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Parasitic capacitance
From Wikipedia,
the free encyclopedia
In electrical
circuits , parasitic capacitance , stray capacitance or, when relevant, self-capacitance (of an inductor), is
an unavoidable and usually unwantedcapacitance that exists between the parts of an electronic
component or circuit simply because of their proximity to each other. All actual circuit
elements such asinductors , diodes , and transistors have internal capacitance, which can cause their behavior to depart from
that of 'ideal' circuit elements. In addition, 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 boardtraces.
Contents [hide] ·
1 Example ·
2 Effects ·
3 Parasitic capacitance of interconnects in
integrated circuits |
Example
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 at
different potentials are affected by each other's electric
field . 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 be significant.
Coils for high frequencies are often basket-wound to minimise parasitic capacitance.
Effects
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. [ 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.
Parasitic capacitance of interconnects in integrated circuits
Main article: parasitic
extraction
In early integrated
circuits the impact of the
wiring was negligible, and wires were not considered as electrical elements of
the circuit. However below the
0.5- micrometretechnology node resistance and capacitance of
the interconnects started making a significant impact on circuit performance. [ 2 ]
Major effects of interconnect parasitics include signal delay and noise .
See also
§ Parasitic element (electrical networks)
References
1. ^ Alley, Charles L.; Atwood, Kenneth W. (1973). Electronic
Engineering, 3rd Ed. . New York: John Wiley & Sons.. p. 199. ISBN 0471024503 .
2. ^ "Automatic Layout Modification", by Michael Reinhardt, p. 120
번호 | 제목 | 닉네임 | 조회 | 등록일 |
---|---|---|---|---|
기생 캐패시턴스 | dwkim |
18410 | 2011-11-09 |