Parasitic capacitance or stray 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. When two electrical conductors at different voltages are close together, the electric field between them causes electric charge to be stored on them; this effect is capacitance.
All practical 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 some capacitance between any two conductors; this can be significant with closely spaced conductors, such as adjacent wires or printed circuit board traces. The parasitic capacitance between the turns of an inductor (e.g. Figure 1) or other wound component is often described as self-capacitance. However, in electromagnetics, the term self-capacitance more correctly refers to a different phenomenon: the capacitance of a conductive object without reference to another object.
Parasitic capacitance is a significant problem in high-frequency circuits and is often the factor limiting the operating frequency and bandwidth of electronic components and circuits.
When two conductors at different potentials are close to one another, they are affected by each other's electric field and store opposite electric charges like a capacitor.[1] Changing the potential
V
i
i=C
| dV | |
| dt |
,
C
Coils for high frequencies are often basket-wound to minimize 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.
In high frequency amplifiers, parasitic capacitance can combine with stray inductance such as component leads to form resonant circuits, also leading to parasitic oscillations. In all inductors, the parasitic capacitance will resonate with the inductance at some high frequency to make the inductor self-resonant; this is called the self-resonant frequency. Above this frequency, the inductor actually has capacitive reactance.
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.
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.
See main article: Miller effect.
Assume the ideal inverting amplifier with gain of
A\nu
(Z{=}Cparasitic)
Z
iZ=Cparasitic{d\overdt}(Vi-Vo)
iZ=Cparasitic{d\overdt}(Vi+A\nuVi)
iZ=\underbrace{(1+A\nu)Cparasitic
1+A\nu
A\nu
CMiller
If the input circuit has an impedance to ground of
Ri
Vo=
| A\nu | |
| 1+j\omegaRiCMiller |
Vi,
\omega=2\pif
fcutoff={1\over2\piRiCMiller