So I'm doing this new laser noise canceller that I was talking about in the "differential signal detector" thread.
The canceller is probably my best gizmo ever, and iw actually pretty simple as well. If you're interested, check out "ultrasensitive laser measurements without tears", . The seven circuits in that paper use MAT04 supermatch quad NPNs, which work great at lowish frequency (
The collector curves are flat, or negative.
I could use that as a signal-pickoff emitter follower, base hung on a
50 ohm microstrip. Any guess what the equivalent loading impedance would be from DC to maybe 4 GHz?I'll get some and try the pickoff thing. Maybe a Colpitts oscillator too.
I still need to try the SAV551 as a follower too.
-- John Larkin Highland Technology, Inc Science teaches us to doubt. Claude Bernard
Not very different from just the pad, given C_CB ~0.2 pF and very high beta.
Works brilliantly as a bootstrap, which is sort of a follower on steroids.
I'm having much worse problems with the photodiodes themselves. The SFH2400 is small and has low capacitance, but even with >10V reverse bias, it slows down grossly around 300uA of photocurrent. (Its rise time goes from ~10 ns to ~250 nw.)
Cheers
Phil Hobbs
>-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
I did some higher speed Early measurements using a cascode transistor to adjust V_CE while allowing current measurement.
With a collector current of 0.5 mA, changing V_CE from 0.35V to 2.85V produced a fast change of about 1.0% in I_C, for a measured Early voltage of
V_A = 2.5V/0.01 = 250V.
As the part warmed up, about half of the peak delta decayed away with a TC of 2.2 ms, which is why slow measurements indicated a 500-V V_A. (The exact amount depends on the absolute power dissipation, of course.)
Doing the math, this gives a Theta_JA of about 200 W/m/K, which is reasonable. Interestingly there was no thermal-diffusion structure on the leading edge of the pulse--at the 10-us level there's no thermal speedup visible.
That's a good 20 times better than your average 10-GHz transistor, and this is a 45-GHz one. It's good enough (and the thermal is slow enough) that I can mimic monolithic behaviour by equalizing the dissipation of the two halves of the diff pair by dorking the V_CE of one side.
That makes a whole lot of things possible that weren't before.
I took a bunch of logging data today, so I can figure out what the R_ee' and beta linearity contributions are doing vs. voltage and current, so we can generate a tweaking algorithm for the real unit.
Fun.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
There is an old Tek trick, apparently re-invented by many others, to put a bypassed resistor in the collectors of a discrete diff pair, so the power dissipation is mostly constant with signal. That eliminated some thermal hooks in vertical amplifiers.
I've seen slow bipolars and fets with negative collector current slopes. I suspect that was bad measurement, device heating messing with the curves.
Yup. Makes a huge difference. (The Tek "Vertical Amplifiers" book is still a good read.)
The present case is a bit more complicated, because I need to be able to split photocurrents very accurately up to ~5 mA, in ratios from 10/90 to
90/10. A bit of degradation is OK at the edges, but in the sweet spot, I want to get the relative error down below 10**-3 from DC to ~10 MHz with no user adjustments.That's very hard with a 100-MHz f_T MAT14, but might be easier with
45-GHz BFP640s, if the temperature tracking can be got round.If the transistors are at different temperatures, the splitting is degraded because the same delta V_BE produces different ratios at different tail currents. Thus a symmetrical layout combined with dorking V_CE as a function of I_C has a lot of charm.
It's happening at millisecond timescales, too, which makes it a good match for some simple MCU magic.
Almost for sure. The Early effect is base thinning due to changes in the width of the CB depletion zone--you can reduce that, but it would be hard to imagine it changing sign.
The noise canceller gets its log ratio output by measuring Delta V_BE of the main differential pair, which has the usual 2 mV/K dependence on temperature differences. (You have to worry about the effect of the other device--some effects add and some subtract.)
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Can you replicate the old IC trick of symmetrical transistors configured to cancel temperature gradients here?
Like four transistirs in a square, with diagonal units paralleled?
Joe Gwinn
With discretes? That would be interesting.
Most of the dual transistors being sold these days are two die in an epoxy package, with bad thermal coupling.
Yes. The BFP640 transistors are about 2 mm square, including leads.
