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Concept
Capacitive coupling
Summary
Capacitive coupling is the transfer of energy within an electrical network or between distant networks by means of displacement current between circuit(s) nodes, induced by the electric field. This coupling can have an intentional or accidental effect.
In its simplest implementation, capacitive coupling is achieved by placing a capacitor between two nodes. Where analysis of many points in a circuit is carried out, the capacitance at each point and between points can be described in a matrix form.
In analog circuits, a coupling capacitor is used to connect two circuits such that only the AC signal from the first circuit can pass through to the next while DC is blocked. This technique helps to isolate the DC bias settings of the two coupled circuits. Capacitive coupling is also known as AC coupling and the capacitor used for the purpose is also known as a DC-blocking capacitor.
A coupling capacitor's ability to prevent a DC load from interfering with an AC source is particularly useful in Class A amplifier circuits by preventing a 0 volt input being passed to a transistor with additional resistor biasing; creating continuous amplification.
Capacitive coupling decreases the low frequency gain of a system containing capacitively coupled units. Each coupling capacitor along with the input electrical impedance of the next stage forms a high-pass filter and the sequence of filters results in a cumulative filter with a cutoff frequency that may be higher than those of each individual filter.
Coupling capacitors can also introduce nonlinear distortion at low frequencies. This is not an issue at high frequencies because the voltage across the capacitor stays very close to zero. However, if a signal passing through the coupling capacitance has a frequency that is low relative to the RC cutoff frequency, voltages can develop across the capacitor, which for some capacitor types results in changes of capacitance, leading to distortion. This is avoided by choosing capacitor types that have low voltage coefficient, and by using large values that put the cutoff frequency far lower than the frequencies of the signal.
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