I've posted this once before, but here ya go...
BTW, I didn't write it, it's copied and pasted
--Audio power amplifiers are classified primarily by the design of the output stage. Classification is based on the amount of time the output devices are made to operate during each cycle of swing. Amplifiers are also defined in terms of output bias current (the amount of current flowing in the output devices with no signal present).
Class A operation is where both devices conduct continuously for the entire cycle of signal swing, or the bias current flows in the output devices at all times. The key ingredient of Class A operation is that both devices are always on. Because of this, Class A amplifiers are single-ended designs with only one type of polarity output device. Class A is the most inefficient of all power amplifier designs, averaging only 20%. Because of this, Class A amplifiers are large, heavy and run very hot. All of this is due to the amplifier constantly operating at full power. the positive effect of all this is that Class A designs are inherently the most linear with the least amount of distortion.
Class B operation is the opposite of Class A. Both output devices are never allowed to be on at the same time, or the bias is set so that current flow in a specific output device is zero when not stimulated with an input signal. Each output device is on for exactly one half of a complete sinusoidal signal cycle. Due to this operation, Class B designs show high efficiency, but poor linearity around the crossover region. This is due to the time it takes to turn one device off and the other device on, which translates into extreme crossover distortion. All of this restricts Class B designs to power consumption critical applications such as battery operated 2-way radios and other communications equipment.
Class AB operation allows both devices to be on at the same time (like in Class A), but just barely. The output bias is set so that current flows in a specific output device appreciably more than a half cycle, but less than the entire cycle. That is, only a small about of current is allowed to flow through both devices, unlike the complete load current of Class A designs, but enough to keep each device operating so they respond instantly to input voltage demands. Thus the inherent non-linerarity of Class B designs is eliminated, without the gross inefficiencies of the Class A design. It is this combination of good efficiency (around 50%) with excellent linearity that makes Class AB the most popular audio amplifier design.
Class AB+B design involves two pairs of output devices; one pair operates Class AB, while the other (slave) pair operates Class B. Class AB+B designs are just as efficient as Class AB, but have even less distortion. The Class AB+B design was developed by Crown.
Class D operation is switching, hence the term switching power amplifier. Here, the output devices are rapidly switched on and off at least twice for each cycle. Since the output devices are either completely on or completely off, theoretically they do not dissipate any power. Consequently, a Class D operation should be 100% efficient. This, however, would require zero on-impedance switches with infinitely fast switching times-a product we are still waiting for; in the meantime there are Class D designs with true efficiencies approaching 90%.
Class G operation involves changing the power supply voltage from a lower level to a higher level when larger output swings are required. there have been several ways to do this. The simplest involves a single Class AB output stage that is connected to two power supply rails by a diode or a transistor switch. the design is such that for most musical program material, the output stage is connected to the lower supply voltage, and automatically switches to the higher rails for large signal peaks. Another approach uses two Class AB output stages, each connected to a different power supply voltage, with the magnitude of the input signal determining the signal path. Using two power supplies improves efficiency enough to allow significantly more power for a given size and weight. Class G is becoming common for pro audio designs.
Class H operation takes the Class G design one step further and actually modulates the higher power supply voltage by the input signal. this allows the power supply to track the audio input and provide just enough voltage for optimum operation of the output devices. The efficiency of Class H is comparable to Class G designs.--
There are also class "T" amps slowly coming into the market. They're basically class D amps that a company called Tripath (hence, class "T") has designed so that they can be used for all frequencies.
