compendium
nonlinear transmission characteristic curve of amplifiers

An ideal amplifier amplifies the input signal by a constant factor. Even if the signal propagation delay through the amplifier is constant in the most important frequency range, non-sinusoidal signals (for instance the mixture of signals of speech and music inside an audio-amplifier) are beeing transmitted unaltered.
A non-linear transmission characteristic curve can be approximated by a power polynomial:

 
(60)
When amplifying a sinusoidal alternating voltage this non-linear tansmission characteristic curve leads to constant components and higher even-numbered harmonics up to the order n of the power polynomal:

 
(60a)
Look at the experiments on linear distortion to find out more about the occurence of constant components and higher harmonics in the amplitude spectrum during the approximation of of the transmission characteristic curve arround the operating point by a power polynomal up to the 3rd order.