VC capacitor?

Magnus Danielson magda at it.kth.se
Wed Jan 15 22:13:46 CET 1997


> Hi!
> 
> can anybody tell me if there exists something like a voltage
> controllable capacitor ?
> 
> I need one with a large range (and pretty precise if possible).

To trow my 2 cents in...

There exists such things... in the PN-junction you get a small capacitance
(from a few to a few tens of picofarad) which will vary with the applied voltage. This behaiviour exists in all PN-junctions (like normal diodes and
transistors) but mostly it is used in devices called capacitance diodes.
These have tailored relation between applied voltage and capacitance... if
they are doped with a a^(-3/2) curve (if my memory serves me rigth) you will
get a linear frequency dependency. However, I beleive that the capacitance range is limited to about a decade for most devices...

Also, in order for the signal not to modulate too much at low frequencies one
must limit the maximum signal... but then why? it can get bizzare :)

>From a range perspective I would not use them for audio...

Try a impedance scaler curcuit and control an resistor instead...

Cheers,
Magnus





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