Editing Equivalent circuit (section)

===Two-port networks===
{{main|Two-port network}}
Linear four-terminal circuits in which a signal is applied to one pair of terminals and an output is taken from another, are often modeled as [[two-port network]]s.  These can be represented by simple equivalent circuits of impedances and dependent sources.  To be analyzed as a two port network the currents applied to the circuit must satisfy the [[Port (circuit theory)|''port condition'']]:  the current entering one terminal of a port must be equal to the current leaving the other terminal of the port.<ref name=Gray>
{{cite book 
|author1=P.R. Gray |author2=P.J. Hurst |author3=S.H. Lewis |author4=R.G. Meyer |title=Analysis and Design of Analog Integrated Circuits 
|year= 2001 
|edition=Fourth 
|publisher=Wiley
|location=New York 
|isbn=978-0-471-32168-2 
|pages=§3.2, p. 172
|url=http://worldcat.org/isbn/0471321680}}
</ref> By [[small-signal model|linearizing]] a nonlinear circuit about its [[Q-point|operating point]], such a two-port representation can be made for transistors: see [[hybrid-pi model|hybrid pi]] and [[Bipolar junction transistor#h-parameter model|h-parameter]] circuits.