Parasitic capacitance

회로의 여러가지 주위 환경 때문에 발생하는 캐패시턴스(Capacitance) 성분을 말합니다.

캐패시터(Capacitor)는 양쪽에 금속판이 있고 중간에 유전물질을 삽입시켜 AC(교류)가 인가되면 반대편에 유도전류가 흐르게 됩니다.

이와 같이 회로에서 인접한 도선 사이에 AC가 인가됨에 따라 도선과 도선 사이에 캐패시턴스(Capacitance) 성분이 있는 것처럼 보이는 것을 기생캐패시턴스(용량)이라고 합니다.

From Wikipedia, the free encyclopedia

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.

Contents  [hide] 1 Example 2 Effects 3 Approximate parasitic capacitance values 4 Footnotes

[edit]Example

For example, an inductor often acts as though it includes a parallel capacitor, because of its closely spaced windings. When apotential difference exists across the coil, wires lying adjacent to each other at different potentials are affected by each other'selectric field. They act like the plates of a capacitor, and store charge. Any change in the voltage across the coil requires extracurrent to charge and discharge 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.

[edit]Effects

Therefore at low frequencies parasitic capacitance can usually be ignored, but in high frequency circuits it is a major problem. Inamplifier 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 ampscan 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.^[1]

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.

[edit]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

[edit]Footnotes

1. ^ Alley, Charles L.; Atwood, Kenneth W. (1973). Electronic Engineering, 3rd Ed.. New York: John Wiley & Sons.. p. 199. ISBN 0471024503.

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