Q.Define Class A, Class B and Class C power amplifiers.

Power amplifiers are classed by how much of the input cycle the transistor conducts, which depends on where the operating point (Q-point) is fixed on the load line. …

[!TLDR]

Class A, B and C power amplifiers are defined by the fraction of the input cycle during which collector current flows — 360°, 180°, and less than 180° respectively — set by the position of the Q-point.

A power amplifier is classified by the conduction angle, i.e. the portion of the input cycle for which the output (collector) current flows, which is decided by the position of the operating point on the DC load line.

1. Class A amplifier: the Q-point is fixed at (or near) the centre of the load line, in the active region. Collector current flows during the entire 360° of the input signal cycle, so the output is a faithful, distortion-free copy of the input. Efficiency is low (about 25%, up to 50% for transformer coupling).

2. Class B amplifier: the Q-point is fixed at cut-off. Collector current flows for only one half (180°) of the input cycle; the transistor is off during the other half. A single transistor therefore reproduces only half the wave (a push-pull pair is used for the full wave). Efficiency is higher (about 78.5%). …