In an electric circuit, a nonlinear element or nonlinear device is an electrical element which does not have a linear relationship between current and voltage. A diode is a simple example. The current I through a diode is a non-linear function of the voltage V across its terminals:
Other examples of nonlinear elements are transistors and other semiconductor devices, vacuum tubes, and iron core inductors and transformers when operated above their saturation current. Examples of linear elements are resistors, capacitors, and air core inductors.
Many circuit elements that operate linearly at low signal levels will show nonlinearity at higher signal levels. An example of this can be seen in many audio systems by turning up the volume. At a high enough volume level, the transistors or other amplifying elements will begin to operate nonlinearly and distort the sound. Conversely, nonlinear elements can be made to operate linearly if the signal in them is limited to a low level. If the input of a non-linear device such as a transistor only varies in a small range around a fixed value, then the input/output relation is linearized around this fixed value (usually called the quiescent point, Q-point, or Bias point). This is called a small signal model.
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