Editing Capacitive coupling

{{Short description|Transfer of energy between circuits}}
{{Redirect-distinguish|Coupling capacitor|Decoupling capacitor}}
{{Redirect-distinguish|DC blocking capacitor|Bypass capacitor}}
{{More footnotes needed|date=August 2023}}
'''Capacitive [[coupling (electronics)|coupling]]''' is the transfer of energy within an [[electrical network]] or between distant networks by means of [[displacement current]] between circuit(s) [[node (circuits) | nodes]], induced by the electric field. This coupling can have an intentional or accidental effect.

[[File:Fluorescent tube under electric line.jpg|thumb|upright|alt=Capacitive coupling from high-voltage power lines can light a lamp continuously at low intensity.|Capacitive coupling from high-voltage power lines can light a lamp continuously at low intensity.]]

In its simplest implementation, capacitive coupling is achieved by placing a [[capacitor]] between two nodes.<ref name="J279">{{cite book|last=Joffe, Elya|title=Grounds for Grounding:A Circuit to System Handbook|year=2010|publisher=Wiley-IEEE|isbn=978-0-471-66008-8|pages=277}}</ref>  Where analysis of many points in a circuit is carried out, the capacitance at each point and between points can be described in a [[Capacitance#Capacitance matrix|matrix form]].

==Use in analog circuits==

[[File:Mallory 150 coupling capacitor.jpg|thumb|Polyester [[film capacitor]]s, commonly used for coupling between two circuits.]]In [[analog circuit]]s, a coupling capacitor is used to connect two circuits such that only the [[alternating current|AC]] signal from the first circuit can pass through to the next while [[direct current|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.<ref>{{Cite web|title = Capacitor Characteristics|url = http://sound.whsites.net/articles/capacitors.htm#s23|website = sound.whsites.net|access-date = 2015-06-06}}</ref><ref>{{Cite web|title = Signal distortion from high-K ceramic capacitors|url = http://www.edn.com/design/analog/4416466/Signal-distortion-from-high-K-ceramic-capacitors|access-date = 2015-06-06|first = John|last = Caldwell}}</ref>

==Use in digital circuits==

AC coupling is also widely used in digital circuits to transmit digital signals with a zero [[DC component]], known as [[DC-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 modern [[line code]]s are designed to produce DC-balanced waveforms. The most common classes of DC-balanced line codes are [[constant-weight code]]s and [[paired-disparity code]]s.

==Gimmick loop==

A [[Gimmick capacitor | gimmick loop]] is a simple type of capacitive coupler: two closely spaced strands of wire. It provides capacitive coupling of a few [[picofarad]]s between two nodes. Usually the wires are twisted together.<ref>{{cite book | title = Applications of Electronics | author = Bernard Grob and Milton Sol Kiver | year = 1960 | publisher = McGraw–Hill | pages = 300–01 | url = https://books.google.com/books?id=g9lUleIpznwC&q=gimmick}}</ref><ref>{{cite book | title = The Forrest Mims Circuit Scrapbook | author = Forrest M. Mims | publisher =  Newnes | year = 2000 | pages = 95–96 | isbn = 1-878707-48-5 | url = https://books.google.com/books?id=STzitya5iwgC&q=gimmick+capacitor&pg=RA1-PA71}}</ref>

==Parasitic capacitive coupling==

Capacitive coupling is often unintended, such as the capacitance between two wires or [[printed circuit board|PCB]] traces that are next to each other. One signal may capacitively couple with another and cause what appears to be [[noise (electronic)|noise]]. To reduce coupling, wires or traces are often separated as much as possible, or ground lines or [[ground plane]]s 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 (electronics)|gain]] analog circuits often use circuits that are built over a ground plane to control unwanted coupling. If a high-[[gain (electronics)|gain]] [[amplifier]]'s output capacitively couples to its input it may become an [[electronic oscillator]].

==See also==

* [[Coupling (electronics)]]
* [[DC block]]
* [[Decoupling (electronics)]]
** [[Decoupling capacitor]]
* [[Direct coupling]]
* [[Differential capacitance]]
* [[RC coupling]]
* [[Crosstalk]]

==References==

{{Reflist}}
*{{FS1037C MS188}}

==External links==
*[[Howard Johnson (electrical engineer)|Howard Johnson]]: [http://www.sigcon.com/Pubs/news/4_15.htm When to use AC coupling], [http://www.sigcon.com/Pubs/news/7_09.htm DC Blocking Capacitor Value]
*[[Texas Instruments]]: [http://focus.ti.com/general/docs/lit/getliterature.tsp?baseLiteratureNumber=scaa059 AC-Coupling Between Differential LVPECL, LVDS, HSTL, and CML] (PDF)

[[Category:Capacitors]]
[[Category:Electromagnetic compatibility]]