RF Transistor Noise Source

It'll be good to know what your results are.

Twenty years ago you could buy a noise diode from MA-COM (IIRC; it may have been some other company), home-brew your own circuit to hold it, then send it back to MA-COM for calibration. I don't know if you still can.

A noise diode was, of course, 'just a zener', optimized for use at microwave frequencies.

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Noisecom and Micronetics are the only two I know of. Noisecom used to sell "factory seconds" to hams for pennies on the dollar but that practice seems to have gone by the wayside. They, as you noted, would also do a calibration of your design for a few bucks. Gone also.

It will be fun to get back to experimenting with something where I don't have a real good idea what the answer is going to be.

Jim

I think it's Noisecom that I was thinking of. Dang; I should have taken advantage while I could.

I have thought that if you were building something low-noise enough you could measure the noise figure with a pair of transmission lines terminated in resistors: drop one into ice water (or dry-ice/acetone, or LN2), and heat the other one up (boiling water, or a not-quite-melted- solder heat furnace). Then switch between them. With no current flowing through the resistors, you'd certainly know their noise temperatures!

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A common approach in physics labs is to terminate the input with a 300 kelvin resistor, measure the noise, dunk the resistor in liquid nitrogen, and measure it again. Works great.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs
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ElectroOptical Innovations
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email: hobbs at electrooptical dot net
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That sounds good. A "real" RF transistor might deliver wider bandwidth, from having less junction capacitance. BFS17, BFT25, something like that.

Please let us know what you find.

John

One thing to keep in mind is the transistor is easily damaged when zenered. You should do this with a current limited supply, say 100ua.

If you've ever done ESD testing or fuse testing, invariably the reverse biased diode is the thing that is easily fried. When popping metal fuses, you need to insure that the inductive kick of the zapper is such that after popping the fuse, the diode junction gets forward biased. This does dump current into the device, but that can be controlled by the size of the capacitor used in zapping. For ESD structures where there is no diode to the positive rail, the snap back of the "off" nfet saves the parasitic diode junction.

It ruins their beta, but if the transistor will never be used as anything but a zener, who cares? I suppose the issue is whether the

*zener* properties will change over time. I'm guessing that a transistor, especially an RF transistor, will have a much higher current density than a part designed to be a zener.

This suggests some interesting experiments.

John

Goodness you keep the heat turned up -- or is that in the summer?

It's about 293K in here now, because I can get comfort cheaper with a sweater than by turning up the heat.

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The garden variety transistors (2n3904 for instance) has a very low current when it zeners. It makes a micropower reference. I'm not sure about RF transistors.

The device is easily damaged as in hard failure when zenered. I can't speak for off the shelf transistors, but for IC diode junctions I've tested, what happens is the current flows at a hot spot on the junction, probably at a wafer defect. It gets zapped and goes ohmic. I've never done zener zap trimming, but it might work in that manner.

If you had a bare die and an emission microscope or maybe even a sensitive video camera, you would see the hot spot as it zeners. Once it goes ohmic, you would put liquid crystals on the die and see a boiling effect at the point of failure with applied power. I'm not a reliability engineer, but have done the tests a few times. [I'm guessing reliability people don't hang out on design newgroups. ;-)]

Nope, we reduce waste by keeping the house about 59 F in the winter (55 at night). So since it's 3 AM here, I'll see your 293 and raise you -8 kelvins. ;)

Maybe the kids will move to Texas at some point.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net

Well, all of the bipolar transistors seem to have the_specification_ of a max reverse VBE of 5 volts, but in fact they all zener in the 8 volt region. And the majority exhibit negative resistance and noise from sub-nanoamps to a few milliamps.

Be advised that the damage to a zenered transistor E-B junction is a time * current or dosage product, exactly as if it got radiation damage. Total dosage: a little over a long time = = a lot over a short time. Fairchild uA709s used in the Apollo got "nailed" by that. Turns out the company hired to test and burn them in did not know what an op amp was or how to test them or even burn them in despite a burn-in circuit in the data sheet! Their circuit zenered the inputs and that caused a failure mode during a mission. Fairchild engineers had to teach some basic electronics, and then advance to op amps and test methods as part of proof the 709s were not initially bad. The other part was a setup burning in NIB same date lot code parts in

2 batches: one using the nasty circuit and the other using the datasheet circuit. Oh yes; the cure is to anneal out the damage in an oven.

a

time.

in

Hello Robert,

I did some experiments with low current reversed bias to the BE junction of BC847. Even at low reverse current (10uA for several hours), the HFE at low collector current (

Geesh! I try falling asleep at home without a space heater and wake up all sore, apparently from shivering all night. Has to be at least 68F in here to be comfortble, I'd guess.

'Course, at school I'm spoiled, because the hot water radiators are always cooking at a toasty 72 or so.

Tim

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Indeed. Within a transistor the current density is much greater, and the zener voltage degrades... accelerated by high temperature, such as under the hood of a car... noted while developing integrated alternator regulators in the mid '60's. Also accelerated by mounting in cheap phenolic packaging. ...Jim Thompson

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That sort of frugality would get me divorced. You've met Mo and probably noticed her unfavorable mass/surface area ratio.

Car seat warmers are another great marriage-saving invention.

All of which explains this week's automation project.

John

And Joerg was coming unglued because I said we have seat warmers/coolers in our car ;-) ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
                    Help save the environment!
             Please dispose of socialism responsibly!

"Reference zeners" like the 1N935 sort of guys, are remarkably stable over time. They are usually a stack of a zener in series with one or two forward diodes, giving a TC that is zero at some current around

7.5 mA, usually. You can tune the current to hit zero TC.

The best way to get a really good current source is to use the reference zener to make its own current source, using a simple bootstrap circuit. Just make sure it starts up!

I once bought a 6.2 volt zener from Motorola for $35, which was a lot of money then. It came in a presentation-quality tube with a 1000-hour graph of stability, signed by all sorts of important people.

Some good stuff here:

John

Lots of NPNs zener around 5 volts. If you use the collector and emitter, you get a "reference zener", a zener in series with a forware-biased diode, around 6.2 volts with a very low TC.

For some reason, PNP transistors sometines have higher zener voltages,

10-12 volts maybe.

John