When an amplifier’s stage devices are passing current at all times, including when the amplifier is at idle (no music playing), whether the amplifier is single ended or push-pull, the amplifier is said to be biased in Class A. Because the current is flowing at all times, an input signal causes the current to be immediately diverted to the speakers, and therefore, the sound is very “fast”. In the case of a push-pull amplifier, there is also less crossover distortion when the signal passes from the positive to the negative or negative to positive, since each side of the push-pull section is already “on”. If all stages of the amplifier are biased in Class A, and the amplifier operates in Class A to full output (enough current flowing at idle that could be required for full output), it is said to be a “Pure Class A” amplifier. Pure Class A designs are understandably expensive to build and are usually only found in high-end boutique amps.
Class B differs from Class A in that there is no current flowing when the output devices are at idle, and as a result, they have to turn on from a zero current state when signal is present. In a push-pull Class B design the output devices would each produce half of the audio waveform (one set for the positive half, and another for the negative half) and would not have any current flow when the other half is operating. Class B designs tend to have a slower slew rate and more crossover distortion but are less expensive and require less robust power supplies.
As its name implies, this is sort of a combination of Class A and Class B operation. If an amplifier operates in Class A mode for only a portion of its output, and has to turn on additional current in the devices for the remainder of its output, it is said to operate in Class AB. Most amplifiers are in this category since they operate in two classes. In class AB and B, the amplifier is slower than in Class A because there is a finite time between the application of the input signal and when the devices are turned on to produce a flow of current to the speakers. However, Class AB and Class B are more efficient than Class A and do not require such large power supplies.
A Class D amplifier is one in which the output transistors are operated as switches. When a transistor is off, the current through it is zero. When it is on, the voltage across it is small, ideally zero. In each case, the power dissipation is very low. This increases the efficiency, thus requiring less power from the power supply and smaller heat sinks for the amplifier. These are important advantages in portable and battery-powered equipment.
The “D” in class-D is sometimes incorrectly said to stand for “digital.” The Class D amplifier is based on analog principles; there is no digital coding of the signal.
If an amplifier has more than one voltage rail (DC voltage delivered by the power supply), then it is designated Class H. This is a very efficient type of amplification. The output transistors of an amplifier have to dissipate, in heat (watts), the difference between the rail voltage and the voltage across the speaker terminals, multiplied by the current (as stated in Ohm’s law). So, when there is a low rail voltage for use during periods of low volume, and a high rail voltage for use during loud volume, the output transistors don’t have to dissipate very much power when the volume is low. This causes less drain on the power supply and makes it possible to build a very lightweight design. The drawback is distortion at mid-volume when the amplifier has to go back and forth between the two (or more) rail voltages.