17-11-2012, 02:42 PM
Two-port network
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A two-port network (or four-terminal network or quadripole) is
an electrical circuit or device with two pairs of terminals (i.e., the
circuit connects two dipoles). Two terminals constitute a port if
they satisfy the essential requirement known as the port condition:
the same current must enter and leave a port.[1][2] Examples include
small-signal models for transistors (such as the hybrid-pi model),
filters and matching networks. The analysis of passive two-port
networks is an outgrowth of reciprocity theorems first derived by
Lorentz[3].
A two-port network makes possible the isolation of either a
complete circuit or part of it and replacing it by its characteristic
parameters. Once this is done, the isolated part of the circuit becomes a "black box" with a set of
distinctive properties, enabling us to abstract away its specific physical buildup, thus simplifying
analysis. Any linear circuit with four terminals can be transformed into a two-port network provided
that it does not contain an independent source and satisfies the port conditions.