Parasitic capacitance

For example, an inductor often acts as though it includes a parallel capacitor, because of its closely spaced windings.

Any change in the voltage across the coil requires extra current to charge and discharge their small capacitances.

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 minimize parasitic capacitance.

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.

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 minimize the effects of unwanted capacitance.

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.

in Figure 2 has a parasitic capacitance between the amplifier's input and output as the feedback impedance

is: Even a small parasitic capacitance is problematic because the Miller effect multiplies it by

This acts as a low-pass filter with a cutoff frequency that limits the amplifier's bandwidth to: The voltage gain of modern transistors can be 10–100 or even higher, and for op amps are orders of magnitudes higher, so Miller capacitance (first noted in vacuum tubes by John Milton Miller in 1920) is a significant limitation on the high frequency performance of amplifying devices.

The screen grid was added to triode vacuum tubes in the 1920s to reduce parasitic capacitance between the control grid and the plate, creating the tetrode, which resulted in a great increase in operating frequency.

[3] In bipolar junction transistors, the parasitic capacitances between the base and collector or emitter have voltage dependence too.

Figure 1: Stray capacitances are often drawn with dashed lines. This equivalent circuit of part of a Tesla coil has stray capacitance between each winding and one to ground.
Figure 1: The Miller effect causes a feedback impedance between the input and output of an amplifier to apparently be multiplied by a little more than the amplifier's gain when viewed as an input impedance .