Linearity is the behavior of a circuit, particularly an amplifier , in which the output signal strength varies in direct proportion to the input signal strength. In a linear device, the output-to-input signal amplitude ratio is always the same, no matter what the strength of the input signal (as long it is not too strong).
In an amplifier that exhibits linearity, the output-versus-input signal amplitude graph appears as a straight line. Two examples are shown below. The gain, or amplification factor, determines the slope of the line. The steeper the slope, the greater the gain. The amplifier depicted by the red line has more gain than the one depicted by the blue line. Both amplifiers are linear within the input-signal strength range shown, because both lines in the graph are straight.
In analog applications such as amplitude-modulation ( AM ) wireless transmission and hi-fi audio, linearity is important. Nonlinearity in these applications results in signal distortion, because the fluctuation in gain affects the shape of an analog output waveform with respect to the analog input waveform.
Even if an amplifier exhibits linearity under normal conditions, it will become nonlinear if the input signal is too strong. This situation is called overdrive. The amplification curve bends towards a horizontal slope as the input-signal amplitude increases beyond the critical point, producing distortion in the output. An example is a hi-fi amplifier whose gain is set to the point where the VU (volume-unit) meter needles kick into the red range. The red zone indicates that the amplifier is not operating in a linear fashion. This can degrade the fidelity of the sound.