Two dies would not be able to cancel temperature gradients. Especially if the thermal cnnection is molding plastic, not a good thermal condusctor lile silicon. Sort of defeats the purpose, doesn't it?
Four chips in a square mounted on an alumina substrate would be far better.
Joe Gwinn
That picture is possible because there was no 'hat' over the package. Does it get better coupling if you glue a heat spreader (the pyrolitic graphite type, or a simple copper slug) atop the unit?
SOT143 is only one millimeter top-to-bottom, so sticking a spreader on top can definitely give some coupling beyond what the package has intrinsically.
SOT666 from Nexperia is only 0.55mm top-to-bottom.
That would help with the externally-imposed gradients, sure. It's less helpful with the differential dissipation, because the thermal coupling is heartbreakingly poor.
If you have a squint at the BC61C current mirror datasheet, you'll find a cryptic note on P. 4 that indicates that the maximum input current to the mirror for which the output device is thermally stable is 5 mA for a
5V V_CE on the output device--25 mW.That is, at 25 mW, a 1-K temperature change will cause the output transistor's current to increase enough that its temperature goes up by another degree, and we're off to the races. We'll call the temperature difference between the two dice Tdiff, and the thermal resistance between them Theta_JJ. We thus have:
dPout/dT = 5V * dIout/dT
dIout/dT = ((2 mV/K) * e/kT * I_out)
dTdiff/dPout = Theta_JJ
And at the stability boundary
dTdiff/dPout * dPout/dT = 1.
Thus near room temperature the device becomes unstable when
(5V * 2 mV/K / 26 mV * 5 mA) Theta_JJ = 1
or
Theta_JJ = 520 K/W.
So even for two dice in the same SOT-343 package, the thermal coupling between them is worse than their coupling to the outside world via the leads. (There's probably a safety factor in the quoted value, but it sure ain't like a monolithic pair.)
In addition there are transient problems--thermal conduction gets quadratically slow with distance, even in the best heat conductors. The thermal response of the BFP640 die just in its own package shows a time constant of 2.2 ms, so coupling between devices would be slower even than that.
Dorking the dissipation to be constant can be done as fast as you like, _provided_ the Early voltage is high enough.
Thanks
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
On a board you can put the two of them on a paddle, which pretty well fixes the problem.
A bit better, for sure, but just the increased distance (and the slow thermal diffusivity of alumina) would make it fairly disappointing at signal frequencies.
Even SOI wafers are noticeably worse than bulk silicon for this sort of job.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
A factor of 2 at the outside. Plastic is a really, really crappy, really, really slow thermal conductor. That's a big help when you want to use nylon screws to clamp a cold plate to a TEC, but usually annoying the rest of the time.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
The "plastic" is a mixture, epoxy filled with some kind of material so it doesn't leak light. A thin layer isn't too bad; STmicro 78xx regulators come in 'fullpack' and bare-metal tabs, with near-equal thermal resistance junction-to-case.
I went through the math in another post. IC packages are generally made of Sumitomo novolac epoxy, which like other plastics has a thermal conductivity near 0.1 W/m/K and a thermal diffusivity hundreds of times slower than silicon's.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Ahh. I was not thinking of that, I was thinking only of externally imposed gradients.
Thermal runaway. Cute.
That makes sense, I guess. I should read up on Early voltage. Something clever this way comes.
Joe Gwinn
...
The thermal servo trick is originally due to Tek? Interesting. I learned it from Nikolay Ukhanski some years ago. He and his colleaques published it, too, in a physics journal.
Regards, Mikko
The paper I'm thinking is older and more obscure, but judging by the abstract, doi:10.1063.12335630 looks looks like same stuff.
Regards, Mikko
The Tek idea is to bias the transistors at the maximum-dissipation point, so that small excursions don't cause thermal transients of any appreciable size. That's an open-loop thing.
The thermal servoing idea may well not be original with me, but at least I invented it independently in about 2010 or 2011. It's not much use with normal RF transistors, because their Early voltages are so low, but SiGe devices have VA ~ 250V at signal rates. (The datasheet curves even tilt the other way, because the thermal effect on V_BE is bigger than the Early effect.)
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
The reference doesn't seem to work. Is there something after the slash?
Thanks
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.