Editing Electrical impedance (section)

== Variable impedance ==
{{see also|Network analysis (electrical circuits)#Time-varying components}}

In general, neither impedance nor admittance can vary with time, since they are defined for complex exponentials in which {{math|1= −∞ < ''t'' < +∞}}. If the complex exponential voltage to current ratio changes over time or amplitude, the circuit element cannot be described using the frequency domain. However, many components and systems (e.g., [[varicap]]s that are used in [[Tuner (radio)|radio tuners]]) may exhibit non-linear or time-varying voltage to current ratios that seem to be [[LTI system theory|linear time-invariant (LTI)]] for small signals and over small observation windows, so they can be roughly described as if they had a time-varying impedance. This description is an approximation: Over large signal swings or wide observation windows, the voltage to current relationship will not be LTI and cannot be described by impedance.