In electronics, gain is a measure of the ability of a circuit (often an amplifier) to increase the power or amplitude of a signal from the input to the output. It is usually defined as the mean ratio of the signal output of a system to the signal input of the same system. It may also be defined on a logarithmic scale, in terms of the decimal logarithm of the same ratio ("dB gain"). A gain greater than one (zero dB), that is, amplification, is the defining property of an active electronic component or circuit, while a passive circuit will have a gain of less than one.
Thus, the term gain on its own is ambiguous. For example, "a gain of five" may imply that either the voltage, current or the power is increased by a factor of five, although most often this will mean a voltage gain of five for audio and general purpose amplifiers, especially operational amplifiers, but a power gain for RF amplifiers, and for directional aerials will refer to a signal power change compared with a simple dipole. Furthermore, the term gain is also applied in systems such as sensors where the input and output have different units; in such cases the gain units must be specified, as in "5 microvolts per photon" for the responsivity of a photosensor. The "gain" of a Bipolar transistor normally refers to forward current transfer ratio, either hFE ("Beta", the static ratio of Ic divided by Ib at some operating point), or sometimes hfe (the small-signal current gain, the slope of the graph of Ic against Ib at a point).
In laser physics, gain may refer to the increment of power along the beam propagation in a gain medium, and its dimension is m-1 (inverse meter) or 1/meter.
Logarithmic units and decibels
Power gain, in decibels (dB), is defined by the 10 log rule as follows:
where Pin and Pout are the input and output powers respectively.
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