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 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.