Impedance bridging

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For the amplifier configuration, see bridged amplifier.

In electronics, especially audio and sound recording, an impedance bridging or voltage bridging connection is one which maximizes voltage transfer to the load. The other typical configuration is an impedance matching connection, which maximizes power delivered to the load.

When the output (output impedance) Z_out of a device (the source) is connected to the input (input impedance) Z_in of another device (the load), it is a bridging connection if the second device does not appreciably load the previous device.

$Z_\mathrm{load} >> Z_\mathrm{source} \,$

Essentially no power is transferred. The second device is sensitive to the output voltage of the first device, and this is maximized when loading is minimized. All good audio connections are bridging, even the connection between the power amplifier and the loudspeaker, since it is more important to accurately control the loudspeaker than to drive it with the maximum amount of power available. (Maximizing power transfer is more important in applications such as driving electric motors.)

In systems involving very long lines (such as telephone or cable TV systems), the source, line, and load impedances must be matched to prevent reflections of the signal at the ends of the line from causing reflections and echoes.

A circuit is commonly said to be bridged if the load impedance is at least ten times the source impedance.

[edit] Audio amplifiers

It has been suggested that this article be split into articles entitled Damping factor, accessible from a disambiguation page. (Discuss)

In audio system specifications, the value of the low output impedance is hidden in the word damping factor, DF, which is:

$DF = \frac{Z_\mathrm{load}}{Z_\mathrm{source}}$

Now it is easy to calculate Z_source

$Z_\mathrm{source} = \frac{Z_\mathrm{load}}{DF}$

In audio systems, Z_load is typically the nominal impedance of an 8 ohm loudspeaker. The output impedance of the amplifier is typically in the neighborhood of the impedance of the cables connecting it to the speaker (<0.1 ohm), so DF will drop rather easily.

Applying negative feedback also lowers the damping factor, because the source resistance is effectively reduced. In this way, damping factors into the hundreds can be reached.