In the world of electronic, 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 to the signal converted from some power supply. 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 same voltage level and impedance is also known as unity gain. This is a simple Biasing methods for an N-channel MOSFET to form a unity-gain non-inverting amplifier