Impedance bridging

For the amplifier configuration, see bridged amplifier.

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 line level audio connections are bridging, to ensure the high fidelity transfer of a voltage signal from one device in the chain to the next. If the bridging criterion is not satisfied, there may be distortion, altered frequency response or both.

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

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 same order of magnitude 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.

Although the load impedance is significantly greater than the source impedance in this situation, the purpose here is not to transmit a voltage signal with great fidelity from input to output. Therefore, this situation is not an instance of impedance bridging. (If voltage transfer mattered in driving a loudspeaker, it would be easy to simply design the loudspeaker to have a high impedance.) Furthermore, the goal is to transfer power to the speaker, whereas the goal of impedance bridging is the exact opposite: to minimize transmitted power, allowing devices not to require powerful current sources to drive their outputs.

The reason an amplifier can transmit maximum power to a loudspeaker even though the speaker's impedance is much higher is that the power amplifier amplifies current. Its output impedance is kept as low as possible to minimize the current's power dissipation in the amplifier itself, thereby maximizing dissipation in the speaker. The low output impedance also dampens the speaker's motion.

See also

• DI box

References

Bartlett, B, IMPEDANCE FAQ, http://www.tape.com/resource/impedance.html