The Class of an amplifier refers to the design of the circuitry within the amp. For audio amplifiers, the Class of amp refers to the output stage of the amp (in practice there may be several classes of signal level amplifier within a single unit). There are many classes used for audio amps. The following is brief description of some of the more common audio amplifier classes.
Class A
Class A amplifiers have very low distortion (lowest distortion occurs when the volume is low) however they are very inefficient and are rarely used for high power designs. The distortion is low because the transistors in the amp are biased such that they are half "on" when the amp is idling (this is the point at which the semiconductor devices are most linear in behavior). As a result of being half on at idle, a lot of power is dissipated in the devices even when the amp has no music playing! Class A amps are often used for "signal" level circuits (where power requirements are small) because they maintain low distortion. High end Class A audio amplifiers are sometimes used by the most discriminating audiophiles. Distortion for class A amps increases as the signal approaches clipping, as the signal is reaching the limits of voltage swing for the circuit. This topology is least efficient at about 20%.
Class B
This type of amplifier operates in the opposite way to Class A amplifiers . The output devices only conduct for half the sinusoidal cycle (one conducts in the positive region, and one conducts in the negative region), or in other words, if there is no input signal then there is no current flow in the output devices. This class of amplifier is obviously more efficient than Class A, at about 50%, but has some issue with linearity at the crossover point, due to the time it takes to turn one device off and turn the other device on. Class B amplifiers are used in low cost designs or designs where sound quality is not that important.
Class AB
This type of amplifier is a combination of the above two types, and is currently one of the most common types of power amplifier in existence. Here both devices are allowed to conduct at the same time, but just a small amount near the crossover point. Hence each device is conducting for more than half a cycle but less than the whole cycle, so the inherent non-linearity of Class B designs is overcome, without the inefficiencies of a Class A design . Efficiency for Class AB amplifiers is about 50%.
Class D
The concept of a Class D amp has been around for a long time (~ 50 years or so), however only fairly recently have they become more commonly used in consumer applications. Due to improvements in the speed, power capacity and efficiency of modern semiconductor devices, applications using Class D amps have become very attractive. Class D amplifiers use a completely different method of amplification as compared to Class A, B and AB. Whereas the aforementioned classes of amplifier operate the semiconductor devices in the linear mode, Class D amplifiers operate the output semiconductor devices as switches (ON or OFF); this is why sometimes they are also called "digital". In a Class D amplifier, the input signal is compared with a high frequency triangle wave, resulting in the generation of a Pulse Width Modulation (PWM) type signal. This signal is then applied to a special filter that removes all the unwanted high frequency by-products of the PWM stage. The output of the filter drives the speaker. As the switches are either fully on or fully off, significantly reducing the power losses in the output devices. Efficiencies of 90-95% are possible.
Class D architecture has been around for many years, but historically efficiency gains have been at the expense of audio quality. Now, the performances of the new modulator IC's together with the last generation Mosfets, deliver the advantages of Class D with sonic performance that truly rivals that of prestige linear amplifiers.