Help Reading JFET Datasheets - Page 1
Forum section: electronics / beginners

author: eev_carl
author_rank: Regular Contributor
timestamp: September 07, 2018, 01:50:47 pm
content: Hi,I was comparing these two JFETs and was wondering if I was reading their data sheets correctly.The 2N4392 JFET has a bunch of tr parameters on the order of 5 ns. The J211 has a transconductance value of 12000 uS. Are these the same parameters and units (siemens)?Also, if there is in fact a such a large difference, what does that mean for applications? (Why choose one over the other?)Thanks,Carl___2N4392 datasheet :https://www.mouser.com/datasheet/2/68/2n4391-4393-53044.pdfJ211 data sheet :http://interfet.com/datasheet/J210/

author: eev_carl
author_rank: Regular Contributor
timestamp: September 07, 2018, 02:15:42 pm
content: I found another datasheet that has the ns under turn on parameters, so it must be nanoseconds. I was confused b/c I don't see the transconductance listed on all the datasheets.

author: Benta
author_rank: Super Contributor
timestamp: September 07, 2018, 02:36:42 pm
content: Yeah, watch the units. There's a major difference between S and s.

author: Ian.M
author_rank: Super Contributor
timestamp: September 07, 2018, 02:54:49 pm
content: small s is S.I. unit of time (seconds)big S is S.I. unit of conductance (1/ohms)S.I. prefixes are used with both in the normal way.Centralsemi describe the 2N4392 as a analog switching or chopper JFET, so dont bother giving a mutual conductance for it as they don't expect it to be used in its linear region.

author: David Hess
author_rank: Super Contributor
timestamp: September 07, 2018, 06:12:16 pm
content: Quote from: Ian.M on September 07, 2018, 02:54:49 pmIf only there was some unambiguous legacy unit for Siemens which would avoid confusion with seconds, samples, and whatever else might use S which itself is easily confused with 5. Maybe it could even be mnemonic.QuoteExactly, annoyingly as it is, JFET specifications are generally only given in relation to the intended application. Some of the other specifications can be calculated but some would need to be measured.

author: Benta
author_rank: Super Contributor
timestamp: September 07, 2018, 07:11:23 pm
content: Quote from: David Hess on September 07, 2018, 06:12:16 pmI remember older data sheets from US manufacturers using "mho" instead of "S" for conductance.

author: floobydust
author_rank: Super Contributor
timestamp: September 07, 2018, 07:15:47 pm
content: It changed from mho's to siemens in 1971. I still use mhos for transconductance, more intuitive.

author: rfeecs
author_rank: Frequent Contributor
timestamp: September 07, 2018, 09:02:16 pm
content: Those Central Semiconductor datasheets are not the greatest. They just give a short list of Min/Max.Find a better datasheet, like this one:https://media.digikey.com/pdf/Data%20Sheets/Vishay%20Siliconix%20PDFs/2N,PN,SST4391.pdfTo get a rough order of magnitude idea of gm, you could just divide Idss by Vp.Edit - This note gives a simple way to model the IV behavior including gm from just Idss and Vp:http://www.kennethkuhn.com/students/ee351/jfet_basics.pdfThe transconductance varies a lot depending on operating condition. One number at one point might not be too useful. From the Vishay datasheet: