negative resistance and constant current source

Sure, any electronics book has a negative resistor, usually with an operational amplifier. Limitations are the bandwidth and the usual amplifier limitations.

Rene

Sure. This can be done to improve at-load power supply regulation, to compensate for wire losses, although 4-wire remote sensing is better, because it doesn't need to know the wire resistance.

Linear oscillators use negative resistance to cancel real resistive losses.

Interestingly, connecting a negative resistor in parallel with a platinum RTD makes the net resistance linear on temperature. For a 100 ohm RTD, the value is - don't quote me, it's been a while - something like -2.5K.

Simple hysteretic buck switching regulators often have a negative output resistance... I've never figured out why.

You can build a 2-terminal 1K negative resistor with a couple batteries, an opamp, and three 1K resistors. Then you can build fun things: voltage dividers with gain, interesting R-C circuits, oscillators.

John

Some DC motor control circuits use this principle to cancel the armature resistance, thus providing constant speed with varying load for given supply voltage. I think some model train supplies work this way so the trains don't slow down when going up hills. Some Minarik controllers also use this principle.

Sort of.

You insert a resistor in series with motor (in ground lead) equal to armature resistance.

Measure this voltage and add TWICE this value to control voltage for the amount applied to motor.

...Jim Thompson

Hello Jon,

Since you mentioned tunnel diodes: These were in almost any electronics project book when I was young. It all looked so deceptively easy. What the authors did not tell their fellow readers was that it was next to impossible to obtain any. At least not unless you name was Rockefeller.

Regards, Joerg

Some of the GE germanium TDs used to cost around $1.50, as I recall. I did a lot of TD circuits when I was a kid... bought them from Allied. My senior paper at Tulane was "The Tunnel Diode Slideback Sampling Oscilloscope."

I think that GPD still makes fast - 20 ps - TDs, but the last time I bought one, some years ago, they were about $80 each. They acquired the old GE mesh-etch process, truly astounding.

John

Hello John,

I lived in Europe in those days. Maybe the project books consisted in part of schematics from the US. I had tried to buy tunnel diodes a few times and the electronics stores always had the same answer. Too expensive, nobody would buy them so they didn't stock them. There was no such thing as Digikey back then and a special order would have required me to ask dad for a substantial loan :-(

BTW it was the same with unijunction transistors. Lots of project ideas published but the things were unobtanium. Oh well, that way we all learned how to design with what's there, no with what's the rage.

Regards, Joerg

I wonder if the post-war boom in electronics in the USA was partly driven by the ability of kids to get parts. I used to buy surplus radars and stuff from Fair Radio Sales, for pennies per pound, and I had an infinite supply of old tube-type TV sets to scavenge. I think it was deliberate government policy to dump all that WWII electronics on the surplus market.

When semiconductors arrived, you could buy a CK722 or a 2N107 and more exotic stuff from Allied.

The Soviets probably damaged themselves by being so restrictive about ham radio. It's best to learn anything, tennis or dancing or electronics, when you're young.

John

Hello John,

Europe wasn't too bad either. WWII surplus was available from both sides, mostly at hamfests. Old Ge transistors (OC series) could be scavenged from discarded hearing aid amplifiers or radios. Scratch off the lacquer and you had a photo sensor. But I build more with tubes. They had better oomph and were usually free.

They had ham radio but they had to be careful what they said. Big brother was omnipotent. Their skills might have even been honed better than in the west since they really had to live on scarce supplies. Everybody over there must have known how to create things like a stable voltage without ever dreaming of laying their hands on a zener diode. The purchase of something like the uA723 was unthinkable.

Regards, Joerg

Oy, my aching wallet! Years ago around our area you could get old CDC 3200 computer cards for 5 cents each. Each card had about 6 TO-18 transistors, a few 1/8 watt resistors, and often a tunnel diode or two. Back then we'd use to salvage the "useful" components, like the 2N706 transistors, and toss away everything else. Wish I'd saved the tunnel diodes!