[Elecraft] Negative Resistance????

Thu Jul 17 20:04:06 2003

Dear Group,

I am not going to write a book here, but I can show you mathematically that
any oscillator made from real components, i.e., L's and C's with finite Q,
has a negative resistance generated by the oscillator amplifier to move the
poles to the "Y" axis, thus allowing sustained oscillation.  The interesting
thing is, power must be applied somewhere in the circuit to overcome the
losses in the oscillator components.  I know of NO passive component
oscillator.  As soon as one shows up, let me know, please, because perpetual
motion has been accomplished, and I want to be in on the initial
offering!!!!!!!

Bill
NA7Y

----- Original Message ----- 
From: "Ron D'Eau Claire" <[email protected]>
To: "'Lee Buller'" <[email protected]>; "'Elecraft Reflector'"
<[email protected]>
Sent: Thursday, July 17, 2003 3:15 PM
Subject: RE: [Elecraft] Negative Resistance????

Negative resistance is a very common phenomena in electronics. Someone
mentioned "tunnel diodes" which use negative resistance to oscillate in the
microwave range. But even much more mundane devices such as a common "neon"
bulb exhibit negative resistance.

First, let's define negative resistance starting with er.... "positive"
resistance. In normal of "positive" resistance, the current varies directly
with the voltage applied to the resistor according to ohms law. You increase
the voltage and the current increases. You decrease the voltage and the
current decreases. It's a linear function.

"Negative" resistance is a situation in which when you reduce the voltage
the current goes up (and vice versa). Take a common neon bulb - a glass
envelope with two electrodes that come out to leads. The envelope is filled
with neon gas.

As you increase the voltage across it, the current increases very slightly
until the "ignition" voltage of the bulb is reached. At ignition, the
current jumps up very fast, even if the voltage stays exactly the same! That
is, it exhibits "negative resistance". Even if you reduce the voltage,
within limits, the current increases just the same. That's because once the
neon ionizes (and starts producing light) it's resistance drops
dramatically.

A common oscillator circuit used for many years produced a nice sawtooth
wave using just such a bulb. You hook a capacitor across the bulb. One end
of the bulb/capacitor goes to the negative terminal of a battery with enough
voltage to light the bulb (usually 40 to 60 vdc). The other bulb/capacitor
terminal goes through a BIG resistor (100 k or so typically, depending upon
the size of the capacitor) to the + side of the battery. The bulb is an open
circuit at first. The battery charges up the capacitor through the resistor
until the ignition voltage of the bulb is reached.

WHANG! The bulb lights and starts drawing a LOT of current, discharging the
capacitor. The resistor in series with the battery is too large for it to
supply enough current, so the voltage across the capacitor starts dropping.
But because the bulb, once lit stays lit about 50 volts typically stays lit
until the voltage across it drops to about 20 volts, the current keeps
flowing even though the voltage is dropping (there's the "negative
resistance" effect). Finally, the voltage drops too low and the neon
extinguishes, stopping the heavy current drain and the battery starts
recharging the capacitor again.

More exotic circuits using tubes produce oscillations with "negative"
resistance. One of those was the "klystron" oscillator - a key element in
the early Radar sets from the WWII period. In those cases the "negative
resistance" effect comes from the manipulation of clouds of electrons in the
spaces between the elements of a tube, much in the same way a tunnel diode
achieves it manipulating carriers in a solid state material.

Ron AC7AC
K2 # 1289

-----Original Message-----
From: [email protected] [mailto:[email protected]]
On Behalf Of Lee Buller
Sent: Thursday, July 17, 2003 1:55 PM
To: Elecraft Reflector
Subject: [Elecraft] Negative Resistance????

What am I missing here?  Negative resistance?  What are we talking about
here?  I always thought that you either had resistance or there was no
resistance.  You cannot have no resistance because you will always have
some...at least in the physics in my head.  To me, negative resistance is
one the other side of the Y coordinate and could only happen in mathmatics
and not in real life.  Conductance?  No, I do not thing that is what we are
talking about either.  HMMMMM.  Negative resistance doesn't make sense to me
although I guess if they are using those special ceramic compounds cooled
down to -254 degree or something.  Even then, I just can't get idea of
negative resistance.  Negative resistance strikes me a bit funny.

What am I missing here?

Lee - K0WA

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