 | This articleneeds additional citations forverification. Please helpimprove this article byadding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "Direct coupling" – news ·newspapers ·books ·scholar ·JSTOR(February 2014) (Learn how and when to remove this message) |
In electronics,direct coupling orDC coupling (also calledconductive coupling[1] andgalvanic coupling) is the transfer of electricalenergy by means of physical contact via a conductive medium, in contrast toinductive coupling andcapacitive coupling. It is a way of interconnecting two circuits such that, in addition to transferring the AC signal (or information), the first circuit also providesDC bias to the second. Thus, DC blocking capacitors are not used or needed to interconnect the circuits. Conductive coupling passes the full spectrum offrequencies includingdirect current.
Suchcoupling may be achieved by awire,resistor, or commonterminal, such as abinding post or metallicbonding.
The provision of DC bias only occurs in a group of circuits that forms a single unit, such as anop-amp. Here the internal units or portions of the op-amp (like the input stage, voltage gain stage, and output stage) will be direct coupled and will also be used to set up the bias conditions inside the op-amp (the input stage will also supply the input bias to the voltage gain stage, for example). However, when two op-amps are directly coupled the first op-amp will supply any bias to the next - any DC at its output will form the input for the next. The resulting output of the second op-amp now represents an offset error if it is not the intended one.
This technique is used by default in circuits likeIC op-amps, since large couplingcapacitors cannot be fabricated on-chip. That said, some discrete circuits (such aspower amplifiers) also employ direct coupling to cut cost and improve low frequency performance.
One advantage or disadvantage (depending on application) of direct coupling is that any DC at the input appears as a validsignal to the system, and so it will be transferred from the input to the output (or between two directly coupled circuits). If this is not a desired result, then the term used for the output signal isoutput offset error, and the corresponding input signal is known asinput offset error.
Temperature drift and device mismatches are the major causes of offset errors, and circuits employing direct coupling often integrate offset nulling mechanisms. Some circuits (like power amplifiers) even use coupling capacitors—except that these are presentonly at the input (and/or output) of the whole system but not between the individual circuit units inside the system.
The advantage of direct coupling is very good low frequency response, often from DC to the highest operating frequency that the system will allow. All applications that require monitoring of slowly changing signals (such as those fromthermistors,thermocouples,strain gages, etc.) must have a very good DC amplification with minimum offset errors and hence they must be directly coupled throughout, and have offset correction or trimming incorporated into them.
The circuits we have considered so far may be regarded as conductively coupled, because one loop affects the neighboring loop through current conduction. When two loops with or without contacts between them affect each other through the magnetic field generated by one of them, they are said to be magnetically coupled.
This article incorporatespublic domain material fromFederal Standard 1037C.General Services Administration. Archived fromthe original on 2022-01-22. (in support ofMIL-STD-188).