[sdiy] Filter Structure Question

[Magnus Danielson]

From: Scott Gravenhorst <music.maker at gte.net>
Subject: [sdiy] Filter Structure Question
Date: Fri, 16 Jul 2004 08:01:34 -0700
Message-ID: <200407161501.i6GF1YZ32465 at linux6.lan>

> This may seem daft to many, but I have evil, subversive and nefarious
> reasons for asking:
> 
> Are there lowpass voltage controlled filters that instead of using the
> standard integrator, use a high pass filter structure in the feedback loop
> of an OpAmp?  If so, are there viewable examples?

Try something like this:

buffer the highpass filter with an op-amp and let the buffered version drive
the negative input of the opamp. Replace the resistor with an OTA which has 
its neg input grounded and its pos input properly divided from the buffered
signal. This should make the OTA operate as a resistor to ground with CC
(Control Current) which needs the classical drive.

Why you ask for it is beyond me... but I am curious to find out.

Cheers,
Magnus