Editing Output impedance (section)

== Audio amplifiers ==
[[File:Source and load circuit Z.png]]

The real output impedance (Z<sub>S</sub>) of a [[power amplifier]] is usually less than 0.1&nbsp;Ω, but this is rarely specified. Instead it is "hidden" within the [[damping factor]] parameter, which is:

: <math>
DF = \frac{Z_\mathrm{L}}{Z_\mathrm{S}}
</math>

Solving for ''Z''<sub>S</sub>,

: <math>
Z_\mathrm{S} = \frac{Z_\mathrm{L}}{DF}
</math>

gives the small source impedance (output impedance) of the power amplifier.  This can be calculated from the ''Z''<sub>L</sub> of the loudspeaker (typically 2, 4, or 8 ohms) and the given value of the damping factor.

Generally in audio and [[hifi]], the input impedance of components is several times (technically, more than 10) the output impedance of the device connected to them.  This is called [[impedance bridging]] or voltage bridging.

In this case, ''Z''<sub>L</sub>>> ''Z''<sub>S</sub>, (in practice:) ''DF'' > 10

In video, RF, and other systems, impedances of inputs and outputs are the same. This is called [[impedance matching]] or a matched connection.

In this case, ''Z''<sub>S</sub> = ''Z''<sub>L</sub>, ''DF'' = 1/1 = 1 .

The actual output impedance for most devices is not the same as the rated output impedance.  A power amplifier may have a rated impedance of 8 ohms, but the actual output impedance will vary depending on circuit conditions. The rated output impedance is the impedance into which the amplifier can deliver its maximum amount of power without failing.