# Amplifiers

Normal operating classes are A, AB, B and C.

Class A

The input signal is applied to the straight part of the characteristic. Input voltage signal and collector current are in phase. Input voltage signal and output voltage are 180° out of phase. A collector current flows at all times. It is used mainly as a small signal amplifier with a practical operating efficiency of less than 25%.

Class B

Requires less bias, and current flows in the output circuit for alternate half-cycles only. A practical operating efficiency is about 50%.

Class C

Often used in oscillators and tuned RF amplifiers. Expected efficiency 65-90%. A class C amplifier is rich in harmonics and can be used as a frequency multiplier. A collector load of a tuned circuit, resonant at the required harmonic, will produce the required frequency (highest multiplication factor used is normally three).

The overall gain of an amplifier is the product of the gains of each stage.

The total gain of an amplifier in dB is the sum of the individual gains (in dB) of each stage.

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