What is gain?
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 by adding energy converted from some power supply to the signal. It is usually defined as the mean ratio of the signal output of a system to the signal input of the same system. It is often expressed using the logarithmic decibel (dB) units (“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.
The term gain alone is ambiguous, and can refer to the ratio of output to input voltage, (voltage gain), current (current gain) or electric power (power gain). In the field of audio and general purpose amplifiers, especially operational amplifiers, the term usually refers to voltage gain, but in radio frequency amplifiers it usually refers to power gain. 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 ofIc 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 gain has a slightly different meaning in antenna design; antenna gain is the ratio of power received by a directional antenna to power received by an isotropic antenna.