[sdiy] the most simple semicon VCO ever built

[Czech Martin]

Yes, this is the idea.

The IC people call this npn snap back.
It's an avalanche on heavily doped collector/base junction.
This will turn on the (unconnected) base, thus emitter
injection with current gain can happen.

Since normal transistors have low doped collectors,
the transistor must be used reverse, i.e. emitter swapped with
collector.

On CMOS ICs the npn is parasitic, collector and emitter
are equivalent.

Usually this is used as discharge device for ESD,
but also a capacitor can be discharged with it.
If you use a coax cable as "capacitor", this technique
can deliver ns pulses with sharp shape.
A very old RF technique.

Of course, frequency and amplitude vary together if the
base terminal is used for voltage/current control.

But if this doesn't matter, it's still a usefull relaxation
device. And it is fast.

If you think about a PS3000 Korg style aparatus, this could be
enough. (Is this the correct name of these multi vco, vcf
Korg things?).


m.c.

-----Original Message-----
From: Steve Ridley [mailto:spr at spridley.freeserve.co.uk]
Sent: Dienstag, 25. März 2003 23:38
To: Czech Martin
Subject: Re: [sdiy] the most simple semicon VCO ever built



> Yesterday I browsed through a 1970 something issue of the
> "Funkschau" mag. I saw a simple VCO, one cap , one
> npn one resistor. Obviously the transistor was working
> in tunnel diode breakdown, thus creating negative
> impedance characteristic.
> 
> The base was left open, but it was said that it could be used
> for frequency control.

There's an article about using transistors as negistors at:

http://www.keelynet.com/zpe/negistor.htm

Terence Thomas gave a basic negistor oscillator circuit in 
his book "Sound Synthesis - Analog & Digital Techniques" 
but decided it had insufficient range for a musical oscillator.  

I briefly built a dual expo VCO using a 3046 as two negistor 
and two expo current sources, but it didn't seem worth taking 
any further


Steve Ridley