2. Non-linearities

There are two kinds of non-linearities: on the one hand, limitations that lead to signal clipping above a certain threshold, and on the other, imperfect linearity of the transfer characteristic. Threshold limitations only occur at high levels, whereas "soft" non-linearities are present whatever the level applied. At very low levels, however, it is clear that any characteristic can be considered linear.

Measurable quantities such as composite gain, input and output impedances, defined as small signals, can be extended to large signals. However, these quantities are much less useful, as they vary according to the average power and peak value of the applied signal. For example, knowledge of the parameter S ₂₁ (composite gain between terminations of Z ₀ = 50 Ω) for a given average...