Updated March 19th 2010
Class a/b/c/d amplifier
In spite of modern digital technology, transistors as well as tubes are still much in the same way. Certain characters by each amplifier class may accomodate for different purposes.
In recent years, class d amplifiers have been developed mainly because of thebenefits of smaller size and also lower weight, and therefore they have become better. So first a look at this one
Class D amplifier
A class d amplifier is a switching amplifier using pulse-width modulation (PWM) circuitry. Although the circuitry performs on-off switching, the "D" does not stand for digital, even if digital electronics may be used. The switching is rather binary and usually by MOSFET transistors.
The switching frequency is much higher than audio, at least 2-300kHz or over 1MHz. The principle can be described with a pulse width, being "on" 25% of a certain period length, the output becomes + or -0.5 of a full signal. If the pulse width is 50%, no output will occur. The switching speed has to be at least twice the highest signal frequency but most designs utilize a much higher sampling rate. The effiency can get as high as 90 or even 95% but most are between 80 - 90%. Since less heat will be developed as other amplifiers, smaller heatsinks can be used.
The benefits are obvious, less power consumption, smaller and less weight. Examples include use in automobiles and for other mobile or portable use. The size and weight get even lower for class D amplifiers up to ca 5 - 10 W because heatsinks aren't needed.
Class D however, may not be used for very high quality audio is or in applications where EMI has to be as low as possible.
Class D amplifier links:Class D Audio Amplifiers - Theory and Design
Class D Audio Amplifier FAQ (pdf)
Class D Audio Amplifier - Wikipedia Class D Audio Amplifier - Main article Wikipedia
Class A amplifier
The amplifier conducts 100% of the signal period. The effiency is low, not more than 25% at full load, and lower with lower output.
There is no crossover distortion. Class A can be used in small signal circuits where power dissipation is negligible, yet to obtain optimal linearity.
Class A amplifier links:Class A - Wikipedia
Class B amplifier
Is conducting 50% of the time and uses two complementary active elements that conduct one at a time (push-pull).
Class B amplifiers are subject to crossover distortion if there
is any mismatch, hence a notch distortion might occur unless the bias for each of the element is shifted which would cause an overlap as in class AB.
Class B amplifier links:Class B and AB - Wikipedia
Class AB amplifier
Class AB is a combination of A and B. It was created to leave one element conducting while the other was conducting. This may create an overlap between the two signals. Hence, the signal would become larger in the overlap. The efficiency is higher than of class A and lower than of class B.
Class C amplifier
Is conducting less than 50% of the signal period. A tuned filter after can filter out the distortion. It is mostly used in RF applications where a linear amplification is not neccessary such as in FM transmitters. However the voltage can be AM-modulated, hence the RF output too, with fairly low distortion.
Class C amplifier links:Class C - Wikipedia
Class E, F, G and H amplifiers
I do not include these here but Wikipedia has information about class E, F, G and H amplifiers in the electronic amplifier article.
Links:Amplifier classes - simplified overview
Amplifier classes - Duncan's amp pages