Maintaining a Vbe Multiplier's bias value

To further clarify this question, I already know that some manufacturers provide 2N2222A's with 40V and with 60V max Vce specifications. I assume this is a function of differences in the FAB processes they choose to apply in making their parts. That's what I'm talking about... not only for differences in model parameters but _useful_ differences, too.

And how do I learn the salient details of various FAB processes?

Thanks, Jon

If you drive both adjust pins with the signal input, the 317 output is Vin+1.25 and the 337 output is Vin-1.25. Connect them to the output through a couple of resistors, valued to set the idle current. Where's the deadband?

Or you can take the output from the 317 output pin, with the 337 now acting like a constant-current sink to the 317.

I like to use LM1117s as power emitter followers, inside the loop of an opamp. That makes a cheap, well protected power driver, for load cell excitation and such. I did a bunch of tests to see whether flailing the adj pin can damage the regulator, and never managed to break one.

John

Once upon a time there was JEDEC, and all 2N2222A's had to be the same in regards to essential specifications.

But I'd use the OnSemi model, irrespective... the LTspice version lets too many variables drop back to their default values... might not matter, but who knows. ...Jim Thompson

Yep, I got the REF polarity reversed in my mind, but you still have an issue if you expect this to source any significant power. Thru resistors ?:-)

Show us that configuration in class-B, say 10 Watt output into 8 Ohms :-) ...Jim Thompson

"Jon Kirwan" schrieb im Newsbeitrag news: snipped-for-privacy@4ax.com...

Hi Jon, I like your approach coming from equations to analysing practical circuits. But then follows the verification with real parts. I wouldn't worry with small details, as long as the general idea is understood and followed. When you have performed some measurements on real parts, it is good to understand the important parameters, so you can tweak them yourself. ciao Ban

Okay. That's gone, then.

I'd still like to _learn_ about FAB processes, geometries, mask steps, subtrates (and if any BJTs include a bond to such things), and differences between them. For example, I've heard you talk about processes that include gold as a step (or more?) I'd like to know what does what. I can (and have attempted) a few 2D spatial integrals aka Hauser's analysis of crowding on r_b many years ago, and I'm vaguely aware of the fact that he neglected to account for lateral base diffusion which happens when the crowding and some local base widening takes place. I actually _did_ take measurements of real Hamamatsu diodes, years ago, and reversed out from the measurements what the dopant concentrations had to have been so that I could better model the behavior over a wide range of temperature operations (Hamamatsu flatly refused to give me any such information.) The resulting model I created _did_ model that photodiode at -40C to 55C better than I'd expected it to do and much better than the gross models I had at the time were able. So at some point, I'd like to study these things to get a better feel... but I'd like to know who has what FABs and what the processes are capable of and produce.

I hope it's not as difficult as pulling dopant numbers out of Hamamatsu was! I'm not wanting to know specific recipes or anything -- just process capabilities. Hopefully, FAB and process capabilities and locations are something that is known about and published. I can hope.

Jon

It's the thinking processes that I'd like to encourage in myself. Being able to deduce to specific cases from theory seems a vital part of not just copying others but being able to think on my own, at times. Of course, theory doesn't necessarily tell me what the constants are -- to get those, I can always scout for models... but as this part of this thread clearly shows, that only gets you within some very vague area. But theory remains VERY important all the same, even if in practice the detailed constants applied may be just about anything. Even with the same part numbers, as the recent discussion shows.

Yes! Of course. Theory can be used to deduce math models and models to express the math order of error terms ignored in them or the frequencies involved (for example, theory can tell you whether or not even order harmonics are possible from the deduction in hand), sans calibrated constants. To compute quantities, though, it helps either to have accurate model constants or else go to the bench and test out the facets you care about at the time.

I have some idea how to measure most useful parameters of BJTs. Problem is, it can be pretty hard to get for some. (Read: "work+equipment.") One of the things I'd like to do, when I get further along, is to design and build a BJT tester with a micro that can extract parameters and generate models at the EM1, EM2, EM3, and GP levels, at least. Maybe even VBIC, if I can grasp for it. That will teach me so much and I can take it in steps, so long as I can think out what I need in the first place. For example, I will need to be able to set voltages, monitor pin currents; or set currents, and monitor pin voltages; or just observe voltage after setting high impedance; etc. It would be nice to do a lot more than just curve tracing. And I'd learn a great deal from all this, spread out over a time by which I can assimilate each part in mind.

Thanks, Jon

=A0 =A0...Jim Thompson

I'm not sure how you can sleuth out doping profiles in that manner. When I've had the need to get such numbers, I would use an outside lab. They can actually profile the device, so you don't just see the net dopant, but how it changes. I've managed to discover a few trade secrets doing this. In the days before using epi for CMOS was common, a company where I worked would ion implant the wafer from the back. That was like a faux epi. A piece of cake for the outside lab to spot.

