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Assuming A sinuaoidal output:
Class A: Output device conducts for 360 degrees of the cycle
Class B: Output device conducts for 18 degrees of
. the cycle (applies to push-pull output only)
Class AB: [180 degrees] >Output device conduction angle <= [360 degrees]
Class C: Output device conduction angle is less than 180 Degrees Useful only for RF circuits where a resonant tank circuit completes the sine wave cycle. Class C amplifier can be viewed as a synchronous switch that pumps energy into the tank circuit that is tuned to the input frequency.
Class G: A scheme in which the power supply voltage automatically varies depending on the output signal amplitude. This is done to reduce the dissipation att low power outputs.
Class D: Switching amplifier that produces a pulse width modulated output at a fixed "carrier" frequency. The carrier is filtered out to produce an amplified replica of the input.
Efficiencies (listed in increasing order)
Class A (Lowest Efficency and distortion)
Class AB (Higher distortion than A, but lower than B)
Class C (Not useful for audio)
Added after 2 minutes:
"Class B: Output device conducts for 18 degrees of " should read "Class B: Output device conducts for 180 degrees of "
Chrisbarty makes a good point about class B. Since the conduction angle is 180 degrees, the output stage must be push-pull in order to avoid severe distortion. This also applies to class AB. Distortion is not an issue for class C, since it is used to amplify an RF carrier, and the resonsnt tank circuit cleans up the waveform to produce a sine wave.