In electronics, direct coupling or DC coupling (also called conductive coupling and galvanic coupling) is the transfer of electrical

energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of heat a ...

by means of physical contact via a conductive medium, in contrast to

inductive coupling In electrical engineering, two conductors are said to be inductively coupled or magnetically coupled when they are configured in a way such that change in current through one wire induces a voltage across the ends of the other wire through el ...

and

capacitive coupling Capacitive coupling is the transfer of energy within an electrical network or between distant networks by means of displacement current between circuit(s) nodes, induced by the electric field. This coupling can have an intentional or accidental ...

. It is a way of interconnecting two circuits such that, in addition to transferring the AC signal (or information), the first circuit also provides

DC bias In signal processing, when describing a periodic function in the time domain, the DC bias, DC component, DC offset, or DC coefficient is the mean amplitude of the waveform. If the mean amplitude is zero, there is no DC bias. A waveform with no DC ...

to the second. Thus, DC blocking capacitors are not used or needed to interconnect the circuits. Conductive coupling passes the full spectrum of

frequencies Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...

including

direct current Direct current (DC) is one-directional flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor such as a wire, but can also flow through semiconductors, insulators, or even ...

. Such

coupling A coupling is a device used to connect two shafts together at their ends for the purpose of transmitting power. The primary purpose of couplings is to join two pieces of rotating equipment while permitting some degree of misalignment or end mov ...

may be achieved by a

wire Overhead power cabling. The conductor consists of seven strands of steel (centre, high tensile strength), surrounded by four outer layers of aluminium (high conductivity). Sample diameter 40 mm A wire is a flexible strand of metal. Wire is c ...

resistor A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active el ...

, or common

terminal Terminal may refer to: Computing Hardware * Terminal (electronics), a device for joining electrical circuits together * Terminal (telecommunication), a device communicating over a line * Computer terminal, a set of primary input and output devic ...

, such as a

binding post A binding post is a connector commonly used on electronic test equipment to terminate (attach) a single wire or test lead. They are also found on loudspeakers and audio amplifiers as well as other electrical equipment. History A binding post ...

or metallic bonding.

DC bias

The provision of DC bias only occurs in a group of circuits that forms a single unit, such as an

op-amp An operational amplifier (often op amp or opamp) is a DC-coupled high-gain electronic voltage amplifier with a differential input and, usually, a single-ended output. In this configuration, an op amp produces an output potential (relative to c ...

. 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.

Uses

This technique is used by default in circuits like IC

s, since large coupling

capacitor A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals. The effect of ...

s cannot be fabricated on-chip. That said, some discrete circuits (such as

power amplifier An audio power amplifier (or power amp) is an electronic amplifier that amplifies low-power electronic audio signals, such as the signal from a radio receiver or an electric guitar pickup, to a level that is high enough for driving loudspea ...

s) also employ direct coupling to cut cost and improve low frequency performance.

Offset error

One advantage or disadvantage (depending on application) of direct coupling is that any DC at the input appears as a valid ''signal'' 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 is ''output offset error'', and the corresponding input signal is known as ''input offset error''.

Error correction

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 present ''only'' at the input (and/or output) of the whole system but not between the individual circuit units inside the system.

Advantages

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 from

thermistor A thermistor is a type of resistor whose resistance is strongly dependent on temperature, more so than in standard resistors. The word thermistor is a portmanteau of ''thermal'' and ''resistor''. Thermistors are divided based on their conduction ...

thermocouple A thermocouple, also known as a "thermoelectrical thermometer", is an electrical device consisting of two dissimilar electrical conductors forming an electrical junction. A thermocouple produces a temperature-dependent voltage as a result of the ...

strain gage A strain gauge (also spelled strain gage) is a device used to measure strain on an object. Invented by Edward E. Simmons and Arthur C. Ruge in 1938, the most common type of strain gauge consists of an insulating flexible backing which supports ...

s, 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.

References

*{{FS1037C MS188

External links

Common Impedance Coupling
Electromagnetic compatibility Electronic amplifiers