how can i differentiate a class B and a class AB pushull amplifier in just looking in the ckt??

Look at the bias in relation to the current flow (Ip, Ik or Ic, Ie) during the quiesent state.

I'm sure the definitions of Class B and Class AB already told you this.

Class-AB circuits have two diodes in series between the bases of the output transistors.
Class-B amplifiers have the bases shorted together.

as mr audioguru mentioned, class b amps have a terminal of each (push & pull) transistor shorted together, could be emitters or bases depending on the configuration, with the inputs coming off a center tap transformer. while a class ab will have some components (diodes, resistors) providing bias to move the q point off zero, eliminating zero crossover distortion.

Center-tapped driver transformers have not been used in audio amplifiers for 50 years!
The output transistors are a complimentary pair. One NPN and the other PNP.
They are both emitter-followers and have their emitters connected together or each has a low value resistor in series with its emitter.

When their bases are shorted together then they don't conduct any current when at rest. So it is class-B with horrible crossover distortion in the "dead" center of the waveform.

When the transistors have two diodes in series between their bases then they always conduct a little so the waveform flows smoothly from one transistor to the other in class-AB.

While not much in the audio spectrum, push-pull amplfiers are still around, in the higher power applications ... even those using center tapped transformers.

Althought not in commercial service, there are push-pull parallel circuits used in other applications to achieve higher power levels.

Class A, AB1, and AB2, B, and C are still characterized by the current flow during quiesence or the current flow during the input cycle.