Your comment about be being way below 0.65v quiescent tells me you're running class B. Like that it can be stable enough. If you ran it AB as I thought you were it would be very unstable. Why...
Trs idle with 0.65v Vbe, much of opamp's Iq going through the Rs, some to tr bases.
As Trs warm up under exercise, their Vbe drops
You then have more Ib and less I through the psu/base R
So the trs turn on more, get hotter and turn on more. Vicious cycle with no way to reduce quiescent current or overlap of drive to the 2 trs.
LT Spice lets you declare a diode to have a constant forward votage drop, which I guess the zener models have. That can be handy sometimes but it's not real.
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John Larkin Highland Technology, Inc
lunatic fringe electronics
They allow the opamp to run within its voltage rating, but still shoot its supply currents up to the +-16 volt rails.
Those just let us shut down the output stages when the customer wants zero magnetic field, like when he's shimming the superconductive magnet.
D6 and D7 add an output deadband that further reduces current and noise when we want zero field. NMR has parts-per-billion sensitivity, so tiny zero offsets matter.
We don't do NMR instrumentation any more. Agilent acquired our customer, Varian, and eventually killed off the NMR operation. I think other analytical chemistry techniques have mostly replaced NMR; the big magnets were really expensive to buy and operate. Agilent also killed off the Varian FTMS products, which needed even bigger magnets. We were developing a cool FTMS controller too.
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Science is fun, as long as you don't have to do the grad-postgrad-PhD-postdoc-publish thing yourself.
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John Larkin Highland Technology, Inc
lunatic fringe electronics
I met a few people laid off from Varian's NMR group. I know about quadrupole* mass spectroscopy.. kinda cool how it all works. I'm not sure about the Fourier transform part.
I suck at writing, but like to make pretty pictures for publication. The worst part of research is grubbing after the money.
George H.
*I want to spell that quadrapole, quadrupole sounds like you've multiplied your mass spectrometers by four.
I'm bored so here's a very brief intro to FTMS. It is based on ion cyclotron resonance. Start with an ion in a vacuum in a uniform magnetic field. In the direction of the field there is no force on the ion so it is free to keep drifting with whatever velocity it started with. Perpendicular to the magnetic field the ion experiences the usual E cross B force, making it's path curve. If the B field is strong and the velocity low relative to the mass to charge ratio m/q the path becomes a circle and you can show that the time to traverse that circle is 2pi m / q B or the cyclotron frequency in hertz is q B / 2 pi m. Put the ion in a box and bias the ends that are perpendicular to the magnetic field with a few volts of the same sign as the charge on the ion relative to the other walls and you get a potential well inside the box that traps the ion along the magnetic field, and the magnetic field does the trapping along the other two axes so now you have a trapped ion cell. Apply rf perpendicular to the magnetic field and the ion will absorb energy and increase its cyclotron radius. If there is more than one ion of the same m/q they will each move in phase with the applied rf, and if you pump them up to a final cyclotron radius that is much larger than the initial radius before the excite they will each have approximately the same final cyclotron radius and the cloud will orbit coherently. Pick a point on one of the cell walls perpendicular to the magnetic field and as the cloud approaches and then recedes it will induce an image charge that can be detected. The simplest trap is a cube, with six sides. Two end plates do the axial trapping, one pair of opposing side plates is used to apply the excitation rf, and the other pair of opposing side plates is connected to a differential preamp to record the signal. For magnets in the 1-7 tesla range and m/q ratios in the 18 to say 10,000 range the frequencies are in the 1-5 kHz to 5-10 MHz range, and for ion populations of say 1000-1000000 the raw signal is in the microvolt range with cyclotron radii of .5 to 2 cm. In the second picture John posted the tubular object with the ruler next to it is a trapped ion cell. There are two end segments 2" long and a central segment 3" long. The end segments can be simple cylinders but are often split into four quadrants, and the central section is split into four quadrants to provide the excite and detect plates. Cells can be cubic, rectangular, cylindrical, and other shapes, and the length to diameter aspect ratio can vary also, and that discussion fills books and fuels arguments. FTMS supercon magnets usually have room temperature bores of
4-6" so by the time you stuff a vacuum chamber into one a rectangular cell winds up maybe 1.5 to 2.5" across and a cylindrical cell maybe 2-3" diameter.
