Explanation
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 of the load device is much greater than the output impedance 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: 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 ZS and ZL 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 ZL 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 ZL, one can maximize both the voltage and current (and therefore, the power) at the load by minimizing ZS. 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.
Read more about this topic: Impedance Bridging
Famous quotes containing the word explanation:
“We live between two worlds; we soar in the atmosphere; we creep upon the soil; we have the aspirations of creators and the propensities of quadrupeds. There can be but one explanation of this fact. We are passing from the animal into a higher form, and the drama of this planet is in its second act.”
—W. Winwood Reade (1838–1875)
“There is no explanation for evil. It must be looked upon as a necessary part of the order of the universe. To ignore it is childish, to bewail it senseless.”
—W. Somerset Maugham (1874–1965)
“The explanation of the propensity of the English people to portrait painting is to be found in their relish for a Fact. Let a man do the grandest things, fight the greatest battles, or be distinguished by the most brilliant personal heroism, yet the English people would prefer his portrait to a painting of the great deed. The likeness they can judge of; his existence is a Fact. But the truth of the picture of his deeds they cannot judge of, for they have no imagination.”
—Benjamin Haydon (1786–1846)