Article : Andy Collinson
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Introduction
To connect analogue circuits together there are three basic methods, Direct Coupling, Capacitive Coupling and transformer coupling. All have different properties and can be used in any linear or non-linear circuit.

Direct Coupling
With direct coupling, or DC coupling, two components are connected directly to each other. In the diagram, the collector of Q1 is wired direct to the base of Q2. The bias level of Q1 is calculated first, and this will also be the bias for the base of Q2. The advantages are, minimum parts and a good low frequency response, down to dc.

Capacitive Coupling
With capacitive coupling, two stages are connected using a coupling capacitor,Cc in the diagram. Using this method, the bias levels on the first stage, F1 are isolated from the next stage. Here resistor Rb would be used to bias this stage. The capacitor Cc and Rb and the input impedance at Q1 form a high pass filter, allowing AC current to pass, but blocking DC.

Transformer Coupling
Here, the two stages are isolated with a transformer. Bias for the second stage is via a biasing network. The advantage of this method, is that the turns ratio of the transformer can be used to provide gain, and also a frequency selective response can be obtained. This technique is used widely in RF applications.