Editing Output impedance (section)

== Description ==
All devices and connections have non-zero resistance and reactance, and therefore no device can be a perfect source. The output impedance is often used to model the source's response to current flow. Some portion of the device's measured output impedance may not physically exist within the device; some are artifacts that are due to the chemical, thermodynamic, or mechanical properties of the source. This impedance can be imagined as an impedance in series with an ideal [[voltage source]], or in parallel with an ideal [[current source]] (''see'':  [[Series and parallel circuits]]). 

Sources are modeled as ideal sources (ideal meaning sources that always keep the desired value) combined with their output impedance. The output impedance is defined as this modeled and/or real impedance in series with an ideal voltage source. Mathematically, current and voltage sources can be converted to each other using [[Thévenin's theorem]] and [[Norton's theorem]].

In the case of a [[nonlinear device]], such as a [[transistor]], the term "output impedance" usually refers to the effect upon a small-amplitude signal, and will vary with the [[bias point]] of the transistor, that is, with the direct current (DC) and voltage applied to the device.