[sdiy] Sallen-Key Filter & 4069

[René Schmitz]

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


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uzs159 at uni-bonn.de
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