# Capacitive coupling

The capacitive coupling provides in the field of electrical engineering the transfer of energy between two physically non-interconnected conductors due to a mutual different electric potential . It may in this case is an undesired parasitic effect (z. B. crosstalk two cable lines) or to a desired Act (e.g. for a contactless measurement method).

The capacitive coupling is frequency dependent and increases with increasing frequency. Since the electrical capacitance between conductors decreases with increasing distance, the capacitive coupling is very dependent on the distance and can therefore only be used for small distances.

The unwanted capacitive coupling is (in addition to inductive coupling , galvanic coupling and radiation coupling ) one of the coupling types that plays a role in electromagnetic compatibility (EMC). It is a cause of undesired crosstalk between parallel telephone lines. As a desired effect, it is used in circuit technology for coupling two circuits under certain conditions; appropriately selected capacitors are used as coupling capacitance .

## Electromagnetic compatibility

Principle of capacitive coupling, with field description (A) and as a network model (B)

In the context of EMC, capacitive coupling relates to various spatially adjacent electrical circuit parts which are coupled as a result of stray capacitances. If there is a potential difference between two circuits , i. H. if they are at different potentials, an electric field E is created . When changed over time, this electric field can cause displacement currents which manifest themselves as voltage drops in an interference voltage superimposed on the useful signal . The field effect is modeled in networks, as in the figure on the right, as a capacitance with the stray capacitance C s .

## Circuit application

Transistor amplifier stage with coupling capacitors at input and output

If two parts of a circuit have different direct voltage potentials, but an alternating voltage signal (example: audio signal) is to be transmitted from one part of the circuit to the other, a capacitor is connected in between. If it has a sufficiently high capacitance, its impedance for the alternating voltage signals to be transmitted is negligible or at least controllable; the DC voltage component, which is required, among other things, for setting the operating point of the amplifier stage, is blocked.

## Individual evidence

1. Capacitive coupling. German Society for EMV Technology, accessed on August 10, 2020 .
2. Heinz Zenkner, Markus Haller: Phenomena in EMC: Electromagnetic coupling and signal composition. In: elektroniknet.de. April 16, 2018, accessed August 10, 2020 .
3. Adolf Schwab, Wolfgang Kürner: Electromagnetic Compatibility . Springer, Berlin / Heidelberg 2011, ISBN 978-3-642-16609-9 , pp. 123 , doi : 10.1007 / 978-3-642-16610-5 .
4. Nicolai Korshenewsky, Wilhelm T. Runge, F. Schröter (eds.): Textbook of wireless communications technology: television technology 2nd part - technology of electronic television . Springer, Berlin / Heidelberg 1963, ISBN 978-3-642-92863-5 , p. 530 , doi : 10.1007 / 978-3-642-92863-5 .
5. Measures against capacitive coupling. In: Planning Compendium Energy Distribution. September 10, 2017, accessed August 10, 2020 .
6. Stefan Finkbeiner, Wolfgang-Michael Müller, Wolfgang Welsch, Hartmut Kittel, Christian Bauer: Sensor measuring principles . In: Sensors in Motor Vehicles . Springer Fachmedien, Wiesbaden 2016, ISBN 978-3-658-11210-3 , p. 26–111 , doi : 10.1007 / 978-3-658-11211-0_2 .