How to: Amplifier classes
By definition, the output device(s) in a Class A amplifier is always conducting or “on” and provides output current during the complete signal cycle (over a 360-degree interval). Because the device is always turned on, the theoretical maximum efficiency is only 12.5% for a resistively biased single-ended output stage, to 25% for a current source biased output stage, to 50% for a “push-pull” output stage. Also, maximum efficiency is only approached at maximum output; at low output the efficiency is much worse.
In Class B operation, the output device only conducts for 180 degrees of the signal cycle, so it is only on or conducting half of the time. To make a useful audio amplifier two Class B devices must be used in a “push-pull” configuration to generate output over the full 360 degree waveform cycle. The amplifier’s output devices are connected to positive and negative voltage supplies and are driven alternately to “push” current and “pull” current from the loudspeaker. A small forward bias on the output devices must be adjusted precisely to turn on and cutoff exactly at the zero crossing of the waveform. Too little bias and the amplifier will conduct for less than 180 degrees and slip into class-C operation, resulting in offensive “crossover distortion”. Too much bias and the amplifier will conduct more than 180 degrees, dissipate more power and also increase distortion. The higher efficiency of the Class B amplifier is due to the fact that when one output device is conducting the other is cutoff and therefore no power dissipated by the cutoff transistor. Theoretical maximum efficiency is about 78.5%.
In between class A and class B operation is class AB. The output devices are biased to that the output devices are conducting for more than 180 degrees of the signal cycle but less than 360 degrees. Some amplifiers are heavily overbiased to give a few or several Watts of class A operation at low output power. When the amplifier output is increased, it slips into class B operation, where only one of the output devices is conducting and the other is cutoff. The theoretical maximum efficiency falls in between class A and class B (50 -78.5%). Actually, most consumer grade class B amplifiers are actually marketed and described as class AB amplifier to avoid the negative connotations of class B amplifiers that persists from the early days of transistor amplifiers where thermal drift and immature transistor technologies produced many amplifiers that exhibited very audible crossover distortion.