What is A or C biasing?

Answer 1

The three standard forms of analog amplifier biasing are:

1. class A - the amplifier is biased in the center of its linear operating range, this is the most linear but least efficient type of amplifier (because the transistors or tubes are always conducting, even when there is no signal to ampliify).
2. class B - the amplifier is biased at the cutoff point, this is an efficient amplifier but is only linear if operated as a push-pull amplifier (because the transistors or tubes are in cutoff and not conducting when there is no signal and through one half of every cycle, a class B push-pull amplifier has two sections that operate on alternate halves of the cycle).
3. class C - the amplifier is biased in hard cutoff so that only the peaks of the input signal are amplified, this is the most efficient amplifier (because the transistors or tubes may be in cutoff and not conducting for more the 85% of the time) but it is not capable of linear amplification. An amplifier biased as class C is only suitable for use in RF transmitter power stages, where a resonant LC tank circuit will be excited into oscillation by the output of the amplifier and complete the missing parts of the cycle.

There are other forms of biasing (e.g. class AB) analog amplifiers that get some of the advantages of two of the standard forms. There are also forms of amplifiers having other nonstandard classes (e.g. class D) that are not analog amplifiers, but instead operate by amplifying pulses.