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A power amplifier (PA) is defined by the ratio between the power supplied to the load and the power provided to the circuit by the source. A PA is categorized on the basis of its power rating, which is measured in watts (W). The power rating can be stated in terms of load impedance (ohm) across which the output power is measured.
Modern electronic circuits give high power amplification when low impedance is used (in the case of speakers), but this is not always recommended. Generally, the rated power output of an amplifier is said to be its peak output value. For example, if the PA is rated at 20 W, then the output can range from 0 to 20 (i.e., the maximum rated value). In the case of an overdriven input, the PA can give an output power that is many times higher than the maximum rated value, but, at the same time, the output signal quality decreases. Power amplifiers are used in a number of everyday appliances such as audio speakers and microwave ovens.