Tuned coil

[DPJ]

Hi
Can any one help??
I Have a tuned circuit which needs to be varible (25K-100K)
I've tried varactor diodes which only seem to go up to 100pF
which is not enough I need at least 2nF, is there a way of increasing the capacitance. Or a simple way of making a bandpass filter which is controled by voltage or resistance
Ive tried the semiconductor filters but they are to exspensive.

Many thanks for any help
DPJ

 

[don01]

hi
my memory says there is a cct used in power suppliies where a capacitor is used to bias a transistor. This gives you a multiplier effect for the capacitor. It would seem a logical extension that you could vary a divider cct to get an effective variation of the capacitance value. Put this into your filter cct and you should have a variable filter.
Now this is based on my memory and I cannot find a specific reference. But you should be able to find reference to it in books on power supplies.

How about playing around with an op amp (cheap one ) say lm741 or TL072
and a capacitor resistor cct on the feed back. This is how some of the tone controls are done in guitar amps.

Let us know how it goes.
Don

 

[nbucska]

Senf me your FAX# and frequency range.
<nbucska@pcperipherals.com>

 

[jbartos]

Suggestion: What about adding a suitable capacitor to the varactor?

 

[ambertec]

Please try to find a copy of Electronic Design, December 5, 1991 where on page 113 is an Idea For Design entitled &quot;Vary Capacitance To Positive Or Negative&quot;. I submitted that item. It describes making variable capacitances of values to as many µF as you'd like, including negative values if need be. You may be able to use that with your coil to get the tuning you need. If you can't find that item, let me know by e-mail and I'll send it to you in a Word file.

Good luck.

John Dunn
ambertec@ieee.org