Direct coupling

In electronics, direct coupling or DC coupling (also called conductive 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 (or information), the first stage [what does this mean?] also provides DC bias to the next. Thus, there is no need for a DC blocking capacitor to be used in order 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.

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 (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 like IC op-amps, since large coupling capacitors cannot be fabricated on-chip. That said, some discrete circuits (such as power amplifiers) also employ direct coupling to cut cost and improve low frequency performance.

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