Direct coupling


In electronics, direct coupling or DC coupling is the transfer of electrical energy by means of physical contact via a conductive medium, in contrast to inductive coupling and capacitive coupling. It is a way of interconnecting two circuits such that, in addition to transferring the AC signal, the also provides DC bias to the next. Thus, DC blocking capacitors are not used or needed to interconnect the circuits. Conductive coupling passes the full spectrum of frequencies including direct current.
Such coupling may be achieved by a wire, resistor, or common terminal, such as 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. Here the internal units or portions of the op-amp will be direct coupled and will also be used to set up the bias conditions inside the op-amp. 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 op-amps, since large coupling capacitors cannot be fabricated on-chip. That said, some discrete circuits also employ direct coupling to cut cost and improve low frequency performance.

Offset error

One advantage or disadvantage 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. 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 even use coupling capacitors—except that these are present only at the input 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 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.