[sdiy] Negistor article, Popular Electronics, Dec. 1975

[Aaron Lanterman]

On Jun 6, 2007, at 7:55 AM, Paul Perry wrote:

> Since the negistor circuit relies on undocumented properties of the
> transistor, it is not suprising that some versions of the 2N222A  
> work and
> some don't.
> Another problem is that the reverse breakdown voltage - a crucial  
> part of
> the operation - varies between units.
> So sometimes you might need more than 15V supply.
> I havn't had any direct experience, but it has been discussed on  
> the diy
> stompbox board.

One of my students play with that in my synth class in the Fall  
semester. She did get a negistor effect out of a 2N2222A, and used  
two back-to-back to make a symmetricish waveshaping nonlinearity...  
two negistors in parallel and in parallel with a resistor in the  
feedback loop of an op amp (iirc). Needed a pretty small input  
resistor to generate enough current to make it do anything interesting.

However, it was very hard to get working. The amount of current  
needed to drive it to get the full effect was more than a garden  
TL07x/08x sort of op amp could do, so she found a circuit that used  
two BJTs (I think it was BJTs, might have been FETs, can't remember  
now) on the output to provide more drive current.

Come to think of it, I've been meaning to post the student project  
reports from the Fall semester... I'll see if I can hunt up her report.

- Aaron