On May 12, 2009, at 12:38 AM, Stephan Boettcher wrote: > Joerg <[email protected]> writes: > >>>> The topper was a professor at my university who said that soon >>>> everything will be ICs, that transistors and most of that discrete >>>> stuff would go away. I burst into laughter in the auditorium, a bit >>>> embarrassing ... >>> >>> Well, in case you haven't noticed, it's headed that way. My CCD >>> measurement chains of 10-20 years ago were full of discrete >>> transistors. IC's were not up to the job at the power levels >>> required >>> for a space instrument. Present day versions have no discretes, but >>> use less power, and are faster and quieter. It's physics: the >>> scaling >>> laws tell you that in most cases, smaller transistors with shorter >>> interconnections are better. You can only go so far down this road >>> with discretes. Still need a big power transistor? Those are mostly >>> IC's, too: millions of tiny transistors in parallel. > > Our charge sensitive preamps for the Radiation Assessment Detector > (RAD) > on the Mars Science Lab mission (launch 2011) use a discrete input > FET. > 17 bit dynamic range are still tough in an integrated circuit, when > the > noise level is supposed to be 1000 electrons, and the largest expected > signal is 150M electrons. The problem is less with the power, but > mass. > 1cm² board space per preamp is significant weight, if it needs to > go to > Mars. Even more expensive was the paperwork to get that FET (BF862) > qualified. >
Yep. A couple of years ago, the MIT folks I work with were doing some experiments with x-ray detection using APDs. The system I designed for them used a BF862 at the front end. But how much longer will this last. The process folks keep improving their ability to mix technologies. I think somebody will come up with a process mixing low voltage JFETs with bipolar, some designer (maybe even you or me) will then put a preamp (or maybe a whole measurement chain) in a six bump BGA, and this part of the discrete game will be over. It's had an awfully long run though: the Amptek hybrid that is the "industry standard" here is a slight modification of a preamp my old MIT colleague Bob Goeke designed in the early 1970's. I have a copy of the original schematic around here somewhere... As for qualification, the whole thing is a disaster, and the harm it has caused is immense. Superstitious bureaucrats together with ignorance born of lazy hyperspecialization produce complete nonsense. There's enough recent experience with industrial parts in space to show that, if anything, qualified parts tend to be *less* reliable than the alternatives. John Doty Noqsi Aerospace, Ltd. http://www.noqsi.com/ [email protected] _______________________________________________ geda-user mailing list [email protected] http://www.seul.org/cgi-bin/mailman/listinfo/geda-user
