Dual N-Channel JFET replacement - Page 1
Forum section: electronics / projects

author: todorp
author_rank: Regular Contributor
timestamp: September 16, 2020, 12:54:22 pm
content: Hi all, I am trying to run through the J-Fet lab in the second edition of 'Learning the Art of Electronics'. At a certain point a dual n-channel jfet is used (2N3958) to build a j-fet follower with a current sink as active load...The 2N3958 is obsolete and I am unable to find a suitable replacement (neither throu-hole nor SMD). I tried to look for LSK389, LS5912 and IFN146 but these too seem un-onbtainium from standard distributors: mouser/digikey...Could anyone suggest a possible substitue for the 2N3958 which is easily sourced from standard distributors?Thanks a lot,Tod

author: Kleinstein
author_rank: Super Contributor
timestamp: September 16, 2020, 01:19:30 pm
content: Dual JFETs are getting rare and expensive. There are still a few available at a premium price (e.g. $20).Mouser in Europe still has the 2N3958 as one of the few types still available.If it is just for learning one could probably use 2 selected single JFETs. E.g. get some 10 pieces of a affordable common type (e.g. MMBFJ201, mmbf309 - depends on details of use) and select 2 with a reasonable similar threshold.

author: TheMG
author_rank: Frequent Contributor
timestamp: September 16, 2020, 01:21:44 pm
content: I might have some in my stash of old obsolete components.I think such a component has been made obsolete by JFET input opamps.Anyways, you could also replace it with two individual JFETs. Get a bunch of them and find two with closely matched characteristics. Stick them together with thermal adhesive if they need to be kept at matched temperatures too.

author: tggzzz
author_rank: Super Contributor
timestamp: September 16, 2020, 01:27:12 pm
content: Whether some of thesehttp://www.linearsystems.com/product.htmlare suitable is left as an exercise for the student.Finding known parts can be done using octopart or findchips.

author: todorp
author_rank: Regular Contributor
timestamp: September 16, 2020, 02:50:12 pm
content: Ok, I will try to match single jfets as I am not prepared to spend $20 for the matched pair :-).Quote from: tggzzz on September 16, 2020, 01:27:12 pmtggzzz, does LinearSystems sell only through their site? Niether Mouser nor Digikey seem to stock any of their parts...

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: September 16, 2020, 03:38:01 pm
content: Quote from: todorp on September 16, 2020, 02:50:12 pmHmm, LSK389 are available fromTrendSetter electronics for $5 per one dual unit. Just FYI.https://www.trendsetter.com/discrete-semiconductors/transistors-jfet.html

author: tggzzz
author_rank: Super Contributor
timestamp: September 16, 2020, 03:48:55 pm
content: Quote from: todorp on September 16, 2020, 02:50:12 pmNo idea. Use octopart.com.

author: David Hess
author_rank: Super Contributor
timestamp: September 18, 2020, 04:42:53 am
content: I agree with earlier posts; if you cannot find a dual, then buy like 10 single JFETs and select a pair with matched Vgs or Id.

author: bdunham7
author_rank: Super Contributor
timestamp: September 18, 2020, 05:08:29 am
content: https://www.utsource.net/itm/p/1854804.html?utm_source=google&adgroupid=100104688518&keyword=2n3958&adposition=&gclid=CjwKCAjwkoz7BRBPEiwAeKw3q-8TjUkUSv-Wj818fVh4IrWEsWTxtjYP1Mxz0aWf9XGu8HKsZLds2BoCrYAQAvD_BwEThey even have a toll-free telephone number in Italy, so they must serve that market.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 18, 2020, 05:30:45 am
content: Just use a handful of 2N3819 or the like. Sort them by I_DSS, find a pair that are fairly close together.Measure I_DSS by tying G to S, and apply some voltage (probably >6V) to D; measure the current flow. (This is a "current regulating diode" configuration.)Tim

author: todorp
author_rank: Regular Contributor
timestamp: September 18, 2020, 08:16:31 am
content: Thanks guys, since this is just to lear I will follow the advice and buy descrete jfets and try to match them.I was just curious: do people not use matched jfets at all since they are so difficult to source?Thanks,Tod

author: Kleinstein
author_rank: Super Contributor
timestamp: September 18, 2020, 09:14:34 am
content: The matched FETs were popular in times before good FET input OPs were available. So there are few applications left that still want / need precision matched JFETs. The FET input OPs just got so much better: even an OPA140 (130 µV offset and < 1 µV/K) is relatively cheap compared to precision matched JFETs (some 5-10 mV offset and some 10 µV/K) .For DC wise less demanding applications like audio there are relatively cheap dual JFETs (e.g. SK2145) with likely reasonable matching, but not as good as the expensive precision part.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 18, 2020, 02:13:37 pm
content: Plenty useful and common -- just not in that form factor. They're integrated in JFET op-amps.There are very few applications that can't be served by off-the-shelf op-amps; some quite excellent performance is available, in terms of whatever you might want: bandwidth, input offset voltage or bias current, supply current, input noise, etc. (Especially, the performance per supply current / power dissipation keeps improving, an inevitable consequence of miniaturization.) For the few remaining applications, there are specialty parts, such as listed above, and old stock.Tim

author: Cerebus
author_rank: Super Contributor
timestamp: September 18, 2020, 06:21:14 pm
content: Quote from: tggzzz on September 16, 2020, 01:27:12 pmThe student wouldn't have to try too hard as they list a 2N2958 directly. Price from their UK distributor is £4.50ea for TO-71, or £3.78ea in a SOIC-8 package.

