[sdiy] Sallen-Key Filter & 4069

[Scott Gravenhorst]

=?iso-8859-1?Q?Ren=E9?= Schmitz <uzs159 at uni-bonn.de> 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=20
>>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=20
>get six voltage controlled resistors from it, they're all tied=20
>together at one end. I.e. a series connection isn't possible.

But couldn't the forced tie point simply be where the two input
resistors connect together?  Granted, you would 'waste' four
of the 6 resistors.

   partial schematic:

  >---R1---+---R2--+--opamp---+-->
           |       |          |
          cap     cap         |
           |       |          |
           |      gnd         |
           +------------------+     

I see two resistors, connected at one point (common tie point
for the 4069) and then two 'floating ends', one for the 
filter input and one connected to the opamp input.

>But, you could do that using a 4007 in which the individual=20
>transistors are more freely accessible.
>
>Cheers,
> Ren=E9
>
>
>--=20
>uzs159 at uni-bonn.de
>http://www.uni-bonn.de/~uzs159
>
>=20
>

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