In the common case of a circuit in which the components' values are constant and don't change with time, an alternate definition of linearity is that when a sinusoidal input voltage or current of frequency f is applied, any steady-state output of the circuit (the current through any component, or the voltage between any two points) is also sinusoidal with frequency f.[1][4] A linear circuit with constant component values is called linear time-invariant (LTI).
Informally, a linear circuit is one in which the electronic components' values (such as resistance, capacitance, inductance, gain, etc.)
Linear time-invariant circuits are important because they can process analog signals without introducing intermodulation distortion.
These also give an intuitive understanding of the qualitative behavior of the circuit, characterizing it using terms such as gain, phase shift, resonant frequency, bandwidth, Q factor, poles, and zeros.
They must be analyzed using approximate numerical methods by electronic circuit simulation computer programs such as SPICE, if accurate results are desired.
The behavior of such linear circuit elements as resistors, capacitors, and inductors can be specified by a single number (resistance, capacitance, inductance, respectively).
In contrast, a nonlinear element's behavior is specified by its detailed transfer function, which may be given by a curved line on a graph.
Nonlinear elements such as transistors tend to behave linearly when small AC signals are applied to them.