in class A amplefiers Idc=Ic(i.e collector current at no input condition ,but in class B amplefiers at no input ,collector current is zero so why ,in class B amplefiers ,Idc(DC current)=2/pi.I(p).

 

you must understand the transistor bias relationship to the dc load line. class a amplifiers are biased such that the operating or "Q" point is centered on the load line, always consuming power. but also able to swing both positive and negative from that point in equal amounts. class b amps are biased at cut off, Vce = 0v, very power frugal. and the signal actually provides the base bias to turn the transistor on. in this case the amp can only provide one direction of signal output, say positive, but in full amplitude. and there is signal distortion for the first .7 volts when the transistor is turning on. i do not understand your last formula, can you explain it's application conditions?