JFET - is it worth studying them? - Page 1
Forum section: electronics / beginners

author: Moriambar
author_rank: Unknown
timestamp: April 16, 2019, 08:01:56 am
content: Hi there,recently I am studying on the art of electronics (3rd edition), just to expand my knowledge (I usually use the book as a reference when I have a specific problem to solve).Anyway I see that on the FET chapter, around 30 pages are dedicated specifically to JFET and circuits regarding them. Perhaps I'm too much a noob, but when I think about FETs I most certainly do not think of JFETs, and also it seems to me that JFETs are not used that much as discrete components, especially when compared to the past (this opinion derives from reading here and there on the internet).So my question is: are all of these circuits really useful to learn or study? Do they have a specific didactic purpose?Thanks!

author: bd139
author_rank: Super Contributor
timestamp: April 16, 2019, 08:16:39 am
content: Yes they are definitely worth learning about even if you never use one. They still have uses in signal switching, sample and hold, feedback circuits and oscillators.

author: Moriambar
author_rank: Unknown
timestamp: April 16, 2019, 08:28:44 am
content: thanks

author: tszaboo
author_rank: Super Contributor
timestamp: April 16, 2019, 09:19:15 am
content: Yes, it is worth learning. Discrete components and analog is a not a mainstream knowledge anymore, everyone is investing their time in programming. JFETs are excellent for precision analog.

author: lordvader88
author_rank: Frequent Contributor
timestamp: April 16, 2019, 09:34:53 am
content: Are they easier to model than BJT's ? I've never seen any equations for JFETs yet, but I will learn them as well.

author: Zero999
author_rank: Super Contributor
timestamp: April 16, 2019, 09:37:53 am
content: SiC JFETs look really cool.Check out the KT12FN008:1200V80mΩ-60°C to +240°C temperature rangeThe VGSrange of -15V to +2V is interesting. With plain old silicon JFETs, the maximum gate voltage is under 0.5V, otherwise it starts conducting. Presumably the forward voltage of the silicon carbide gate diode is much higher than the usual 0.6V for a plain silicon diode? I couldn't find any information about this by doing a quick Google.http://caly-technologies.com/wp-DATASHEETS/JFET/DS-00005-16-KT12FN008.pdfhttps://unitedsic.com/appnotes/USCi_AN0004-Cascode-Configuration-Eases-Challenges-of-Applying-SiC-JFETs.pdfOne thing about JFETs is they need a negative get voltage, with respect to the source, to turn off. The classic way to do this is to lift the source voltage up, keeping the gate at 0V, using a MOSFET: a configuration known as a cascode. The MOSFET can be fairly low voltage, but it needs to carry the full load current.

author: rhb
author_rank: Super Contributor
timestamp: April 16, 2019, 10:43:15 pm
content: blueskull has enumerated many of the reasons to learn about JFETS. I'll add one more, LNAs. It is very hard to find them now, but some now obsolete JFETs provided really good noise figures.

author: SilverSolder
author_rank: Super Contributor
timestamp: April 17, 2019, 12:04:24 am
content: The latest update of LTSpice included a whole bunch of JFET models provided by Linear Systems, which seems to cover both their own devices as well as many "classic" jfets - well worth a look if you want to have a play!