Ivory Towers in the Land of CMOS - Page 1
Forum section: electronics / general

author: c4757p
author_rank: Super Contributor
timestamp: September 12, 2013, 06:31:49 pm
content: My semiconductor devices prof thinks BJTs are obsolete.QuoteMaybe in the dream he had last night! Or perhaps he's secretly a time traveler from 2050, where discrete semiconductors are gone, all transistors are MOS, all circuits are produced on a single chip 500um square that runs at 2THz on 60pW recovered from gnat and housefly farts, and wars and hunger are no more?The same guy didn't even pause to comment about JFETs, thoughBy the way, free_electron, he talks about you an awful lot... Are you friends? Perhaps you could have a talk with him...

author: IntegratedValve
author_rank: Regular Contributor
timestamp: September 12, 2013, 06:54:58 pm
content: Does your professor know that BJTs switch faster than CMOS?

author: c4757p
author_rank: Super Contributor
timestamp: September 12, 2013, 07:00:05 pm
content: Careful there.... I'm pretty sure switching applications are one area where the BJT has been pretty much replaced.... and CMOS circuits run blazingly fast these days...

author: tszaboo
author_rank: Super Contributor
timestamp: September 12, 2013, 07:04:15 pm
content: I've just used a BJT to invert a logic level signal and boost the current which costs 0.01 Euro in high quantities. BC846. Am I making obsolete board? Or does he have a suggestion to handle this easier way?

author: IntegratedValve
author_rank: Regular Contributor
timestamp: September 12, 2013, 07:11:16 pm
content: Quote from: c4757p on September 12, 2013, 07:00:05 pmWell at least BJTs have higher bandwidth and maximum frequency limit make them more suitable for RF stuff...

author: c4757p
author_rank: Super Contributor
timestamp: September 12, 2013, 07:11:56 pm
content: Quote from: NANDBlog on September 12, 2013, 07:04:15 pmI'd probably use a 2N7002 for that, myself...(Assuming it's 5V. There must be a similarly priced MOSFET with a threshold guaranteed valid for 3.3V logic, though, I just don't know off the top of my head)

author: oPossum
author_rank: Super Contributor
timestamp: September 12, 2013, 07:17:20 pm
content: Quote from: c4757p on September 12, 2013, 07:00:05 pmInsulated GateBipolarTransistors (IGBT) dominate high power switching.Quote from: c4757p on September 12, 2013, 07:00:05 pmAlmost as fast as 30 year old ECL.CMOS is popular due to thermal issues with BJT, not because it is fast - it isn't.

author: casper.bang
author_rank: Frequent Contributor
timestamp: September 12, 2013, 07:22:46 pm
content: QuoteI gotta ask; is this a US vs. Europe thing? I noticed BCXXX is often preferred by Europeans (I bought my first BC547 transistors from a physics teacher when I was 13-14) and I have never come by a 2NXXXX.

author: c4757p
author_rank: Super Contributor
timestamp: September 12, 2013, 07:25:31 pm
content: I believe it is. 2N's are very common here and BC's rare as hen's teeth. (I do have a bunch in the junk bin, though...)But the point is that 2N7000 is roughly BS170 - a MOSFET.

author: IntegratedValve
author_rank: Regular Contributor
timestamp: September 12, 2013, 07:30:55 pm
content: Maybe this analogy may help.BJT to MOSFET is what Vacuum tube to Transistor.BJT is the solid-state version of vacuum tube.MOSFET is the solid-state version of a relay.BJT is a lot more "dynamic" in this sense.

author: c4757p
author_rank: Super Contributor
timestamp: September 12, 2013, 07:35:54 pm
content: MOSFET is equivalent to a relay? The last CMOS op amp I used begs to differ!And the JFET is a solid-state equivalent to a tube! BJTs are pretty unique.

author: komet
author_rank: Unknown
timestamp: September 12, 2013, 07:49:08 pm
content: Quote from: casper.bang on September 12, 2013, 07:22:46 pm1N..., 2N... are JEDEC (ex EIA) designations, an American association.BC, BF, etc. are Pro Electron (https://en.wikipedia.org/wiki/Pro_Electron), the European association.The Japanese have their 2SC, 2SK...

