Equivalent circuit

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An equivalent circuit refers to the simplest form of a passive circuit that retains all of the electrical characteristics of the original (and more complex) circuit.

Any circuit consisting of passive components can be reduced to a basic equivalent circuit containing a power source and a single impedance representing total impedance. This process is called reduction to an equivalent circuit.

There are two types of equivalent circuits:

Thévenin equivalent - reduces the circuit to a single voltage source and an impedance
Norton equivalent - reduces the circuit to current source and an impedance

This is also done to describe and model the electrical properties of materials or biological systems like the cell membrane. The latter is modelled as a capacitor (i.e. the lipid bilayer) in parallel with resistance-battery combinations (i.e. ion channels powered by an ion gradient across the membrane).