for: Amplifier (band)
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for: 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. 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.
In popular use, the term usually refers to an electronic amplifier, often as in audio applications to operate a loudspeaker that is being used in a PA system to make the human voice louder or play recorded music. Amplifiers may be classified by the input (source) they are designed to amplify (such as a guitar amplifier to perform with an electric guitar), or named for the device they are intended to drive (such as a headphone amplifier), or by the frequency range of the signals (Audio, IF, RF and VHF amplifiers for example), or grouped by whether they invert the signal (inverting amplifiers and non-inverting amplifiers, or by the types of device used in the amplification (valve or tube amplifiers, FET amplifiers, etc.).
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.
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)). RF amplifiers are often specified in terms of the maximum power gain obtainable, while the voltage gain of audio amplifiers and instrumentation amplifiers will be more often specified (since the amplifier's input impedance will often be much higher than the source impedance, and the load impedance higher than the amplifier's output impedance).
- Example: an audio amplifier with a gain given as 20dB will have a voltage gain of ten (but a power gain of 100 would only occur in the unlikely event the input and output impedances were identical).
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 nowrap: −3 dB bandwidth. Bandwidths (otherwise called "frequency responses") for other response tolerances are sometimes quoted (nowrap: −1 dB, nowrap: −6 dB etc.) or "plus of minus 1dB" (roughly the sound level difference people usually can detect).
