Amplifier

Generally, an amplifier or simply amp, is any device that changes, usually increases, the amplitude of a signal. The "signal" is usually voltage or current. In popular use, the term today usually refers to an electronic amplifier, often as in audio applications. The relationship of the input to the output of an amplifier — usually expressed as a function of the input frequency — is called the transfer function of the amplifier, and the magnitude of the transfer function is termed the gain.
A related device that emphasizes conversion of signals of one type to another (for example, a light signal in photons to a DC signal in amperes) is a transducer, a transformer, or a sensor. However, none of these amplify power. The bandwidth (BW) of an amplifier is the range of frequencies for which the amplifier gives "satisfactory performance". The "satisfactory performance" may be different for different applications.
However, a common and well-accepted metric are the half power points (i.e. frequency where the power goes down by half its peak value) on the power vs. frequency curve. Therefore bandwidth can be defined as the difference between the lower and upper half power points. This is therefore also known as the −3 dB bandwidth. Bandwidths for other response tolerances are sometimes quoted (−1 dB, −6 dB etc.). A full-range audio amplifier will be essentially flat between 20 Hz to about 20 kHz (the range of normal human hearing.)
In minimalist amplifier design, the amp's usable frequency response needs to extend considerably beyond this (one or more octaves either side) and typically a good minimalist amplifier will have −3 dB points <> 65 kHz. Professional touring amplifiers often have input and/or output filtering to sharply limit frequency response beyond 20 Hz-20 kHz; too much of the amplifier's potential output power would otherwise be wasted on infrasonic and ultrasonic frequencies, and the danger of AM radio interference would increase. Modern switching amplifiers need steep low pass filtering at the output to get rid of high frequency switching noise and harmonics.