Editing Capacitive coupling (section)

==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>