[sdiy] NIC-based wave folder/shaper

[Scott Gravenhorst]

Thanks Ian,

Allow me to post an ASCIImatic:


                        100K
                   +--/\/\/\/\/\/---+
                   |                |
                   |                |
                   |       |\       |
                   |       | \      |
         47K       |       |  \     |
   o---/\/\/\/\/---+-------| - \    |
                           |    \   |
 Input                     |     >--+--+-----o Output
                           |    /      |
                   +-------| + /       |
                   |       |  /        |
                   |       | /         |
                   |       |/          |
                   |                   |
                   |                   |
                   +----/\/\/\/\/\/----+
                   |       100K
                   /
                   \
                   /
                   \
               50K /<--+
                   \   |
                   /   |
                   \   |
                   /   |
                   |   |
                   +---+
                   |
                   |
                   |
                 -----
                / / /



Is this correct?  I love simple things like this.

Should the 100K resistors be 1% ?

Also, is there anything to be gained by changing the ratio of the 2 100K
resistors to something other than 1 to 1?

I also like the idea of simple FET or MOSFET control of this.




Ian Fritz <ijfritz at earthlink.net> wrote:
>Friends --
>
>Here is an amazing little circuit I just worked out.  It takes a saw input 
>and continuously deforms it from saw to triangle to inverted saw.  It takes 
>just one op amp, three fixed resistors and a pot!
>
>The circuit is a simple voltage divider -- with one catch.  The lower leg 
>of the divider is a variable *negative* resistor over part of its operating 
>range. The negative resistance region folds the waveform.
>
>Here's how to hook it up, if you have a whiteboard and five minutes to spare.
>
>I/O voltages:  Vin to left side of a 47k resistor, Vout to the right side.
>
>Op amp inverting input:  Connect to right side of (above) 47k resistor and 
>through a 100k resistor to op amp output.
>
>Op amp non-inverting input: connect through another 100k resistor to op amp 
>output.  Also connect through a 50k pot to ground.
>
>That's basically it.  You may also need a small stabilizing cap (10 p) 
>across the negative feedback path.  And you may want to buffer the output 
>if you use a long output lead.
>
>This is just a first shot at the circuit. You will probably find that the 
>ends of the waveform don't quite connect. This can be fixed, if desired, by 
>adjusting the fixed resistors slightly.  Additionally, the amplitude 
>changes with shape, which could be fixed with a ganged pot controlling a 
>variable gain stage.
>
>The circuit could easily be voltage controlled using an OTA or a 
>FET-resistor in place of the pot.  A second OTA or FET could manage the 
>amplitude leveling.
>
>Fun, fun, fun!
>
>   Ian
>

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-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
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