Parasitic capacitance

In electrical circuits, parasitic capacitance is the unavoidable and usually unwanted capacitance that exists between the parts of an electronic component or circuit simply because of their proximity to each other. All actual circuit elements such as inductors, diodes, and transistors have internal capacitance, which can cause their behavior to depart from that of 'ideal' circuit elements. In addition, parasitic capacitance can exist between closely spaced conductors, such as wires or printed circuit board traces.

For example, an inductor often acts as though it includes a parallel capacitor, because of its closely spaced windings. When a potential difference exists across the coil, wires lying adjacent to each other at different potentials are affected by each other's electric field. They act like the plates of a capacitor, and attract charge. Any change in the voltage across the coil requires extra current to charge these small 'capacitors'. When the voltage doesn't change very quickly, as in low frequency circuits, the extra current is usually negligible, but when the voltage is changing quickly the extra current is large and can dominate the operation of the circuit.

Therefore at low frequencies parasitic capacitance can usually be ignored, but in high frequency circuits it is a major problem. In amplifier circuits, parasitic capacitance between the output and the input can act as a feedback path, causing the circuit to oscillate. These unwanted oscillations are called "parasitic oscillations". The capacitance of the load circuit attached to the output of op amps can reduce their bandwidth. High frequency circuits require special design techniques such as careful separation of wires and components, guard rings, ground planes, power planes, shielding between input and output, termination of lines, and striplines to minimise the effects of unwanted capacitance.

The parasitic capacitance between the base and collector of transistors and other active devices is the major factor limiting their high frequency performance. The screen grid was added to vacuum tubes in the 1930s to reduce parasitic capacitance between the control grid and the plate, and resulted in a great increase in operating frequency. [cite book|last=Alley|first=Charles L.|coauthors=Atwood, Kenneth W.|date=1973|title=Electronic Engineering, 3rd Ed.|publisher=John Wiley & Sons.|location=New York|isbn=0471024503|pages=199]

In closely spaced cables and computer busses, parasitic capacitive coupling can cause crosstalk, which means the signal from one circuit bleeds into another, causing interference and unreliable operation.

Approximate parasitic capacitance values

*Printed circuit board traces with ground and power plane: 1-3 pF per inch
*Low capacitance coaxial cable: 20-30 pF per foot

Footnotes