We trend to use PHEMTS for fast stuff. The place where we use still bipolars is in positive current sources, because mosfets have too much capacitance and nobody makes p-channel phemts.
Base current sucks. I designed a fast currrent source with base current correction, but it's kinda complex.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
I mostly use BJTs as transconductance devices, and their nice repeatable turn-on is a big plus. They're also much better in low-voltage analogue stuff for the same reason. Besides being much, much quieter.
PNPs are good for positive-to-negative level shifting, too.
Those SiGe:C devices (BFP640/650) are magic--high beta, low noise, effectively infinite VAF, and bandwidth that goes on forever. As you pointed out some time back, they don't switch as fast as pHEMTs that are seemingly slower. That one-diode sampler I did with ChesterW last year originally used a BFP640FESD, which produced a 200-ps sampler. Swapping it out for an ATF55143 got us down to 100 ps, but cost an extra 45 cents ($0.67 vs $0.22).
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Nah nah it was an _isdn_ xfrmr. That's the good shit
Where's Famous Chef Pante?
ISDN transformers are really cool; I've used lots of them (over 20K!) in power supplies and such. My favorite part is 1:1:2:2, which is pretty versatile. But I use home-made transmission-line xfmrs for big fast pulses.
Is ISDN still alive? "It Still Does Nothing"
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
The problem with fast bipolars, especially PNPs, is that they keep getting EOL'd.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
I'm planning to get a few reels for insurance pretty soon. BFP640s are too useful to lose, and they're only 20 cents or so--just like a BF862.
A couple of dollar's worth of fancy transistors can let me build a very swoopy box that sells (hopefully!) for $2k-ish.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
We both sell IP, soldered to PC boards.
One of our customers wants to audit our parts cost to see if we're making too much money. We declined.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
I know it was "Big in Japan" for a while...
I think it's still used by some businesses for POS applications and stuff. The wiki says Verizon no longer offers it in the Northeast as of 2013
BJTs are really more like voltage controlled current sources with a pathology than current controlled current sources
Bwahahahahahaha >:-} ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| STV, Queen Creek, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
Thinking outside the box... producing elegant solutions.
"It is not in doing what you like, but in liking what you do that
is the secret of happiness." -James Barrie
There have been analog designers with a lot more experience than I have that've said much the same thing.
The Ebers-Moll equation relating Ie to Vbe says nothing about Ie dependence on base current.
Gummel-Poon charge-controlled model is useless outside of computer simulation
And with much more gain than MOSFETs!
I like to think of low-Vce(sat) transistors as "MOSFETs with 5x more gain but a really leaky gate". They have a reasonably linear (including 3rd quadrant) "Rce(on)", too.
"Vbe(th)" is also way more repeatable. I was testing almost a hundred boards, recently, and measured a current-limited output on each of them as part of the test spec. The design is guard-banded, so the current limit is high enough never to matter in normal operation, while it's also low enough to dissipate a safe amount of power when shorted. Despite the wide design tolerance, they all measured within 0.3% of each other. (Evidently, room temperature was pretty stable between those days of testing.)
Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
On a sunny day (Wed, 10 May 2017 16:16:56 -0400) it happened bitrex wrote in :
Well, maybe you misread the label, and it was a FET... :-)
On a sunny day (Wed, 10 May 2017 11:30:14 -0400) it happened bitrex wrote in :
Na, was in the kitchen just now baking mushrooms... MY opinion on this, and actually last night there was, on cable here, a professor giving college about CERN comparing it to a super microscope. You know ever higher energies needed to take a picture of ever smaller things. But if you ask me, I did say: 'Do you really expect to get a deeper insight into the Tesla autopilot software by driving those into each other at 1000 km/h?'
So..
Ya, I suppose if you could design a transconductance physics box with an exponential law instead of square that also had a pretty well-defined "gate" threshold of 0.6 volts and drew no "gate" current, there would be little reason to use anything else assuming similar noise performance over a similar bandwidth.
I think (x)HEMTs come pretty close, sadly they are not literally a dime a dozen and likely never will be.
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