From: John Popelish
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Subject: Re: Active Bandpass Filter Oscillation Problem
Date: Mon, 30 Sep 2002 03:24:57 GMT
NNTP-Posting-Date: Sun, 29 Sep 2002 23:24:57 EDT
> Recently, I've experimented with active bandpass filters for filtering
> a certain frequency from a microphone (5 kHz). The filter form from 2
> cascaded Friend Bandpass Filters, with each stage having a Q of 5.
> The overall gain is 65 dB (fixed by other design constraints, namely
> the Q and the center frequency. The supply comes from two 9V
> batteries, and the alimentation is decoupled by a 47uF, a 10uF and a
> When the filter is used alone, it reacts very well, with an acceptable
> SNR (~20-25 dB). However, when I connect the output to an op-amp used
> as a comparator, the filter oscillates. The SNR drops to about ~10
> dB. I've tried to buffer the output of the filter with another op-amp
> used as a voltage follower, with no good result. I wonder why the
> filter is affected by what comes after it, even if it's not directly
> Any ideas ?
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.