An amplifier is a device that increases the strength of an input signal, such as voltage, current, or mechanical motion, but not the way it varies. Amplifiers play a vital role in most electronic devices. Electronic amplifiers, usually based on transistors and electron tubes, can be thought of as a sort of variable switch in which the output from a power source is controlled (modulated) by a weak input signal. An important factor is the fidelity with which the waveform of the output signal reproduces that of the input over the desired bandwidth.