What Is Parasitic Capacitance?

Parasitic capacitance, in electrical circuits, is the extra effect of conductors that serve as plates between a dielectric, which is usually air. It becomes a problem with higher frequencies because the very small distributed capacitances that exist will have lower impedances at these frequencies. This effect can be addressed at circuit design stage, where positioning of components may decrease the effects to a point where satisfactory operation is attainable.

Capacitors are available as lumped or distributed components. As lumped components, these capacitors are deemed as confined to certain components; for distributed capacitance, there is a need for planning in component and circuit design. When an inductor is manufactured, there is always a distributed capacitance involved; this may be considered a parasitic capacitance. An ideal inductor will have zero distributed capacitance; therefore, it will resonate at a frequency in the vicinity of infinity. It is well-known that most inductors will have a non-infinite resonant frequency due to the distributed capacitance of the winding that leads to a measurable resonant frequency.

Parasitic capacitance in radio frequency (RF) amplifiers may cause these amplifiers to have low gain due to parasitic loss. In some cases, it may cause these amplifiers to oscillate. With parasitic capacitance, the actual circuit in the real world is the circuit drawn at design stage plus capacitances to ground or between various points of the circuit. In some cases, the solution is simply to reduce the lumped capacitance for a certain circuit position. For other cases, the solution could be to increase an inductance to maintain a certain frequency passband.

There are instances where the characteristics of the electronic component may compensate for parasitic capacitance. For instance, the decreased RF output due to a parasitic capacitance may be increased by using a higher gain transistor. In some cases, the odd effects of parasitic capacitance may be compensated by adding circuit stages.

A parasitic element may exist due to the proximity of conductors or the lengths of traces, wires, or leads of components. The common approach to lessening the chance of discovering a parasitic element is to shorten conductors and decrease the surface area in components and traces on printed circuit boards (PCBs). Based on the mentioned practices toward avoiding excessive parasitic effects, the miniaturization of components and PCB traces has become a standard practice.

In digital switching circuits, the rise time and fall time of the digital signal greatly affect the maximum speeds achievable. The parasitic capacitance on the inputs and outputs of the digital devices increases the rise and fall times. An alternative is to use output devices that can inject higher currents to compensate for the parasitic capacitances. Unfortunately, this approach increases direct current (DC) power consumption. This explains why very high-speed digital circuits usually require huge amounts of DC currents.