An audio amplifier can be loosely classified as any amplifier that amplifies between the frequency range of 20 and 20000 Hz. They are used to drive a loudspeaker of some sort (like speakers or headphones.)
The different classes commonly used in audio amps are class A, class AB, and class B. There are some other more exotic types but these are most common for homebrewing. Class A means that the active device (transistor or tube) conducts over the whole cycle, class AB over more than 180 degrees but less than the whole cycle, and class B for only 180 degrees.
its mostly used for the amplification of AUDIO signals...........they r installed at the receiver section. audio amplifiers r used in TV receivers, fm receivers etc etc.........they can be classified as class A, class B, CLASS AB, CLASS C,. i think CLASS C audio power amplifier is preferred because of its higher effieciency.
THERE'S A book on BASIC ELECTRONICS by V.K MEHTA or electronics by BOYLSTEAD MORE than enough info.
Besides all the explanations above, could be mentioned also the class D of audio amplifiers, which consists in a PWM amplifier with a low pass filter before the signal is output to the speakers. See more at: **broken link removed**
In the construction or active elements of amplifiers construction we can mention:
- Valve (tubes) amplifiers
- Discrete bipolar transistor amplifier
- Discrete MOSFET amplifier
- Hybrid (IC composed of bipolar transistors or MOSFETs) amplifier