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Amplifier
The electrical signal coming from a microphone is too weak to drive a speaker by itself, but if you plug the mic into an amplifier, it will easily drive a speaker. The amplifier uses the microphone's signal to control electrical power from a battery or other source. It is this added power that drives the speaker to loud sound levels.
Amplifier Classes
Class A amplifiers have a simple design and the best signal accuracy. An amplifier of this type uses a single transistor or vacuum tube, and has a fairly low efficiency, between 12 and 22 percent. A class AB amplifier has nearly the same accuracy as a class A, but has more efficiency, at the cost of a more complex circuit. A class B amplifier design has one or two transistors, and it trades off some accuracy for better efficiency than class A. A class C has even better efficiency ̵2; up to 90 percent ̵2; and poor accuracy. Predictably, a class D has high efficiency and the lowest accuracy of the four types.
Efficiency, Power and Heat
An amplifier delivers power to a speaker or other device, and uses power from a supply of electricity. Ideally, an amplifier that delivers 1 watt of power to a speaker uses 1 watt from the supply, but no amplifier is that efficient. The amplification process itself uses some energy, which becomes heat in the transistors. A class A amplifier having 15 percent efficiency uses 1 watt of power from a source, turns .85 watts into heat, and delivers .15 watts to the speaker.
Simplicity and Cost
Class AB amplifiers have more transistors and other components than a class A, though this adds little cost to the circuit. Because of the class A amplifier̵7;s low energy efficiency, these are much less common than the class AB. Class A amplifiers have uses in radio broadcasting and in specialty audio; the vast majority of audio amplifiers are class AB.