author: rdl
author_rank: Super Contributor
timestamp: September 12, 2013, 07:52:55 pm
content: Yes, the numbering differences are mentioned on Wikipedia under "Transistor"http://en.wikipedia.org/wiki/Transistor#Part_numbering_standards_.2F_specifications

author: madires
author_rank: Super Contributor
timestamp: September 12, 2013, 08:11:29 pm
content: It think your prof is a victim of the latch-up effect. :-)

author: c4757p
author_rank: Super Contributor
timestamp: September 12, 2013, 09:06:20 pm
content: Well, if he were working with BJTs in the first place he wouldn't have that problem, now would he?

author: IntegratedValve
author_rank: Regular Contributor
timestamp: September 12, 2013, 09:20:27 pm
content: New generation prefers newer technologywhile older folks prefer "obsolete" everythingPersonally I think the entire silicon things and probably electric energy will be obsolete after 10 years...The trend is photonics! Your prof will like this.

author: c4757p
author_rank: Super Contributor
timestamp: September 12, 2013, 09:31:52 pm
content: Yeah, but saying they are obsolete is just stupid. I'd agree that their days are numbered. Analog CMOS technology is improving. But saying they are actually obsolete is like saying gasoline cars are obsolete because electric ones are around the corner. Step outside and look at the street - no they are not! And it's a misleading thing to say to a bunch of student engineers who really need to learn how the things work.I will happily declare even things that I like to use "obsolete". Discrete non-RF JFETs are obsolete. (But not ones inside ICs!) Discrete RF JFETs are almost obsolete. Analog oscilloscopes are obsolete. Linear power supplies are very nearly obsolete (lab PSUs doesn't count, there'll always beoneexception.) But BJTs are as obsolete as gasoline.

author: IntegratedValve
author_rank: Regular Contributor
timestamp: September 12, 2013, 09:41:52 pm
content: It's like saying analog is obsolete.

author: sync
author_rank: Frequent Contributor
timestamp: September 12, 2013, 09:51:36 pm
content: profs are obsolete. we have internet now.scnr

author: IntegratedValve
author_rank: Regular Contributor
timestamp: September 12, 2013, 09:53:40 pm
content: Unless he means discrete BJTs, they are rarely seen in any modern circuits.

author: tszaboo
author_rank: Super Contributor
timestamp: September 12, 2013, 10:27:42 pm
content: Quote from: IntegratedValve on September 12, 2013, 09:53:40 pmDave not so long ago opened some 20 Gigahertz Agilent stuff. It was filled with discrete transistors.

author: free_electron
author_rank: Super Contributor
timestamp: September 12, 2013, 10:56:52 pm
content: I don't know any profs. apart from an ex collegue of mineBipolar is not gone. but only in the analog world. for digital BJT is deader than dead. no sane designer would use bjt's for that purpose

author: vk6zgo
author_rank: Super Contributor
timestamp: September 13, 2013, 03:10:56 pm
content: Quote from: c4757p on September 12, 2013, 07:35:54 pmAs a Greybeard,I think of the dual gate devices used as RF preamps & mixers in receivers at up to UHF when I hear the term,MOSFET.Many younger people only know them as high voltage switches.Actually tubes are more like MOSFETs than JFETS in many ways,although youcandraw grid current.In my last job,we were using LDMOS devices at around 400MHz.Each package was a push-pull pair of devices,with the combination producing 300watts.they were in turn,combined to produce 1200 watts.---not bad for MOS devices which are "limited in RF use",as an earlier poster declared.

author: JohnnyGringo
author_rank: Regular Contributor
timestamp: September 13, 2013, 03:35:24 pm
content: Quote from: vk6zgo on September 13, 2013, 03:10:56 pmI am continually astounded as to the collective knowledge and diversity of experience within the EEVBlog community.

author: sub
author_rank: Regular Contributor
timestamp: September 14, 2013, 12:21:20 am
content: Quote from: free_electron on September 12, 2013, 10:56:52 pmThere are still plenty of ECL counters floating about on Digikey.