[sdiy] Negistor Oscillators and web cleanups

[Michael Baxter]

Hi Jurgen,

This is what it looked like to me too, a discrete transistor 
implementation of a SCR. Still very interesting, but different than a 
negistor.

The negistor is actually operating the transistor in inverted mode, where 
the normal terminals are backwards with respect to normal. They actually 
work (differently) in this way, and one typical application from decades 
ago would be low offset voltage choppers. It turns out that some 
transistors also exhibit negative resistance when operated in inverting 
mode, and nearly all make an excellent avalanche diode when used with 
transdiode hookup. Near the avalanche voltage is where the negative 
resistance effect if found. This effect is structurally very similar to 
that of a tunnel diode, with wider voltage swings, volts instead of 
hundredes of millivolts.

In the case of the PN2222A, the avalanche voltage is approximately 6.29V.  
The ratios of voltage swing seen in some of my test circuits were due to
the relative charging current available, as well as the charge that could
be stored in the capacitor, it seems.

I suspect that the basic sawtooth generator with a negistor-configured 
transitor is essentially an integral charge-balancing oscillator. 

Best,
Michael

On Fri, 30 Jan 2004 jhaible at debitel.net wrote:

> I'm not familiar with this negistor stuff, so I cannot
> say if there is any connection to the Korg circuit
> or not.
> 
> But just to avoid confusion: What Korg was using is
> not a side effect of a single BJT. It's a 4-layer
> structure (pnpn): An actual thyristor component
> in some instruments (PE-1000, and VC-10 if memory serves),
> and a composite thyristor built from two BJTs, thus
> formin the pnpn structure. (Ok, there's also a diode
> and a 3rd transistor, but it's basically just a 
> thyristor nevertheless.)
> 
> Gate voltage exceeding a certain level -> the thing
> will conduct
> 
> Positive feedback -> the thing will stay turned on
> 
> Current goes below some level -> feedback loop is
> broken, the thing goes off.
> 
> Works over a wide range of current, too. (MS-20 ...)
> 
> JH.
> 
> 
> 
> 
> > Hi, I don't know if my previous message went through, therefore this
> > is another post in which you will find the URL where to get :
> > Jurgen Haible's recreation of Korg PS3200 VCOs
> > http://www.oldcrows.net/~jhaible/polykorg/sg_1sch.pdf
> > and the shematics of the Korg MS50
> > http://www.hylander.com/moogschematics.html
> > 
> > Cheers
> > 
> > > Hi Yves,
> > >
> > > Many thanks for this input -- could you possibly direct me to the 
> > > schematics
> > > somewhere? I did a quick search, and could not easily find the VCO 
> > > schematics.
> > >
> > > Many thanks!
> > >
> > > Best,
> > > Michael
> > ----------------------------------------------------------
> > Yves Usson
> > 
> > Lab. TIMC UMR5525 CNRS
> > Institut d'Ingéniérie et de l'Information de Santé (IN3S)
> > Faculté de Médecine
> > Domaine de la Merci
> > 38706 La Tronche cedex
> > 
> > email : yves.usson at imag.fr, yves.usson at ujf-grenoble.fr
> > 
> > tel.  (+33) (0)456 520 074
> > fax.  (+33) (0)456 520 022
> > 
> > 
> > 
> > 
> 
> 
> 
> 
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