Q.Explain with a circuit diagram the action of class-C collector tuned power amplifier.

A class-C tuned amplifier biases the transistor beyond cut-off so it conducts only in short pulses, and a parallel LC tank circuit in the collector uses the flywheel effect to rebuild a full sine wave at its resonant frequency. …

[!TLDR]

In a class-C collector-tuned amplifier the transistor conducts only in narrow pulses (less than 180°); these pulses keep a parallel LC tank oscillating by the flywheel effect, so the collector delivers a clean sine wave at the resonant frequency with very high efficiency.

Circuit: a single transistor whose base is biased beyond cut-off (class C, conduction less than 180°). The collector load is a parallel LC tuned (tank) circuit resonant at the signal frequency, connected to $+V_{CC}$; the output is taken across the tank (or through a coupling to the load).

Action: because the base is reverse biased beyond cut-off, the transistor turns on only near the positive peaks of the input, delivering the collector current in short pulses of less than 180° each. On its own such a pulse train is highly distorted, but the LC tank recovers a sine wave through the flywheel effect: each current pulse dumps energy into the tank, and between pulses the energy oscillates back and forth between the inductor's magnetic field and the capacitor's electric field, producing a continuous sinusoidal voltage at the resonant frequency $f_0=1/(2\pi\sqrt{LC})$. Each successive pulse arrives in step to replenish the small losses of the tank, sustaining a steady, clean sinusoidal output. The tank also selects the fundamental (resonant) frequency and rejects the harmonics contained in the pulses, so the output is a pure sine wave despite the pulsed conduction. …