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From: John Popelish
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X-Mailer: Mozilla 4.7 [en] (Win98; U)
Subject: Re: Active Bandpass Filter Oscillation Problem
References: <email@example.com> <3F78F785.9C207565@rica.net> <firstname.lastname@example.org>
Date: Mon, 30 Sep 2002 14:04:19 GMT
NNTP-Posting-Date: Mon, 30 Sep 2002 10:04:19 EDT
> >The only way I can see is back through the supply or common. When the
> >"comparator" saturates, it must bump the supply lines a bit. Isolate
> >the comparator stage by putting a pair of resistors between the + an -
> >supplies before you add bypass capacitors. Use the highest value that
> >you can tolerate based on DC drop.
> That seemed to resolve my problem. With 1K, it seemed to work. I'll
> try lowering further the resistors.
> Thanks very much. I was banging my head on this problem for a while.
Glad to help.
(for the newbies who may be reading)
If you have trouble getting the resistance low enough to get the
output swing you need, you can also add the same RC filter to your
bandpass stages, to increase the effectiveness of their bypass caps.
After all, these are the stages with the high sensitivity. The thing
most people miss is that bypass caps provide current only when the
supply voltage changes. I = C*(dv/dt) The resistors allow for a bit
of dv without letting much current swing get back to the common supply
lines. If the frequencies in question are high enough, a ferrite bead
on a lead can provide the same effect with no significant DC loss.
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