AW: [sdiy] Zener-oscillators

[Magnus Danielson]

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