NPO limits...


Neo Bass

I\'ve wondered what limits NPO capacitor value to 0.1uF without manually stacking multiple devices in parallel. Is there any place I can get this information.


In article <>,
Neo Bass <> wrote:

>I\'ve wondered what limits NPO capacitor value to 0.1uF without manually stacking multiple devices in parallel.

Physics (and money, as usual :) )

The capacitance is determined by the area of the electrodes, the
distance between them, and the dielectric constant of the insulating
material. To increase the capacitance, you must either increase the
area of the electrodes (makes the part bigger), stack up multiple
layers of electrodes (makes the part thicker), reduce the distance
between the electrodes (makes the part thinner but the thinner
insulation means a lower voltage limit), or use an insulator with a
higher dielectric constant.

NP0/C0G caps use an insulator whose dielectric constant is relatively
low, but which doesn\'t change much with temperature or applied
voltage. Hence, you get a relatively low amount of capacitance per
volume, but it\'s quite stable.

High-capacitance ceramic caps use materials which have a higher
dielectric constant, giving more capacitance per layer (all else being
equal). The tradeoff is that these materials are less stable in the
face of temperature change, and the dielectric constant drops a lot
(and so does the capacitance) if they\'re standing off more than a few
volts. Good for bypass caps, bad for predictable low-distortion
signal-path filters.

In principle it should be possible to fabricate NP0/C0G caps of
much larger capacitance than 0.1 uF - but they\'d be so big and
expensive that there isn\'t much market for them. Or, you could
make high capacitance in a small volume by using really thin
dielectric layers... but the maximum voltage would be so low
(and the reliability might be quite poor due to the thin layer
being prone to develop pinhole defects) that once again there
isn\'t enough market demand to have them made as standard

If somebody develops an insulator with a stunningly high
dielectric constant which is temperature- and voltage-
insensitive, physically stable, costs $0.02 per square
meter, and doesn\'t cause fatal beak warts in migratory
song-birds, then things may be very different in the future :)

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