Linearity

Linearity is extent to which the output waveform of an amplifier is a faithful reproduction of input waveform.  In hi-fi equipment, all amplifiers should be as linear as state of art allows.

If you connect a dual trace oscilloscope (one which lets you observe 2 waveforms at the same time) to input and output terminals of a good hi-fi audio amplifier, the output waveform will appear as a vertically magnified duplicate of the input waveform. If you adjust vertical gain control of oscilloscope so the 2 waveforms have the same peak to peak displacement on screen, the input and output waveforms will look alike. When the input signal is applied to horizontal scope input and output signal is applied to vertical scope input, display will be a straight line.
Figures given below illustrate the principle of linearity. The graph and waveforms in the figure show the characteristics of a linear circuit. The graph of instantaneous output versus instantaneous input is the straight line. When the input is a sine wave, the output is a sine wave. Figure given to us shows what takes place in a hypothetical non linear circuit. The instantaneous output versus input function is not a straight line. Thus, the output waveform is not a faithful reproduction of input, and distortion occurs. In this case, as input increases, output-to-input ratio (amplification factor or gain) decreases, as indicated by decreasing slope of curve. At the high input levels, the circuit does not amplify as much as it does at the low input levels, which flattens the peaks of output wave. This happens with all the amplifiers if the input signal amplitude is large enough. In some of the radio-frequency circuits this is all right; in hi-fi audio systems, it is not alright.The linear amplifiers are designed to work with the input signals up to certain peak (maximum instantaneous) amplitude. If peak input exceeds this level, the amplifier will become nonlinear, and distortion will result. In hi-fi system equipped with VU or

distortion meters, excessive input will cause needles to kick into red range of scale during peaks. If input is driven up still more, needle will stay in red range much of the time. Some people might say it is cool to run a hi-fi amplifier in this way. If lousy sound is what you want, maybe this is true. But it is stupid generally, and in some cases can damage the system components.distortion meters, excessive input will cause needles to kick into red range of scale during peaks. If input is driven up still more, needle will stay in red range much of the time. Some people might say it is cool to run a hi-fi amplifier in this way. If lousy sound is what you want, maybe this is true. But it is stupid generally, and in some cases can damage the system components