Q.With a circuit diagram explain the working of class-B push-pull amplifier.

A class-B push-pull amplifier uses two transistors, each biased at cut-off, that conduct on alternate half-cycles and whose outputs are combined by a centre-tapped output transformer to give the full waveform. …

[!TLDR]

A class-B push-pull amplifier has two cut-off-biased transistors that conduct on alternate half-cycles; a centre-tapped output transformer combines their half-cycles into a full sine wave at high efficiency, with crossover distortion as the main limitation.

Circuit: two identical transistors $Q_1$ and $Q_2$ operated in class B (Q-point at cut-off). A centre-tapped input (driver) transformer applies the signal in anti-phase to the two bases, and a centre-tapped output transformer combines their collector currents to feed the load $R_L$; the supply $+V_{CC}$ is connected to the output-transformer centre tap.

Working: because the input transformer provides opposite-phase signals to the two bases, the two transistors are driven alternately. During the positive half-cycle of the input, $Q_1$ is forward driven and conducts while $Q_2$ stays off; $Q_1$'s current flows through the upper half of the output-transformer primary. During the negative half-cycle, $Q_2$ conducts while $Q_1$ is off, and its current flows through the lower half of the primary in the opposite sense. Each transistor thus conducts for 180°, and the output transformer adds the two half-cycles to reproduce the complete sine wave across $R_L$ — one transistor "pushes" while the other "pulls", hence push-pull.

Advantages: high efficiency (about 78.5%), since no DC collector current flows when there is no signal; even harmonics cancel in the transformer, reducing distortion; and the DC components in the two halves oppose, so there is no DC saturation of the transformer core. …