Q.With input and output waveforms explain class-A, class-B, class-C and class-AB power amplifiers?

The four classes differ in where the Q-point is set, which fixes how much of the input cycle the transistor conducts and therefore the shape of the output waveform. …

[!TLDR]

With the same sinusoidal input, class A gives a full 360° output, class B a 180° half-cycle, class AB slightly more than a half-cycle, and class C a narrow pulse of less than 180° — the differences arising from the Q-point position.

For all four, the input is the same sine wave; the output (collector-current) waveform differs according to the conduction angle set by the Q-point:

1. Class A: the Q-point is at the centre of the load line, in the active region. The transistor conducts for the entire 360° of the input cycle, so the output current waveform is a complete, faithful sine wave (no clipping). Least distortion, but low efficiency (about 25%, up to 50%).

2. Class B: the Q-point is at cut-off. The transistor conducts only when the input drives the base-emitter into forward conduction, i.e. for one half (180°) of the cycle; during the other half it is off. The output is only one half-cycle of the sine wave (the other half is a flat line at zero). Efficiency about 78.5%. A push-pull pair of class-B stages is used to rebuild the full wave.

3. Class AB: the Q-point is set slightly above cut-off (a small forward bias). The transistor conducts for a little more than a half-cycle — between 180° and 360°. The output is slightly more than a half-cycle. Its purpose is to remove the crossover distortion of class-B push-pull operation while keeping efficiency high. …