Amplifiers

Generally, an amplifier is any device that will convert one signal (often with a small amount of energy) into a another signal (often with a larger amount of energy). In popular use, the term today usually refers to an electronic amplifier, often as in audio applications. more...

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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. A closely related device that emphasizes conversion of signals of one type to another (for example, a light signal in photons to a current signal in amps) is a sensor.

Figures of merit

The quality of an amplifier can be characterized by a number of specifications, enumerated below.

Gain

The gain of an amplifier is the ratio of output to input power or amplitude, and is usually measured in decibels. (When measured in decibels it is logarithmically related to the power ratio: G(dB)=10 log(P_out/P_in)).

Bandwidth

The 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 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 < 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.

Efficiency

Efficiency is a measure of how much of the input power is usefully applied to the amplifier's output. Class A amplifiers are very inefficient, in the range of 10–20% with a max efficiency of 25%. Class B amplifiers have a very high efficiency but are impractical because of high levels of distortion (See: Crossover distortion). In practical design, the result of a tradeoff is the class AB design. Modern Class AB amps are commonly between 35–55% efficient with a theoretical maximum of 78.5%. Commercially available Class D switching amplifiers have reported efficiencies as high as 97%. The efficiency of the amplifier limits the amount of total power output that is usefully available. Note that more efficient amplifiers run much cooler, and often do not need any cooling fans even in multi-kilowatt designs. The reason for this is that the loss of efficency produces heat as a biproduct of the energy lost during the convertion of power. In more efficent amplifiers there is less loss of energy so in turn less heat.

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