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Class A, B, C and D refer to different design architectures for an amplifier, having different power efficiencies. Class A drives the output throughout a cycle, and is the least efficient. Class B drives the output for only a half cycle. Usually, a second Class B driver drives alternate half cycles. Class C drives the output for less than half a cycle and generally drives a tuned circuit to restore the output (typically in a transmitter) to a pure sine wave. Class D, the most efficient, uses pulse width modulation.The outtput is switched on and off at a rate well above the audio to create the amplified signal.