Impedance Bridging - Explanation

Explanation

When the output of a device (consisting of the voltage source V_S and output impedance Z_S in illustration) is connected to the input of another device (the load impedance Z_L in the illustration), it is a bridging connection if the input impedance of the load device is much greater than the output impedance of the source device.

Given an unchangeable Z_S, one can maximize the voltage across Z_L by making Z_L as large as possible. This also correspondingly minimizes the current drawn from the source device. This has a number of effects including: increased signal level (when the signal in question is completely described by the voltage, as often is the case with audio), reduced distortion due to the source having to output less current, and possibly increased environment noise pickup (since the combined parallel impedance of Z_S and Z_L becomes higher and makes it easier for stray noise to drive the signal node). This situation is typicallly encountered in line or mic level connections where the source device (such as the line-out of an audio player or the output of a microphone) has a fixed output impedance which cannot be changed. In such cases, maximum signal level with minimum distortion is obtained with a receiving device that has as high an input impedance as possible (not considering noise). In the cases of devices with very high output impdances, such as with a guitar or a high-Z mic, a DI box can be used to convert the high output impedances to a lower impedance so as to not require the receiving device to have outrageously high input impedance and thus suffer drawbacks such as increased noise pickup with long cable runs. In such cases, the DI box is placed close to the source device (such as the guitar and mic), and any long cables are attached to the output of the DI box (which usually also converts unbalanced signals to balanced signals to further increase noise immunity). Note that the increased voltage signal gotten from increasing Z_L comes at a cost, since in order to drive other circuits down the chain with the higher voltage, a powered buffering stage is required (such is the basic operating principle of active DI boxes and any high impedance receiving circuit in general).

Given an unchangeable Z_L, one can maximize both the voltage and current (and therefore, the power) at the load by minimizing Z_S. This situation is mostly encountered in the interface between an audio amplifier and a loudspeaker. In such cases, the impedance of the loudspeaker is fixed (a typical value being 8Ω), so to deliver the maximum power to the speaker, the output impedance of the amplifer should be made as small as possible (ideally zero).

Both of the above cases fits the bridging criterion mathematically, however most uses of the term impedance bridging applies only to the first case.

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