In electrical circuits, parasitic capacitance, stray capacitance or, when relevant, self-capacitance (of an inductor), is an 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. Additionally, there is always non-zero capacitance between any two conductors; this can be significant at higher frequencies with closely spaced conductors, such as wires or printed circuit board traces.
When two conductors at different potentials are close to one another, they are affected by each others' electric field and store opposite electric charges like a capacitor. Changing the potential v between the conductors requires a current i into or out of the conductors to charge or discharge them.
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 are at different potentials. They act like the plates of a capacitor, and store charge. Any change in the voltage across the coil requires extra current to charge and discharge these small 'capacitors'. When the voltage changes only slowly, as in low-frequency circuits, the extra current is usually negligible, but when the voltage changes quickly the extra current is larger and can affect the operation of the circuit.
Coils for high frequencies are often basket-wound to minimise parasitic capacitance.
At low frequencies parasitic capacitance can usually be ignored, but in high frequency circuits it can be a major problem. In amplifier circuits with extended frequency response, parasitic capacitance between the output and the input can act as a feedback path, causing the circuit to oscillate at high frequency. 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.
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.
Electronic design automation computer programs, which are used to design commercial printed circuit boards, can calculate the parasitic capacitance and other parasitic effects of both components and circuit board traces, and include them in simulations of circuit operation. This is called parasitic extraction.