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 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 radio frequency amplifiers. 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).

The term has slightly different meanings in two other fields. In antenna design, antenna gain is the ratio of power received by a directional antenna to power received by an isotropic antenna. In laser physics, gain may refer to the increment of power along the beam propagation direction in a gain medium, and its dimension is m−1 (inverse meter) or 1/meter.

Famous quotes containing the word gain:

    Well: what we gain by science is, after all, sadness, as the Preacher saith. The more we know of the laws & nature of the Universe the more ghastly a business we perceive it all to be—& the non-necessity of it.
    Thomas Hardy (1840–1928)

    Let the wise also hear and gain in learning, and the discerning acquire skill, to understand a proverb and a figure, the words of the wise and their riddles.
    Bible: Hebrew, Proverbs 1:5-6.

    ...if you are to gain any great amount of good from the world, you must attain a passive condition of mind. is never to be forgotten that it is the rest of the world and not you that holds the great share of the world’s wealth, and that you must allow yourself to be acted upon by the world, if you would become a sharer in the gain of all the ages to your own infinite advantage.
    Anna C. Brackett (1836–1911)