Amplifier

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For the British rock band of the same name, see Amplifier (band).
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.
Contents
1 Figures of merit
1.1 Gain
1.2 Bandwidth
1.3 Efficiency
1.4 Linearity
1.5 Noise
1.6 Output dynamic range
1.7 Slew rate
1.8 Rise time
1.9 Settling time and ringing
1.10 Overshoot
1.11 Stability factor
2 Electronic amplifiers
2.1 Power amplifier
2.2 Vacuum tube (valve) amplifiers
2.3 Transistor amplifiers
2.4 Operational amplifiers (op-amps)
2.5 Fully differential amplifiers (FDA)
2.6 Video amplifiers
2.6.1 Oscilloscope vertical amplifiers
2.6.2 Distributed amplifiers
2.7 Microwave amplifiers
2.7.1 Travelling wave tube (TWT) amplifiers
2.7.2 Klystrons
2.8 Musical instrument (audio) amplifiers
3 Other amplifier types
3.1 Carbon microphone
3.2 Magnetic amplifier
3.3 Rotating electrical machinery amplifier
3.4 Johnsen-Rahbek effect amplifier
3.5 Mechanical amplifiers
3.6 Optical amplifiers
3.7 Miscellaneous types
4 See also
5 References
//
Figures of merit
The quality of an amplifier can be characterized by a number of specifications, listed 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(Pout /(Pin)).
Bandwidth
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 20Hz to about 20kHz (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 <> 65kHz. Professional touring amplifiers often have input and/or output filtering to sharply limit frequency response beyond 20Hz-20kHz; 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 1020% 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 3555% efficient with a theoretical maximum of 78.5%. Commercially available Class D switching amplifiers have reported efficiencies as high as 97%. Amplifiers of Class C-F are usually known to be very high efficiency amplifiers. 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 efficiency produces heat as a by-product of the energy lost during the conversion of power. In more efficient amplifiers there is less loss of energy so in turn less heat.
Linearity
An ideal amplifier would be a totally linear device, but real amplifiers are only linear within certain practical limits. When the signal drive to...(and so on)

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