In general, neither impedance nor admittance can be time varying as they are defined for complex exponentials for –∞ < t < +∞. If the complex exponential voltage–current ratio changes over time or amplitude, the circuit element cannot be described using the frequency domain. However, many systems (e.g., varicaps that are used in radio tuners) may exhibit non-linear or time-varying voltage–current ratios that appear to be linear time-invariant (LTI) for small signals over small observation windows; hence, they can be roughly described as having a time-varying impedance. That is, this description is an approximation; over large signal swings or observation windows, the voltage–current relationship is non-LTI and cannot be described by impedance.
Read more about this topic: Electrical Impedance
Other articles related to "impedance, variable impedance":
... As mentioned above, can be set to any desired impedance without affecting the input impedance ... The input port still presents an impedance of (provided that the output is terminated in ) but the output port no longer presents a constant impedance ... are capable of being reversed so that is then presented at the output and the variable impedance is presented at the input ...
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