AW: [sdiy] Zener-oscillators

[Czech 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.



m.c.

-----Original Message-----
From: Magnus Danielson [mailto:cfmd at swipnet.se]
Sent: Montag, 31. März 2003 15:46
To: np at inverse-entertainment.de
Cc: synth-diy at dropmix.xs4all.nl; micke at bmh.nu
Subject: Re: AW: [sdiy] Zener-oscillators


From: "Nils Pipenbrinck" <np at inverse-entertainment.de>
Subject: AW: [sdiy] Zener-oscillators
Date: Mon, 31 Mar 2003 16:22:29 +0200

> Magnus,

Nils,

> Dumb question maybe but:
> 
> When the zener starts conducting because the cap has reached his 5.6V, how
> long will it stay conducting?
> Afaik the zener will pass current until the voltage across is less than 5.6V
> again, and I would expect the voltage across the cap to be around 5.6V +
> some significant noise-floor.
> 
> Or didn't I get the point?

Well... this all started back in the reversed transistor oscillator, and I just
interprented it as the breakdown "Zener" action. This was obviously taking it
a little too quick. But if you have an avalance started, then it must be that
it continues while the capacitor discharges. However, this is a temporal issue
and not a DC issue. The arguments I've heard so far relates to the DC V-I
curve and if the reversed transistor setup actually works, then there is
another effect which goes beyond the DC V-I curve. This is then the effect we
would like to see in action. If this is only available in reversed transistors
or not is another interesting feature. There are such properties known, but
they are rarely used in LF. In HF they build oscillators like these all the
time, but they use more exotic devices, partly due to the high frequency.

Does all that make sense?

I will make some tests soon (maybe even tonight, but I gotta run now...)

Cheers,
Magnus
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