[sdiy] NIC-based wave folder/shaper

[harrybissell]

No whiteboard... but some loose-leaf paper
and EWB...

The simulation looks quite interesting. My values were a 25K resistor
for the series path and a 100K pot...

I couldn't get the triangle with your values....

nice idea, thanks (soon to be a 'new' module from you-know-who)

H^) harry

Ian Fritz 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