In electrical engineering and science, an equivalent circuit refers to a theoretical circuit that retains all of the electrical characteristics of a given circuit. Often, an equivalent circuit is sought that is the simplest form of a more complex circuit in order to aid analysis. In its most common form, an equivalent circuit is made up of linear, passive elements. However, more complex equivalent circuits are used that approximate the nonlinear behavior of the original circuit as well. These more complex circuits often are called macromodels of the original circuit. An example of a macromodel is the Boyle circuit for the 741 operational amplifier.
Equivalent circuits of descrete components also can describe and model the electrical properties of continuous materials or biological systems in which current does not actually flow in defined circuits. For example, a cell membrane can be 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).
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