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

energy Energy () is the physical quantity, quantitative physical property, property that is transferred to a physical body, body or to a physical system, recognizable in the performance of Work (thermodynamics), work and in the form of heat and l ...

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 ele ...

and

capacitive coupling Capacitive coupling (electronics), coupling is the transfer of energy within an electrical network or between distant networks by means of displacement current between circuit(s) node (circuits) , nodes, induced by the electric field. This coup ...

. 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 value of the waveform. A waveform with zero mean or no DC bias is known as a ''DC balanced'' or ''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. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...

including

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

. Such coupling may be achieved by a

wire file:Sample cross-section of high tension power (pylon) line.jpg, 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 d ...

resistor A resistor is a passive two-terminal electronic 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 e ...

, or common terminal, 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 electronic voltage amplifier with a differential input, a (usually) single-ended output, and an extremely high gain. Its name comes from its original use of performing mathem ...

. 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 nextany 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 op-amps, since large coupling

capacitor In electrical engineering, a capacitor is a device that stores electrical energy by accumulating electric charges on two closely spaced surfaces that are insulated from each other. The capacitor was originally known as the condenser, a term st ...

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

power amplifier An audio power amplifier (or power amp) 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 loudspeakers or headphones. Audio power a ...

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 semiconductor type of resistor in which the resistance is strongly dependent on temperature. The word ''thermistor'' is a portmanteau of ''thermal'' and ''resistor''. The varying resistance with temperature allows these devices ...

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 ...

s, 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.

References

*{{FS1037C MS188

External links

Common Impedance Coupling
Electromagnetic compatibility Electronic amplifiers