Editing Equivalent circuit (section)

===Thévenin and Norton equivalents===
One of linear circuit theory's most surprising properties relates to the ability to treat any two-terminal circuit no matter how complex as behaving as only a source and an impedance, which have either of two simple equivalent circuit forms:<ref name="Johnson (2003a)"/><ref name="Johnson (2003b)">{{cite journal|last=Johnson|first=D.H.|title=Origins of the equivalent circuit concept: the current-source equivalent|journal=Proceedings of the IEEE|year=2003b|volume=91|issue=5|pages=817–821|doi=10.1109/JPROC.2003.811795|url=http://www.ece.rice.edu/~dhj/paper2.pdf}}</ref> 
*[[Thévenin's theorem|Thévenin equivalent]] – Any linear two-terminal circuit can be replaced by a single [[voltage source]] and a series impedance.
*[[Norton equivalent]] – Any linear two-terminal circuit can be replaced by a [[current source]] and a parallel impedance.
However, the single impedance can be of arbitrary complexity (as a function of frequency) and may be irreducible to a simpler form.