Distortion can occur because the device characteristics is not linear. In which case nonlinear amplitude or phase distortion occur.This can occur with all the classes of amplifier operation. Distortion can also occur because the circuit elements and devices respond to the input signal differently at various frequencies.This being frequency distortion.

One technique for describing distorted but periodic wave forms uses Fourier analysis.it is the technique that describes any periodic waveform in terms of its fundamental frequency components and frequency components at integer multiples these components are called as harmonic components or harmonics.The harmonic distortion suggest the presence of those frequency component in the output which are absent on the input side of the amplifier.

The frequency component which has same frequency of the input is known as the fundamental frequency component. The other frequency component in the output, which are integer multiples of fundamental frequency component are known as harmonics.

If the frequency of fundamental component is say fo Hzs then 2 fo, 3 fo ,4 fo, etc. are the harmonic components , 2fo is called second harmonic distortion.3 fo is called third harmonic distortion.and so on.

If the input signal is a pure sine wave of frequency fo then the output should consist of only fundamental component i.e fo all other harmonic components are actually unwanted components.When they get added to the fundamental component ,the output waveform is distorted as shown in the figure: The shape of these signal is decided by amplitudes and relative phases of the harmonics that are added with the fundamental .

Usually the fundamental component has the highest amplitude .The amplitude of harmonic components decrease with increase in the order of harmonics.second has the highest harmonic than all other harmonics

Third harmonic amplitude is lower than that of the second harmonic component but higher than the rest of harmonic component.

The total harmonic distortion can be calculated by following formula.

A signal is considered to have harmonic distortion when there are harmonic frequency components (not just the. If the fundamental frequency has an amplitude A1 and the n th frequency component has an amplitude An, a harmonic distortion can be defined as

The figure for harmonic distortion is as shown: