AW: [sdiy] Zener-oscillators

[harrybissell]

The four layer (npnp) device family does what you are
trying to do by design, rather than by accident (like transistor
avalanche).  These would include the four layer diode (now very rare)
and  (with a gate added), the SCR, Unijunction and Programmable Unijunction
Transistor, and maybe the Diac (though I'm not sure of that one).

The Unijunction is probably the most workable of these devices for a good
VCO... it has complementary pulse outputs (but VERY narrow) and a good
sawtooth (but very high impedance).  I suppose that any of these devices could
do the trick... but the avalanche transistor is (as someone pointed out) the closest
analogue
to the old neon relaxation oscillator...

The Korg-style VCO that JH posted on his site shows an npnp transistor fabricated
with
an npn and pnp pair...

Now what we need is a good neon buld with (let's say) about a 10V breakdown
voltage... :^P

H^) harry

Czech Martin wrote:

> No Harry, just three layer semiconductor (npn)
> side effects...
>
> Is there some relation to UJT? I never dealt with those.
>
> m.c.
>
> -----Original Message-----
> From: harrybissell [mailto:harrybissell at prodigy.net]
> Sent: Dienstag, 1. April 2003 03:17
> To: Magnus Danielson
> Cc: Czech Martin; np at inverse-entertainment.de;
> synth-diy at dropmix.xs4all.nl; micke at bmh.nu
> Subject: Re: AW: [sdiy] Zener-oscillators
>
> Congratulations gentlemen
>
> I think you are well on your way to inventing the four-layer diode...
> maybe even the UJT ???    :^P
>
> H^) harry
>
> Magnus Danielson wrote:
>
> > From: "Czech Martin" <Martin.Czech at Micronas.com>
> > Subject: RE: AW: [sdiy] Zener-oscillators
> > Date: Mon, 31 Mar 2003 17:09:33 +0200
> >
> > Martin,
> >
> > > I've included a drawing of the transistor DC curve,
> > > as well as a curve tracer snap shot.
> > >
> > > It's only visible in heavily doped collector junctions,
> > > thus reverse mode operation.
> > > The thing is that the collector (it is the emitter!)
> > > base junction breaks down
> > > in avalanche. Once a certain amount of holes is in the base,
> > > it will turn on the emitter base junction in normal
> > > conduction and diffusion (emitter is really collector here).
> > > This current will diffuse to the collector where
> > > it will push the avalanche. Positive feedback!
> > > The voltage across the transistor will collapse,
> > > it will then look like a resistive branch.
> > > Go under the minimum voltage to sustain the avalanche
> > > and it will turn off.
> > >
> > > There is avalanche, "pushed avalanche", high injection
> > > and normal current amplification involved at the same
> > > time.
> > >
> > > Circuits that only want emitter to base zener will
> > > therefore not connect the collector pin.
> > >
> > > Before reaching the break over point, the leakage due to
> > > beginning avalanche will rise, if the current source
> > > can not live with this the circuit will hang there.
> >
> > OK. This starts to makes sense. The transistor emitter-collector breakdown I-V
> > explains what happends.
> >
> > I've seen two recent reports of this type of oscillator actually working.
> >
> > I've also tossed at BC550C under a 100 ohm resistor and a pretty good sine for
> > straight and X-Y scope. Not a very elegant setup if you want a good I-V curve,
> > but it does gives a hint. The breakdown is clearly visible and when you drive
> > it as hard as -9.1 V it kicks in and bends in the other direction pulling more
> > and more current falling back quickly to about -7.2 V which is a platau under
> > which it pulls current (I drive mine as hard as -25 mA).
> >
> > A Zener-diode (6.2V out of a bag here on the table) has quite a different
> > story to tell. So does the emitter-base setup. No, the emitter-collector stuff
> > is what makes this fun.
> >
> > It's also interesting to see how the voltage of the base changes through the
> > cycle. Near the abrupt slope it has about the same shape, including the abrupt
> > slope. Looking at the base is like probing into the middle of all this, since
> > it is the P between the N of the collector and the N of the emitter.
> >
> > Got to hit bed, oscillation is next lab-assignment for tomorrow!
> >
> > Cheers,
> > Magnus