Useful time-variable circuit-element equivalences

Because time-variable circuit elements have properties not possessed by time-invariant ones, such as the ability to modulate or amplify with low noise, it is useful to have different ways of looking at them. Here we present general equivalences for time-variable inductors, capacitors, resistors and gyrators, using time-invariant elements and time-variable transformers, which allow various properties to be determined and interpreted on physical grounds.