Impedance bridging

Summary

In electronics, especially audio and sound recording, a high impedance bridging, voltage bridging, or simply bridging connection is one in which the load impedance is much larger than the source impedance.[1][2][3] In cases where only the load impedance can be varied, maximizing the load impedance serves to minimize the current drawn by the load, and maximize the voltage signal across the load. Essentially, the load is measuring the source's voltage without affecting it. In cases where only the source impedance can be varied, minimizing the source impedance will maximize the power (and the current) delivered to the load and, as before, maximize the voltage signal across the load.

ExplanationEdit

 

When the output of a device (consisting of the voltage source VS and output impedance ZS in illustration) is connected to the input of another device (the load impedance ZL in the illustration), these two impedances form a voltage divider:

 

One can maximize the signal level VL by using a voltage source whose output impedance ZS is as small as possible and by using a receiving device whose input impedance ZL is as large as possible. When   (typically by at least ten times), this is called a bridging connection and has a number of effects including:

  • Advantages:
    • Increasing the ZL/ZS ratio reduces attenuation of the voltage signal, which helps maintain a high signal-to-noise ratio
    • Increasing ZL reduces current drawn from the source device, which helps avoid wasting power and helps reduce distortion
  • Disadvantages:
    • Increasing ZL possibly increases environmental noise pickup since to the combined parallel impedance of ZS || ZL (dominated by ZS) increases slightly, which makes it easier for stray noise to drive the signal node

ApplicationsEdit

Receiving voltage signals from sources with unchangeable ZSEdit

Bridging is typically used for 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 unchangeable output impedance ZS. Fortunately, the input impedance of modern op-amp circuits (and many old vacuum tube circuits) is often naturally much higher than the output impedance of these signal sources and thus are naturally-suited for impedance bridging when receiving and amplifying these voltage signals.

In the cases of devices with very high output impedances, 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).

Maximizing power at a load with an unchangeable ZLEdit

Given an unchangeable ZL and VS while ZS can be freely changed, one can maximize both the voltage and current (and therefore, the power) at the load through impedance bridging by minimizing ZS. This is because the power delivered to the load in the above circuit (assuming all impedances are purely real) is:

 

As can be seen, to maximize PL, one needs to minimize RS.

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 8 Ω), so to deliver the maximum power to the speaker, the output impedance of the amplifier should be as small as possible (ideally zero).

See alsoEdit

ReferencesEdit

  1. ^ Eargle, John; Foreman, Chris (2002-01-01). Audio Engineering for Sound Reinforcement. Hal Leonard Corporation. ISBN 9780634043550. In all modern usage the microphone looks into an impedance in the 2000 ohm range or higher, and this represents what is called a bridging load, one which is effectively an open circuit load for the microphone.
  2. ^ Davis, Gary D.; Jones, Ralph (1989-01-01). The Sound Reinforcement Handbook. Hal Leonard Corporation. ISBN 9780881889000. A circuit where the input termination impedance is a minimum of some 10 times the source impedance of the output driving that input is said to be a bridging input
  3. ^ Holman, Tomlinson (2012-11-12). Sound for Film and Television. Taylor & Francis. ISBN 9781136046094. In the case of bridging systems, we say the source impedance is low and the load impedances are high