Impedance bridging

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. In cases where only the load impedance can be varied, maximizing the load impedance serves to both minimize the current drawn by the load and maximize the voltage signal across 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 serves to maximize the power delivered to the load. A different configuration is an impedance matching connection in which the source and load impedances are either equal or complex conjugates. Such a configuration serves to either prevent reflections when transmission lines are involved, or to maximize power delivered to the load given an unchangeable source impedance. 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. In cases where only the load impedance can be varied, maximizing the load impedance serves to both minimize the current drawn by the load and maximize the voltage signal across 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 serves to maximize the power delivered to the load. A different configuration is an impedance matching connection in which the source and load impedances are either equal or complex conjugates. Such a configuration serves to either prevent reflections when transmission lines are involved, or to maximize power delivered to the load given an unchangeable source impedance. 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), it is a bridging connection if the input impedance (ZL) of the load device is much greater than (typically at least ten times) the output impedance (ZS) of the source device. Given an unchangeable ZS, one can maximize the voltage across ZL by making ZL as large as possible. This also correspondingly minimizes the current drawn from the source device. This has a number of effects including: This situation is typically 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 (this also optimizes noise as it minimizes attenuation). 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). Given an unchangeable ZL, one can maximize both the voltage and current (and therefore, the power) at the load 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 8Ω), so to deliver the maximum power to the speaker, the output impedance of the amplifier should be as small as possible (ideally zero). Again, this is for a case where ZL cannot be changed while ZS can be freely changed. In tube amplifiers where ZS is high by nature, maximum power delivery to the speaker (a much lower impedance) is achieved through a transformer that matches the high output impedance of the tube amp to the lower impedance of the speaker.