Analog amplifiers are cataloged by how much current flows during each wave cycle. Measured in degrees, 360º means current flows 100% of the time. The more current, the more inefficient and the more heat generated.
The amplifier conducts current throughout the entire cycle (360º). The Class A design is the most inefficient and is used in low-power applications as well as in very high-end stereo. Such devices may be as little as 15% efficient, with 85% of the energy wasted as heat.
The current flows only 180º for half the cycle, or two transistors can be used in a push-pull fashion, each one operating for 180º. More efficient than Class A, it is typically used in low-end products.
Combines Class A and B and current flows for 180º to 200º. Class AB designs are the most widely used for audio applications. Class AB amplifiers are typically about 50% efficient.
Operating for less than half of one wave cycle (100º to 150º), Class C amplifiers are the most efficient, but not used for audio applications because of their excessive distortion.
A variation of the Class AB design that improves efficiency by switching to different fixed voltages as the signal approaches them.
An enhancement of the Class G amplifier in which the power supply voltage is modulated and always slightly higher than the input signal.
The red indicates how much of the time current is flowing through one wave cycle.
Class D is a digital-like amplifier that works by turning a transistor fully on or off, but the "D" technically does not stand for digital. See Class D amplifier
A variation of the Class D technique from Tripath. Class T modulates the pulses based on the individual characteristics of the output transistors.