Impedance bridging

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For the amplifier configuration, see bridged amplifier.

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In electronics, especially audio and sound recording, a high impedance bridging, voltage bridging, or simply bridging connection is one which maximizes transfer of a voltage signal 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, Z_S in illustration) is connected to the input (input impedance) Z_in of another device (the load) (Z_L in the illustration), 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. This is known as terminating the line.

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

[edit] Audio amplifiers

In audio system specifications, the value of the low output impedance is described by the 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 increases the damping factor, because the source resistance is effectively reduced. In this way, damping factors into the hundreds can be reached.