2N3904 in zener mode

In a given process run ("batch") they'll be the same. Next "batch", probably could differ by as much as a volt.

...Jim Thompson

Mmmh... not too bad after all. Thanks!

This reminds me an old (12 years or so) design of mine, where I needed variable gain cells, DC to say 10 MHz, 12 bit, 1:50 factor or so. I used paired 2N4391-s; one for compensation feedback, the other as a variable resistor (having the gates shorted was handy as it allowed me to solder the cans together for temperature coupling). Buying about a 100 transistors inevitably bought me most if not all of them from the same wafer and I could find decent pairs easily enough. Now obviously I did not get 12-bit temperature stability, but I had anticipated this sort of trouble and had put an AD792 temperature sensor next to them :-). I had an MCU and it did turn the fine gain to compensate; this gave me a 10x+ advantage over competitors (who were _awarded_ all major deals in spite of that, but that's another story). Now the rest of the story is that I got no 12-bit integral non- linearity, either. Obviously even the 4391, in the reasonable low-voltage range where I used it, was about 0.5 to may be 1% non-linear, which was not acceptable. Fortunately I had enough processing power (on a 16.67 MHz 68340 :-) to acquire the non-linear data into a somewhat longer spectrum (8192 was the norm; I did something like 8500 or so) and continuously (IIRC 10 times a second) converted it down to a linearized version.... Worked wonderfully, some are still in use. Got carried away by just a phrase, I guess, but I see this as a "heroic" design of mine (there are more reasons except the above for me to think so :-) and I felt like telling about it although being only very remotely related to the context...

Dimiter

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On Jan 29, 3:31 am, Jim Thompson wrote:

The zener voltage for a given transistor type does not seem to change from batch to batch; sometimes one can even change vendors. Changes seen seem to be less than 1%. In the case cited, i believe you will not see oscillation or negative resistance effects. Run the current down toward zero to see some spetacular "bazz-fazz".

I'll say it isn't the same batch to batch and maker to maker. Some Darlingtons have a very cute EB breakdown. They oscillate.

What the hell is bazz-fazz??

Jim

No, there's no guarantee of this between batches. It's only guaranteed to be above a certain minimum- that's what's on the spec sheet.

But if they decide to cook the next batch a little longer, or change the device geometry to improve some other spec, the zener voltage could really change a lot. As long as it remains above the spec, you have nothing to legitimately complain about if the zener voltage zooms up to 15 volts in the next batch.

As an example, I have a batch of MPSA42's, spec says 300 volts maximum, but a half-dozen I tested work fine up to about 830 volts plus or minus about 10 volts. Does this mean I can depend on this? No.

Better to not go into production if you're depending on this!

Some NPN transistors seem to be very consistant at about 5.5 volts Vbe breakdown. If you then use them in collector-emitter zener mode, you get a nearly zero TC 6.2 volt "reference zener."

John

bazz-fazz????

I remember doing that as an experiment in college. My recollection was you only need 100uA or so. Maybe up the resistor a bit, say 47k to be safe that it breaks down. [Of course, you need to do the experiment. ;-) ]

I strongly do not recommend darlingtons for that; too variable and a high zener due to two E-B junctions to break down. I have tested a number of transistors, and for a given brand and type, the berakdown voltage (in the *stable* region for the part) is remarkably "constant" between devices and between batches (even when a year or so seperated). All transistor E-B junctions have a range of currents where they exhibit negative resistance as well as oscillate; some are really nasty. It seems that a majority *start* with a large negative resistance characteristic (even in the nanoamp region), and slowly go towards zero then finally positive. Then again, most zeners that are made also have similar problems.

Oscillates like all get-out over a wide band of frequencies. Reminds one of a hot, fat electric arc.

E-B junctions, not E-C junctions are the ones to use...

Like an electric arc in noise...

E-C isn't even a junction!

Tim

-- Deep Fryer: a very philosophical monk. Website:

Sorry, meant B-C. But if the E-B is forward biased and the B-C is reverse biased, one gets a lower TC zener...