[sdiy] the most simple semicon VCO ever built

[Czech Martin]

Yes. Thank you, this saves a scan for me!

The npn will snap back if collector (arrow!)
avalanche breakdown will turn on the base.
Vdd must be at least 10V, the resistors must
kepp the power down, otherwise the transistor will vaporize.

I think "negative resistance" is not the correct term,
because the npn does not stay in the snap back region.
It just discharges C, then the current trough resistors
will make collector (arrow!) volatge so small that
the aavalanche action can not be sustained. 
Hence the base is turned off, and the collector current is
turned off. The C can charge up again and another cycle
will start. R1+R2 and Vdd must have the correct value,
otherwise the circuit will not oscillate.

m.c.


-----Original Message-----
From: R. Drake [mailto:rdrake at data2action.com]
Sent: Mittwoch, 26. März 2003 00:29
To: synth-diy at dropmix.xs4all.nl; Czech Martin; cfmd at swipnet.se
Subject: Re: [sdiy] the most simple semicon VCO ever built


dunno if this is the same thing, or kind of thing, but here's a circuit
diagramed in the terence thomas book, identified as a negisistor oscillator:

                                     c1
                                -----|(-----
                               |            |
+        r1            r2      |            |
o----/\/\/\/\/\----/\/\/\/\/\-----        ---------
                 |      ^      |   ^     /         |
                 |      |      |    \   /         ---
                  ------       |    -----          - grnd
                               |      |
                               |      NC
                               |
                               |
                                -----|(----o out
                                    c2
r1: 1k
r2: 10k
c1: 1uf
c2: 5uf
q1: 2n2222

he claims that it's "stable and linear", but has a limited frequency range.
i probably could have breadboarded it faster than draw it out in ascii
(expecially with my cat walking on the keyboard as she is), but i haven`t
tried it, so i cant tell you if/how it works...

lbd                


From: "Czech Martin" <Martin.Czech at Micronas.com>
Subject: [sdiy] the most simple semicon VCO ever built
Date: Tue, 25 Mar 2003 13:53:26 +0100

> 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.
> 
> If there is some interest, I could scan this little
> picture. Perhaps one more item to the "most simple modules
> collection" a la Korg.