It's been 20 years now, but I used 1D and 2D models I found, which applied dopant concentrations to predict behavior. I could go find the book, I suppose, but I figured the information must be _generally_ understood. Certainly, when I was looking, it wasn't that hard to find. The issue was in plugging in the numbers. What I did was to take a series of measurements over a few weeks' time and then used a short program I wrote to "tinker" the parameters until the predictions came very close. I knew physical dimensions for the die.

This was a one-off and the entire company was 20 people, including part time. I don't think they could have afforded the price. In my case, I got paid a few hours' work but the data collection just ran on its own. Most of what was needed to set it up was there and we bought a Burr-Brown board for the rest.

I can't say that I knew the dopant levels, for sure. It's a high quality photodiode, though. It's not complicated and the integrals weren't anything to write home about, as I recall. What I needed to model was dV/dI over temp. Not everything else. So maybe I got lucky. That was my focus, though.

In the end, when I used those dopant figures and then re-used the equations from the book to predict behavior at points where I had _not_ measured it, but interpolated as well as extrapolated, and predicted very well. We had taken measurements down to -5C but the model nailed the observed dV/dI at -40C and at 55C, as well. Which is an impressive range to someone like me. Seemed to solve a problem, anyway. Equipment is still in the field and 'doing fine.'

I can imagine.

What had surprised me is that a neophyte like me could pick up some basic books on the subject, cold, and arrive at something that worked reasonably well and allowed me to deduce equations for automatic corrections that worked so well. Although it could be entirely accidental, when you get such results from the application of semiconductor theory it is hard to believe one could be _that_ lucky (so I think something was nailed down right and the physicists working on these things actually know their stuff pretty well.)

Hehe.

I wish I knew more of this stuff. It's interesting.

Jon

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Excellent, Thanks Phil. Crank up the bias.

I think almost everything I build runs class A. I set a DC bias and then modulate on top of it. I run an opamp into a pass element, apply feedback and then make the load 'look' as resistive as possible. (Then cross your fingers and see if that works.)

So how do you do push pull with tubes, or say with only Jon's npn transistors?

George H.

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I only found them last week, I'd never seen a FET biased with a current source. It looked kinda cool...(Though Phil said it was stupid... anyone ever tried it?)

George H.

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Ohh, I like using the ....LM395?? LM375?? I can't remember the part number. You treat it like a power NPN but it has the built in thermal protection of the voltage regualtors.

George H.

Big Grins!

Yeah I applaud your effort, I wait for further posts.

For me, I=92m building electronics to either detect something or drive something that=92s detecting something. So the fun is in making good detectors or drivers.

George H.

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"fast"

Cool! I think I got it... though if I try it in the future and let the smoke out of something... then I might have questions.

George H.

Well, I am wanting, eventually, to build something I need. Something I cannot buy in the market because the need is unique.

This divides into two parts. Design and build. Since the item is unique, I can't just go out and buy it. And getting the features I need cannot just be "hacked" into existing designs without at least knowing _some_ stuff, first. I might as well turn the "design" part into a fair learning experience, as a separate project of its own. Get past that and when it comes time to build what I want I'll be able to build on what I learned and add what I need and then do a modest hobbyist level whack at actually making what I want to make.

If someone else were to do this for me (hire a designer), they'd get all the fun of learning on the job and taking my money with it. They get the money, they get to further their own education, and I get a tool. One tool. Once. Next time, I get to pay someone else to learn for me.

It almost feels like paying someone to go do your exercising for you. No satisfaction and no weight loss. They get all the _real_ benefits.

Part of the fun isn't the destination itself but it is what you see and enjoy while getting there, too. You take a plane when all you need is to "get there" quick, but you drive when you want to enjoy stops along the way. I used to fly to Burbank every week for a year and a half. Slept in a hotel for 3 nights a week, worked day and night in between, flew home. Barely saw anything but hotel room walls, cubical walls, a few cement roads, pollution so thick you couldn't see the Burbank hills from the Lockheed center, and not much else. The destination was important, of course. Paid the bills and I enjoyed the work, too. But there is a lot more to see in the 1000 miles from here to there.

Anyway, I'm driving this time, not flying.

Besides, I'd rather _keep_ the money and _keep_ the education for myself. That way it pays off, again and again.

Jon

=A0 =A0...Jim Thompson

Hmmm must be lotsa good books out there. I've got Streetman and Szu. (sp?)

George H.

"George Herold

So how do you do push pull with tubes, or say with only Jon's npn transistors?

NPN output transistor amps are called "quasi-complementary push pull " - many millions of them made and sold too.

Use Google to find the schems.

Idiot.

.... Phil

IIRC the LM395 is basically an LM309 with the voltage reference removed. Emitter-follower regulators are nearly bulletproof unless you discharge a cap into the output.

Cheers

Phil Hobbs

Usually two "NPN" tubes whose plates drive a center-tapped output transformer.

John