[sdiy] generating an s-shaped curve

[Tom Wiltshire]

The "normal" Tri-to-sine curve is basically a soft distortion function, right?
And we can get that using a couple of diodes in the feedback loop of an op-amp - standard distortion pedal stuff - eg tubescreamer.
The principle is that the feedback resistor creates a large gain, but when the signal gets large enough for the diodes to conduct, the gain is decreased.
If you wanted the opposite, couldn't you put the limiting diodes across the input resistor instead?
Then when the input goes above the diodes threshold, the diodes conduct and the gain *increases*.
Obviously such a circuit is going to be pretty sensitive (poor thing) and likely to slam into the rails, but there's no charity campaigning against abuse of op-amps.

HTH,
Tom


On 27 Jan 2012, at 06:59, Harry Bissell wrote:

> OK i'm tring to make a circuit that expands an envelope. I figured out a solution using a diode
> breakpoint generator. The curve is the opposite of the tri/sine converter, larger values get progressively
> larger, smaller ones get larger in a negative direction
> 
> H^) harry
>