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Capacitivecoupling is the transfer of energy within anelectrical network or between distant networks by means ofdisplacement 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 acapacitor between two nodes.[1] Where analysis of many points in a circuit is carried out, the capacitance at each point and between points can be described in amatrix form.
Inanalog circuits, a coupling capacitor is used to connect two circuits such that only theAC signal from the first circuit can pass through to the next whileDC is blocked. This technique helps to isolate theDC bias settings of the two coupled circuits. Capacitive coupling is also known asAC coupling and the capacitor used for the purpose is also known as aDC-blocking capacitor.
A coupling capacitor's ability to prevent a DC load from interfering with an AC source is particularly useful inClass A amplifier circuits by preventing a 0 volt input being passed to a transistor with additional resistor biasing; creating continuous amplification.
Capacitive coupling decreases thelow frequency gain of a system containing capacitively coupled units. Each coupling capacitor along with the inputelectrical impedance of the next stage forms ahigh-pass filter and the sequence of filters results in a cumulative filter with acutoff frequency that may be higher than those of each individual filter.
Coupling capacitors can also introduce nonlineardistortion 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 RCcutoff 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 lowvoltage coefficient, and by using large values that put the cutoff frequency far lower than the frequencies of the signal.[2][3]
AC coupling is also widely used in digital circuits to transmit digital signals with a zeroDC component, known asDC-balanced signals. DC-balanced waveforms are useful in communications systems, since they can be used over AC-coupled electrical connections to avoid voltage imbalance problems and charge accumulation between connected systems or components.
For this reason, most modernline codes are designed to produce DC-balanced waveforms. The most common classes of DC-balanced line codes areconstant-weight codes andpaired-disparity codes.
A gimmick loop is a simple type of capacitive coupler: two closely spaced strands of wire. It provides capacitive coupling of a fewpicofarads between two nodes. Usually the wires are twisted together.[4][5]
Capacitive coupling is often unintended, such as the capacitance between two wires orPCB traces that are next to each other. One signal may capacitively couple with another and cause what appears to benoise. To reduce coupling, wires or traces are often separated as much as possible, or ground lines orground planes are run in between signals that might affect each other, so that the lines capacitively couple to ground rather than each other. Prototypes of high-frequency (tens of megahertz) or high-gain analog circuits often use circuits that are built over a ground plane to control unwanted coupling. If a high-gainamplifier's output capacitively couples to its input it may become anelectronic oscillator.
This article incorporatespublic domain material fromFederal Standard 1037C.General Services Administration. Archived fromthe original on 2022-01-22. (in support ofMIL-STD-188).