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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.
General characteristics of amplifiersMost amplifiers can be characterized by a number of parameters.
GainThe gain is the ratio of output power to input power, and is usually measured in decibels . (When measured in decibels it is logarithmically related to the power ratio: G(dB)=10log(Pout/Pin)).
Output dynamic rangeOutput dynamic range is the range, usually given in dB, between the smallest and largest useful output levels. Since the lowest useful level is limited by output noise, this is quoted as the amplifier dynamic range.
Bandwidth and rise timeThe bandwidth (BW) of an amplifier is usually defined as the difference between the lower and upper half power points. This is therefore also known as the −3 dB BW. Bandwidths for other response tolerances are sometimes quoted (−1 dB, −6 dB etc.). A full-range audio amplifier will be essentially flat between twenty hertz to about twenty kilohertz (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 < 10 and > 65 kHz. Professional touring amplifiers often have input and/or output filtering to sharply limit frequency response beyond 20-20 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. The rise time of an amplifier is the time taken for the output to change from 10% to 90% of its final level when driven by a step input. Many amplifiers are ultimately slew rate limited (typically by the impedance of a drive current having to overcome capacitive effects at some point in the circuit), which may limit the full power bandwidth to frequencies well below the amplifiers frequency response when dealing with small signals. For a Gaussian response system (or a simple RC roll off), the rise time is approximated by:
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