Negative resistance with BJTs

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This has been well-known for quite some time. The British Journal of Scient ific Instruments (now Measurement and Science and Technology) published a s hort note on the technique in the mid-1960's.

I can't find the journal reference, but IIRR the author used a gold-doped s witching transistor - perhaps the 2N3904.

The residual temperature coefficient (TC) did depend on the current, which had to be somewhere around a couple of milliamps, though the exact current for minimum TC varied from device to device. The author talked about sticki ng a soldering iron on the package to see which way the reference voltage m oved when you heated the part up, and recommended trying a range of curren ts to bracket the point where the excursion went from positive to negative.

--
Bill Sloman, Sydney
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bill.sloman
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cause UHF oscillation."

It isn't and it doesn't have to be. think of standing waves in a transmissi on line. Now think of the plate as a transmission line.

It seemed like the frequency was controlled chiefly by the filament voltage , you could see the phase change on the screen as you tuned through vacant channels.

So now they needed an extra pin for the suppressor grid. Just connecting it to the cathode did not work. I am not sure how it worked, maybe it was cau sing the suppressor grid to pick up some localized secondary emission or so me shit.

Back then it wasn't a issue enough to investigate, so it certainly isn't no w. the separation ad positive bias on G3 did the trick and that's all we ca red. Before that we tried a million things, one that sometimes worked was t o find just the right tube that didn't do that, or at least didn't do it at frequencies used by local stations.

Reply to
jurb6006

Diac and Schockley are close cousins. The 4-layer schockley diode was unidirectional and made in grades from about 7 to 30V often with low TC and close tolerances. The Diac is bidirectional and I think a 3 layer device. Sometimes Diacs are drawn looking like an NPN BJT with two emitters (as in outwards pointing arrow on the "collector").

piglet

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piglet

Wiki has 'em both:

piglet

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piglet

That 'electron' can be an electron in the conduction band going into the wire, or a hole in the valence band coming from the wire. But at the other end of the wire, it's always gonna be the majority carrier for whatever doping is present at the metal/semiconductor junction.

Metal/semiconductor interfaces are just as nonlinear as other junctions, and it is very important, for Si integrated circuits, that aluminum can alloy with either P or N material and make an ohmic connection and do it at a low-ish process temperature. IC manufacture depends on that fact just as much as on SiO2 for passivation and insulation.

Oh, there's a patent that I recall... yep, takes some tracking, but here it is

The idea is to make a current modulation, and use the magnetic field around the (unshielded) wiring to sense wires through walls

Reply to
whit3rd

Oscillation in a power transistor, especially a power MOSFET. (Pease, P. 88.)

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Phil Hobbs

Well, only partly. (I'm 1959 vintage.) Mostly it's because I taught myself circuit design out of old ARRL Handbooks and National Semi app notes, among other things. As a kid I used to build tube circuits that mostly didn't work because I didn't know enough. I got most of the parts by taking apart old 1950s and early '60s TVs that I cadged from repair shops because new parts were way beyond my budget. The last tube circuit was about 1990, where I used an 811A to run an electrostatic grid in an ion drift detector. Its combination of high voltage and low capacitance was beyond that of any semiconductor.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Phil Hobbs

My uncle Sheldon, the TV repair rascal that taught me electronics, called the vertical bars, from HOT oscillations, "squegging."

The circuit that used the negative resistance/noninverting gain behavior of pentodes was called the "phantastron". It was widely used in radar and scopes and timing circuits.

--
John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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John Larkin

I remember squegging. Didn't remember snivet though. Very useful word !

Just found this site....

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snivet

A short burst of spurious oscillatory circuit behavior.

spudger

An adjustment tool of variable capacitors or inductors that is nonmetallic and nonconducting so that it will not influence the adjusted value. Spudgers are usually made of plastic and thus have a permeability close to that of air.

squegging

Spurious frequency modulation in an oscillator caused by noise or instability.

Reply to
boB

Well that's very different from the other answers. You mean this is a characteristic of the transistor by itself, under certain conditions, regardless of the circuit?

Which Pease book? I know of one but I thought there was another.

Reply to
Tom Del Rosso

Rang a bell with me too, though I didn't remember the definition. It is in my 1962 "A Dictionary of Electronics" paperback (7/6d Penguin) whereas "snivet" isn't.

Mike.

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Mike Coon

I assume the trouble shooting one. I bought another Pease book... Analog circuits (world class designs). Totally NOT worth it.

George H.

Reply to
George Herold

Squegging is specifically caused by the bias time constant being too long.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Phil Hobbs

Yes. The input resistance goes negative when you put a resistor in series with the source.

"Troubleshooting Analog Circuits", i.e. the good one. His other two are junk--I bought "Analog Circuits (World Class Designs)" and chucked it out within a day or two, and his book on driving "How to drive into accidents and how not to" is a turgid mess. Ironically Pease died by crashing a horribly unsafe car (a '69 Beetle) into a tree on the way to Jim Williams's wake. :(

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Phil Hobbs

We used to drive past his house now and then. The yard and neighborhood were littered with rusty old VW beetles and vans in various states of disrepair.

The one that he was driving at the time was literally packed with papers and junk; there was one skinny clear spot on the drivers' seat.

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John Larkin         Highland Technology, Inc 

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John Larkin

I encountered some BJT-related snivets a few years ago, when a retiring ham gave me a two-tone audio oscillator intended for doing amplifier intermod-distortion analysis. He said he'd never been able to get it operate stably - the audio frequencies jumped around a lot for no apparent reason.

I looked at it with a fast 'scope and an RF spectrum analyzer, and found that the audio waveforms would break into a burst of VHF oscillation around the point of their minimum output voltage. I could shift the VHF parasitic oscillation (and even change the audio frequency of oscillation!) by waving my fingers near the wiring or lightly touching the components.

The circuit was out of QST magazine, and used a pair of twin-T oscillators based on 2N2222 transistors (real, genuine metal-case jobbies no less). As I recall, the twin-T circuit they'd chosen had a capacitor from the transistor base, to ground... a notorious cause of instability. The transistors were socketed, with fairly long leads (and significant lead inductance)... another warning sign.

I stuck ferrite beads around the base leads (and, I think, the collector leads) of the transistors, to add some VHF loss. The circuit immediately quieted down and behaved like a tame kitten... stable oscillation by both oscillators, no parasitic VHF, no weird jumping-around when I touched the components.

Snivets. A *lovely* word indeed.

Reply to
Dave Platt

Hey speaking of trouble shooting, I found a new one (for me) today. 74HC14 was being powered by it's input pulses. (I forgot to hook up the 5V rail.)

George H.

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George Herold

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Reply to
Rob

That's in every edition of AoE!

Reply to
Tom Del Rosso

I gave that book to a non-driver as a gift. I never had the heart to tell her...

IIRC it was after he left the wake.

Wasn't he assumed to have had a heart attack since no other cars were involved?

Reply to
Tom Del Rosso

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