Rail to Rail output stage for 2A 5V

Clive, Schlumberger, with a large engineering group near to me here in Cambridge, has hundreds of well- experienced, highly-educated (PhDs, etc) engineers and scientists developing downhole stuff. And they have special parts to use that can handle 250C, maybe higher. A good friend of mine works there. They DO NOT tell anyone what they're doing or how they do it.

Clive Arthur wrote in news:qddo5v$5ko$ snipped-for-privacy@dont-email.me:

Make a miniature "submarine" (a plumb bob) to enclose the array of any tool you want down there.

What are the dimensions of such a box? IOW what are my physical limits on the enclosure?

Steel vacuum cans that can also withstand the external pressure you describe are quite possible, as I am sure you know.

And you also had a temp spec. How hot is it down there? If a gas tube could be included in the tether, one could pump a cooling gas down to take some heat off the bob.

Do these go behind a drill head or in place of it or? Is a downhole probe an entirely different animal than the drill shaft?

Ti makes a high temp rail to rail op-amp they claim is good to 210C:

Only 3.3V though...

Schlum stuff is usually good but *very* expensive. Wells are very hostile environments and the risk of destruction or loss is high.

There are several other companies in the game, though mostly 'only' up to 180'C, and anyway, Schlum staff don't stay there forever...

Cheers

Huh, Digikey lists 23 opamps that run at 210 (or even 225 C)

None of them are cheap.

George H.

fredag den 7. juni 2019 kl. 15.50.53 UTC+2 skrev snipped-for-privacy@decadence.org:

there's a world of difference between 1 bar of vacuum and 2000bar pressure

Lasse Langwadt Christensen wrote in news: snipped-for-privacy@googlegroups.com:

I am merely saying that the circuit device must reside inside a vacuum chamber that has that pressure on the outside. You are saying that one out of 2000 is negligible, but the seal on that chamber needs to handle it without failing

I know the difference. The gauge goes the other way and pegs the needle. I get it. Sheesh... now I have peanut gallery commentary crap.

Ignoring the seals for the moment If you make the walls of the vessel

I read a paper once, about using plastic ic's and transistors at high ambient pressure, in an oil bath or potted. They worked but had DC offsets. TO-can parts were crushed.

I have a nice simulation, op-amps wrapped round complementary common source MOSFETs, which looks good over a wide range of loads and to within 250mV of the rails. Class B in fact, no Iq, but very little crossover distortion - it's not meant to be Hi-Fi anyway.

But the LTspice .tran simulation at high loads running from a sine source shows a few spikes on the output with the same sort of amplitude as the signal. Zooming in shows these are between as a few tens of femtoseconds long and a few picoseconds long. The alternate solver doesn't show these.

Clearly these can't be real, but is the LTspice sim trying to tell me something?

Cheers

Let me warn you that MOSFET SPICE models are horrible at modeling linear operating at current well below their rated Ron-limited switching current. They get g_m vs Id completely wrong, by factors of up to 100.

You need bench, not SPICE measurements, before talking about looking good.

Thanks for the heads up, Win.

Cheers

You aren't using the alternate solver always, and you should.

Cheers

Phil Hobbs

You can take simple bench measurements on your preferred MOSFET types, and create accurate SPICE models. Here's a crude draft of our AoE x-Chapter writeup on the subject, read pages 33-39, and also some of the references there. You can create a model that will match your subthreshold measurements. Then SPICE will give the right answers.

(Don't worry about weird figure locations, that's the TEX automated processing trying to decide where to put stuff. When a publisher processes the file, a human does the job.)

Many of use prefer to take gm measurements in the middle of the operating region, or a few regions, and do a set of analytic small-signal calculations, to evaluate our compensation circuitry, etc. Who needs SPICE anyway? Bench measurements rule.

Winfield, many thanks for sharing.

How would you go about modeling the large RF LDMOS FETs? Would these simple bench measurements be useful at all? We are reverse engineering the PCB design of an old VHF (225MHz) linear amp used for analog TV, which used BLF278s. The aim is to create a SPICE model that will allow us to scale it for better operation at 144MHz (though it works ok there already).

Clifford Heath.

Sometimes Spice does goofy things. Your circuit is probably OK.

Interesting stuff, thanks. You may have been spending too much time in Massachusetts, though--on P. 32 you have you have a LaTeX description environment

Avalanche. Although the very term ?avalanche? evokes images of mayhem and destruction,(25) MOSFETs are not damaged by such insults as long as the peak and repetitive avalanche ratings of the device are respected." (and so forth).

Footnote 25 reads, "(25) Which bothers some men. The word itself makes some men uncomfortable."

Actual guys like nothing better than blowing up stuff. ;)

Cheers

Phil Hobbs

Phil Hobbs wrote in news: snipped-for-privacy@supernews.com:

snip

Even God likes smoke release events.

He may not like some of the labels we use either though.

Oh, boy, that's really out of my league. If forced to work on it, I suppose I might try a set of linear parameters to represent operation at various power levels, but to optimize high power operation, you really need much more powerful non-linear modeling.