Circuits are coupled in different ways to perform in specific fashion. There are four different coupling methods used: AC coupling, DC Coupling, Transformer Coupling, and Optical Coupling.

AC Coupling

This coupling will pass AC signals from transistor Q1 to transistor Q2 via the capacitor and block DC signals.

DC Coupling

This coupling will pass AC and DC signals from transistor Q1 to transistor Q2.

Transformer Coupling

This coupling will pass AC signals from transistor Q1 to transistor Q2 and block DC signals.

Optical Coupling

This coupling will pass AC and DC signals from the transmitting stage to the receiver stage. Signals are passed optically and not electrically. This coupling provides immunity against electrical noise. Alignment between the optical transmitter and receiver may be required.

Different couplings have different frequency responses depending on the coupling elements and the stage frequency response. The lowest bandwidth of any of the three elements (stage 1, stage 2, and coupling element) will determine the bottleneck bandwidth of the entire coupling system. Coupling may boost up or reduce (attenuate) the signal while passing from stage to another depending on the circuit setup.

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