A coupling capacitor is a capacitor which is used to couple or link together only the AC signal from one circuit element to another. The capacitor blocks the DC signal from entering the second element and, thus, only passes the AC signal.
Coupling capacitors (or dc blocking capacitors) are use to decouple ac and dc signals so as not to disturb the quiescent point of the circuit when ac signals are injected at the input. Bypass capacitors are used to force signal currents around elements by providing a low impedance path at the frequency.
While coupling capacitors pass through AC signals to output, do pretty much the opposite; decoupling capacitors shunt AC signals to ground and passes through the DC signal in a circuit. Decoupling capacitors are designed to purify DC signals of AC noise.
Its construction is very simple. Just a dielectric is present in between the parallel plate capacitors. This coupling capacitor is good at obtaining final output as AC signals. There exist decoupling capacitors as well in which the output generated is consisting of DC signals. Hence coupling capacitors are preferred in analog circuits.
For example, a coupling capacitor normally is used in an audio circuits, such as a microphone circuit. DC power is used to give power to parts of the circuit, such as the microphone, which needs DC power to operate. So DC signals must be present in the circuit for powering purposes.
In order to place a capacitor in a circuit for AC coupling, the capacitor is connected in series with the load to be coupled. A capacitor is able to block low frequencies, such as DC, and pass high frequencies, such as AC, because it is a reactive device. It responds to different frequencies in different ways.