- Equivalent circuit
An equivalent circuit refers to the simplest form of a circuit that retains all of the electrical characteristics of the original (and more complex) circuit. 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.cite book
author=Richard C. Dorf
title=The Electrical Engineering Handbook
pages=Fig. 27.4, p. 711
There are two very renowned two-terminal equivalent circuits:
*Thévenin equivalent - reduces a two-terminal circuit to a single
voltage sourceand a series Thévenin impedance
Norton equivalent- reduces a two terminal circuit to a current sourceand a parallel Norton impedance
For a restricted set of linear four-terminal circuits, equivalent
two-port networks can be set up. The restriction upon a two-port representation is that of a port: the current entering each port must be the same as the current leaving that port.cite book
author=P.R. Gray, P.J. Hurst, S.H. Lewis, and R.G. Meyer
title=Analysis and Design of Analog Integrated Circuits
pages=§3.2, p. 172
url=http://worldcat.org/isbn/0471321680] By linearizing a nonlinear circuit about its operating point, such a two-port representation can be made for transistors: see hybrid pi and h-parameter circuits.
Equivalent circuits also can 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 gradientacross the membrane).
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