I know for transformer coupled circuits when current becomes zero the back emf acts and the collector to emitter voltage is increased.But why does it becomes exactly 2Vcc.I am actually having trouble figuring out the value of back emf offered
Some presumptions.
1. Symmetrical input signal, e.g. a sine
2. linear amplifier, linear load (speaker)
3. Sufficient large transformer inductance (ideally infinite)
4. Transistor bias current >= Vcc/(N²*RL)
There's no sense in making the positive voltage swing higher than the negative swing. The negative swing is however smaller than Vcc by the RE voltage drop and transistor saturation voltage.
It is not from the back EMF but rather since the Collector DC voltage, Vc =Vcc from ANY COIL with low DCR. It doesn't have to be a transformer, but the impedance on the collect to emitter ratio determines Voltage gain.
The coil can store energy and swing voltage symmetrically around Vcc thus the maximum Vce , Vpp=2Vcc ( minus Ve)