[sdiy] Sallen-Key Filter & 4069

[harry]

OTOH... most of the MOS transistor "variable resistors"
work best in shunt mode... with one end tied to a low impedance
source like... ground ???

It might be possible to do a series method... but it would probably not
perform as well...

H^) harry

René Schmitz wrote:

> At 21:33 06.10.01, Scott Gravenhorst wrote:
> >In the spirit of really simple, I started looking at
> >filter topologies and noticed that the Sallen-Key
> >filter is really simple.  It's a second order filter
> >that uses just one opamp.  I also noticed that the
> >2 resistors responsible for tuning are connected
> >in series.  If a 4069 is used in the voltage controlled
> >resistor mode, might two of the gates be substituted
> >for the tuning resistors to make it a voltage controlled
> >Sallen-Key?  I think this is possible, I just don't
> >know exactly how to do it.
>
> Interesting thought!
>
> The trouble with the 4069 for filters is, that although you can
> get six voltage controlled resistors from it, they're all tied
> together at one end. I.e. a series connection isn't possible.
>
> But, you could do that using a 4007 in which the individual
> transistors are more freely accessible.
>
> Cheers,
>  René
>
> --
> uzs159 at uni-bonn.de
> http://www.uni-bonn.de/~uzs159
>
>