Anyway, if there is just one m/q in the trap then you will get a simple sine wave after the excite. The amplitude tells how many ions and the frequency tells the m/q. As the ions collide with background gas the amplitude will exponentially decay as individual ionis are knocked out of phase coherence or into plates, with a time constant of about 3 seconds at 10-8 torr. Record the damped sine wave, FFT it, convert Hz to m/q, and there's a mass spectrum. In FTNMR the excitation is a pulse of single frequency rf so the final phase is linear and can be easily corrected but here the frequency range is sufficiently large that the simplest excitation is a swept sine wave, which results in a quadratic phase function after the FFT so the usual answer is to use the magnitude spectrum. Excite chirps are in the range of
0-3 MHz in 1-5 msec applied differentially to the trap so maybe 50-400 volts pp across the trap.
It's been a while since I've given an "intro to ftms" and I tried to keep it short and terse, but I hope it answers the basics. Haven't touched one lately but counting grad school I spent about 30 years working with them, building two completely from scratch, building several different vacuum chambers, ionization sources, and transport ion optics for others, along with a good bit of electronics, computer interfacing, and software to go along with them so if you have any questions just ask :-).
I wanted to detect a single molecule orbiting in the cell, which I think is barely possible. Agilent killed the product line (IonSpec originally, acquired by Varian, acquired by Agilent, killed by Agilent) so I didn't get to try.
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John Larkin Highland Technology, Inc
picosecond timing precision measurement
Hah! Thanks Carl. When I was a postdoc we detected hole CR in GaAs quantum wells. Some of my favorite data... I've kept a copy taped to my wall. Detection was complicated. We monitored the visible absorption spectra as we blasted the sample with an FIR laser. ~20 cm-1, 500 um.
Bored? I don't suppose you know anything about atomic physics? Farting around at a workshop (for teaching others to use our equipment) I recreated some work I did ~10 years ago. Observing the Zeeman splitting of a Rubidium absorption line. And now, when I tried to explain why it worked, I just make myself more confused. It will take me a while to organize the data and explain my conundrum. (So not today.) I was going to post on Sci.optics, maybe here too?
Hah! Thanks Carl. When I was a postdoc we detected hole CR in GaAs quantum wells. Some of my favorite data... I've kept a copy taped to my wall. Detection was complicated. We monitored the visible absorption spectra as we blasted the sample with an FIR laser. ~20 cm-1, 500 um.
Bored? I don't suppose you know anything about atomic physics? Farting around at a workshop (for teaching others to use our equipment) I recreated some work I did ~10 years ago. Observing the Zeeman splitting of a Rubidium absorption line. And now, when I tried to explain why it worked, I just make myself more confused. It will take me a while to organize the data and explain my conundrum. (So not today.) I was going to post on Sci.optics, maybe here too?
George H. =======================================================
Sorry, George, can't help with much physics theory.
The ESR of the Zener and PN Diode are related to the inverse of their power ratings and should match the power rating of the power rating of the R_be string being biased to match the string of ESR's or Zzt's.
Also critical for thermal stability is thermally coupling to track NTC Shockley effects of bias diodes to load diodes.
There's more to this but, that's the main criteria, not the arbitrary comparison of a Zener vs an ordinary diode.
I've been visiting Sandia for the last couple of days. I was talking with a low-temperature guy who apparently uses SiGe BJTs at 4K. JFETs deteriorate quite badly down there, and other bipolars become open circuits due to car rier freeze-out, so I was quite surprised to hear that heterojunction bipol ars keep working.
The IonSpec preamp that I saw was uncooled. Here it is:
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It's a pair of fet opamps running as followers. They have no cooling in vacuum except radiation and run at roughly 120C. Gate current was so high that they needed 1Meg resistors to ground on the inputs. Shot and Johnson noise big-time. I figured they gave up at least 30 dB.
Chemists are maybe the worst circuit designers.
I had a pretty promising and suitably weird design but the whole thing died.
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John Larkin Highland Technology, Inc
lunatic fringe electronics
The IonSpec preamp that I saw was uncooled. Here it is:
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It's a pair of fet opamps running as followers. They have no cooling in vacuum except radiation and run at roughly 120C. Gate current was so high that they needed 1Meg resistors to ground on the inputs. Shot and Johnson noise big-time. I figured they gave up at least 30 dB.
Chemists are maybe the worst circuit designers.
I had a pretty promising and suitably weird design but the whole thing died.
--
John Larkin Highland Technology, Inc
lunatic fringe electronics
a low-temperature guy who apparently uses SiGe BJTs at 4K. JFETs deteriora te quite badly down there, and other bipolars become open circuits due to c arrier freeze-out, so I was quite surprised to hear that heterojunction bip olars keep working.
Huh, maybe they would work as low temperature, temperature sensing diodes?
Re: Cable capacitance, sounds like a job for a driven shield. I made a driven shield to look at the Johnson noise (R~1k - 1M) of resistors dwon the bottom of a probe. It didn't make into the final instrument, but worked fine.
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