For those who are less familiar with RF amplifiers, it is a device through which the input can be amplified for a much intensified output. Sound signals by themselves are really weak but can be made stronger using an amplifier. They come in many different types based on their design which indicates the circuitry within it and are represented through classes A, B, AB, C and D.
Class A: Very low distortion, highly inefficient is how you can define class a amplifiers. Distortion level in an amp depends on the transistor; transistors in class A are little bias, and they remain half ON even when the device is idle, lot of power is thus wasted. With this bias are they actually useful? Yes they are mainly used for circuits requiring less power as Class A RF amplifiers will then maintain low distortion. Some extremely discriminating listeners are often found using high end Class A amps.
Class B: Though these are more efficient than Class A amps they too suffer from crossover distortion which takes place when the level of signal is low. They are used in all those low cost designs where the sound quality required is not that high like pocket transistor radios or clock radio circuits (these devices now use IC amplifiers).
Class AB: As you might have guessed Class AB combines all the good things about Class A and Class B. It is the most common variety of RF amplifiers seen in home stereos. With these, low signal would mean worst distortion and usually lowest at the time when signal reaches clipping point. Class AB amplifiers are equipped with two slightly ON transistors to eliminate crossover distortion present in Class B amplifiers.
Class C: Tremendous distortion caused by Class C makes them unsuitable for using in audio circuits; so are used only in RF circuits. Even in RF circuits filters are used to ensure that the final signal received is acceptable. They are more efficient then all the above Classes.
Class D: Though Class D amplifiers have been around for more than 5 long decades they can be seen in recent time semiconductor devices only recently. Advance power capacity, speed and efficiency of latest semiconductors make it less expensive to use Class D amplifiers in them. While all the above mentioned Classes use semi conductor devices in linear mode Class D uses them in OFF and ON mode just like switches.
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