How to match a set of JFETs - Page 1
Forum section: electronics / projects

author: quarks
author_rank: Frequent Contributor
timestamp: April 26, 2013, 09:56:05 am
content: Hello all,right now I am working on a calibrator and found two defect JFETs out of a matched set of 4. Besides the problem to still get the obsolete J108 JFETs, I wonder what else, besides the ON resistance (for a switch application), should be checked/matched and how to do it. Any hint is welcome.The Picture below shows the circuit I am working on.Thanksquarks

author: Psi
author_rank: Super Contributor
timestamp: April 26, 2013, 10:08:16 am
content: Quote from: quarks on April 26, 2013, 09:56:05 amVoltages would be the first thing. (drain-source, drain-gate)Gate-source voltage vs drain currentInput capacitance

author: quarks
author_rank: Frequent Contributor
timestamp: April 26, 2013, 10:42:31 am
content: Quote from: Psi on April 26, 2013, 10:08:16 amthanks for your reply.How can I practically meassure the Input capacitance?In my application the JFETs are used as DC Switches (I added this to my post now). Therefore I guess the ON resistance is still the main influence. At least the appl. wants the ON resistance of each N-channel JFET to be 3-5 Ohms.

author: Psi
author_rank: Super Contributor
timestamp: April 26, 2013, 10:47:11 am
content: If you don't have the datasheet that's a good question.The gate capacitance is important for switching speed and drive current.If your application is a simple on/off switch and is not oscillating or switching fast then you can ignore it.(ie, if switch-on/off lag doesn't matter)

author: fpliuzzi
author_rank: Regular Contributor
timestamp: April 26, 2013, 03:42:51 pm
content: Hello,Maybe the simple test circuit shown in the article at edn.com will help you match your JFETs.www.edn.com/design/other/4370552/Simple-circuit-lets-you-characterize-JFETsAlso, Erno Borbely has a nice document about JFETs that contains some useful info about matching them. Just google "JFETS: THE NEW FRONTIER, PART 1" & "JFETS: THE NEW FRONTIER, PART 2".Regards,Frank

author: quarks
author_rank: Frequent Contributor
timestamp: April 26, 2013, 04:04:52 pm
content: Quote from: fpliuzzi on April 26, 2013, 03:42:51 pmHello Frank,thanks a lot. I had a first look at it and will later read through for the details.I also just had a look at the Peak Atlas DCA 75 Pro tester, which seems to be very good for this job.http://www.peakelec.co.uk/acatalog/dca75-dca-pro.htmlbyequarks

author: fpliuzzi
author_rank: Regular Contributor
timestamp: April 26, 2013, 05:52:03 pm
content: Quote from: quarks on April 26, 2013, 04:04:52 pmHi quarks,The Peak Atlas DCA 75 Pro tester does look like a very useful device to have around. I've gotten a lot of use out of my M-Cubed Electronix Semiconductor Analyzer (no longer available) and will probably upgrade to the DCA 75 Pro at some point myself. A couple of months ago I watched a few youtube videos demonstrating its capabilities and was impressed.One possible minus for me is that it seems to be limited to testing lower voltage components and components that only need a low level of test current for them to be checked out properly. It looks like I would still need to hang onto my curve tracer and my collection of home-brewed test fixtures that I've put together over the years to test the devices that the DCA 75 Pro couldn't handle (and that my M3 Semiconductor Analyzer can't handle either).However, because of the vast number of things that the Peak Atlas DCA 75 Pro looks like it can do so well, it would make a great addition to my test bench.Regards,Frank

author: fpliuzzi
author_rank: Regular Contributor
timestamp: April 28, 2013, 01:25:07 am
content: Hi quarks,I just noticed your post about measuring the input capacitance of a JFET. The pdf that I have attached is a list of the available JFET tests included in a 1994 book by John Lenk that I've had for many years (I've had this pdf file on my hard drive for a long time).If it looks useful to you, I saw some used copies of the hardback book (as low as $5.98) atwww.amazon.com/McGraw-Hill-Electronic-Testing-Handbook-Procedures/dp/0070376026Regards,FrankEDIT: I added a text file version of the pdf's contents for anyone who cannot open the pdf.

author: sorin
author_rank: Frequent Contributor
timestamp: April 28, 2013, 06:43:34 am
content: I think that the bigger problem should be the leakage current

author: Unknown
author_rank: Unknown
timestamp: April 29, 2013, 02:00:37 pm
content: You could build a simple curve tracer, for a scope... not much in the design, see URL:http://electronicdesign.com/displays/simple-circuit-turns-scope-function-generator-jfet-curve-tracerCircuit:

author: quarks
author_rank: Frequent Contributor
timestamp: April 29, 2013, 07:00:55 pm
content: Thanks for the replies.I have ordered the DCA75 Pro and will check the other suggestions while waiting for it.BTW I could not open Franks pdf. Has anyone else had this problem too?

author: Unknown
author_rank: Unknown
timestamp: April 29, 2013, 08:46:21 pm
content: Hi quarks... if your referring to the EDN web-page as below ! it opens in my browser ? otherwise I can't see any pdf file.http://www.edn.com/design/other/4370552/Simple-circuit-lets-you-characterize-JFETs

author: quarks
author_rank: Frequent Contributor
timestamp: April 30, 2013, 04:02:18 pm
content: @ vlf3,no I had trouble opening the pdf in Franks (fpliuzzi) post. But now I can open it.@ all,I received the atlas DCA Pro today and it is quite useful.Thanks for all repliesquarks