, capacitive coupling
is the transfer of energy within an electrical network
by means of the capacitance
between circuit nodes. This coupling can be an intentional or accidental effect. Capacitive coupling is typically achieved by placing a capacitor in series
with the signal to be coupled.
Use in analog circuits
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 known as a coupling or DC blocking capacitor
. Capacitive coupling has the disadvantage of degrading the low frequency
performance 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 each successive filter results in a cumulative filter with a -3dB
frequency that may be higher than each individual filter. So for adequate low frequency response the capacitors used must have high capacitance ratings. They should be high enough that the reactance
of each is at least ten times the input impedance of each stage, at the lowest frequency of interest. This disadvantage of capacitively coupling is largely minimized in directly coupled
Use in digital circuits
AC coupling is also widely used in digital circuits to transmit digital signal 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 codes are designed to produce DC-balanced waveforms. The most common classes of DC-balanced line codes are constant-weight codes and paired-disparity codes.
A "gimmick" is a very simple kind of capacitive coupling: a piece of wire that is placed in proximity to another one, providing a capacitive coupling between two nodes of a few picofarads in value. Sometimes the wires are twisted together for physical stability.
Parasitic capacitive coupling
Capacitive coupling is often unintended, such as the capacitance between two wires or PCB
traces that are next to each other. Often one signal can capacitively couple with another and cause what appears to be noise
. To reduce coupling, wires or traces are often separated as much as possible, or ground lines or ground planes
are run in between signals that might affect each other. Breadboards are particularly prone to these issues due to the long pieces of metal that line every row creating a several-picofarad capacitor between lines. To prototype high-frequency (10s of MHz) or high-gain
analog circuits, often the circuits are built over a ground plane so that the signals couple to ground more than to each other. If a high-gain amplifier
's output capacitively couples to its input it often becomes an oscillator