Direct coupling

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In electronics direct coupling is a way of interconnecting two circuits such that, in addition to transferring the AC signal (or information), the first stage 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, and this method is also known as DC coupling. This technique is used by default in circuits like IC op-amps, since large coupling capacitors cannot be fabricated on-chip. That said, even discrete circuits (such as power amplifiers) employ direct coupling for improved low frequency performance. 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. 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. The advantage of direct coupling is very good low frequency response, often from DC (if the input/output coupling capacitors are not used) to the highest operating frequency that the system will allow. Most industrial applications that require monitoring of slowly changing signals (such as those from thermistors, thermocouples, etc.) must have a very good DC amplification with minimum offset errors and hence they have to be directly coupled throughout by default, and have offset correction or trimming incorporated into them.

‡ – This is only true for 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 not 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.