author: Cerebus
author_rank: Super Contributor
timestamp: September 18, 2020, 06:23:55 pm
content: Quote from: todorp on September 16, 2020, 02:50:12 pmItalian distributor, from Linear System's website:DETAILSSchurter Electronics S.p.A.Country: ItalyPhone:+011 39 02 30465320Fax:Email:info.it@schurter.comWebsite:www.schurter.com

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: September 18, 2020, 06:40:34 pm
content: These distributors would have been nice to know a few years ago when I cleverly managed to blow up all 4 dual JFET input stages in my Tektronix 1A4.

author: David Hess
author_rank: Super Contributor
timestamp: September 18, 2020, 07:31:02 pm
content: Quote from: todorp on September 18, 2020, 08:16:31 amIn a production environment with long lead times, purchasing would be less constrained.Matched JFETs are not the same thing as dual JFETs which are not the same thing as monolithic dual JFETs. Monolithic duals, including operational amplifiers, and not suitable in all applications because of electrical coupling between the JFETs. Some applications absolutely require single parts and some of those applications can take advantage of matched parts. The general trend has been to either use JFET input operational amplifiers where possible or circuit configurations which do not depend on matched JFETs.

author: graybeard
author_rank: Frequent Contributor
timestamp: September 18, 2020, 08:13:12 pm
content: Here are some in-stock dual (common source) JFETs for $0.67/each qty 1, $0.0571/each qty 10https://www.mouser.com/ProductDetail/Toshiba/2SK2145-YTE85LF?qs=iCzJi%2FIZBF6et8IkW2t5vg%3D%3DThe data sheet is here:https://toshiba.semicon-storage.com/us/semiconductor/product/mosfets/detail.2SK2145.html

author: magic
author_rank: Super Contributor
timestamp: September 19, 2020, 05:43:51 am
content: No mention od matching in the datasheet

author: Kleinstein
author_rank: Super Contributor
timestamp: September 19, 2020, 07:05:19 am
content: The SK2145 data-sheet does not mention offset or matching. So it may very well be just the normal selection of chips coming from a single batch. So one may have to accept quite some offset and may need selecting on the actual needs.Anyway with a small plastic SMD case there is a chance the offset could change with soldering.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: September 19, 2020, 12:37:32 pm
content: Here's what the dual JFET from my 1A4 looks like inside. Well after I zapped them. You can see a bond wire is missing. I guess this is a dual matched pair made from two discrete transistors that are matched before being gently put inside the case, and the metal case matches the temperature of the two die, within practical limits, good enough for a scope front end. I know previously they used two TO-92 packages clipped together on their flat sides, which I guess wasn't good enough.This is the datasheet I found in that repair's folder:https://www.mouser.ca/datasheet/2/676/jfet-ifn3954-ifn3955-ifn3956-interfet.r00-1649128.pdf

author: Käsebrot
author_rank: Newbie
timestamp: September 20, 2020, 11:48:58 am
content: You can buy matched Linear Systems JFETs here, but they aren't exactly cheap:https://diyaudiostore.com/collections/jfets/products/linear-systems-matched-jfets

author: Cerebus
author_rank: Super Contributor
timestamp: September 20, 2020, 12:47:35 pm
content: Yeah, but those are matched sets of single TO-92 transistors, a far cry from manufacturer matched JFETs with the dies (often monolithic) in the same package and in intimate thermal contact with each other. So you're actually paying more for the making of matched sets while getting lower matching performance.Linear systems actually do a monolithic matched pair (the LSK389 at around £4 each) with the same specified characteristics as those individual LSK170 JFETs you've linked to at 27.00 for a pair (presumably USD, the site doesn't specify). Heck, the datasheet for the LSK170 even says "For equivalent monolithic dual, see LSK389 family". The site seems to have been created to take advantage of price gouging audiophools who don't know better (and don't bother to read the datasheet).The LSK170 is an equivalent for Toshiba's discontinued 2SK170, much loved for its very low noise (0.9 nV/sqrt(Hz) typ @ 1kHz, 2.5 nV/sqrt(Hz) @ 10 Hz) and consequently revered by audiophools who then proceed to feed it from a grossly inappropriate source impedance from a Johnson-Nyquist point of view.

author: Käsebrot
author_rank: Newbie
timestamp: September 20, 2020, 12:56:05 pm
content: Tell me something I don't already know.It's not like all these people are "stupid audiophools" who want to pay a huge premium, it's just the fact that dual matched JFETs from Linear Systems directly are hardly available to consumers. Most of the reliable sources only sell to businesses and will likely require a minimum quantity as well. I was surprised someone posted a reseller that actually seems to have them in stock and sells them in low quantities, I haven't checked if they would sell to consumers as the website kept failing on me. But sure, it's all stupid audiophools... seriously, get off your high horse.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: September 20, 2020, 01:04:19 pm
content: Maybe someone can risk these and test themhttps://www.ebay.ca/itm/1PCS-New-2N5564-CAN-6-Matched-N-Channel-JFET-Pairs/392370230460?_trkparms=aid%3D1110006%26algo%3DHOMESPLICE.SIM%26ao%3D1%26asc%3D20200520130048%26meid%3Da5ee56c54f1d4760b9600f7dec384da7%26pid%3D100005%26rk%3D1%26rkt%3D5%26mehot%3Dco%26sd%3D162688119806%26itm%3D392370230460%26pmt%3D1%26noa%3D0%26pg%3D2047675%26algv%3DSimplAMLv5PairwiseWebWithDarwoV3BBEV2b%26brand%3DUnbranded&_trksid=p2047675.c100005.m1851

author: Käsebrot
author_rank: Newbie
timestamp: September 20, 2020, 01:10:51 pm
content: The fact that apparently the writing on the photo had a different store name on it and was replaced already seems fishy. I'd at least ask for an updated product picture with a handwritten note of the store name next to a closeup of the product. But even that's relatively worthless if you don't know what to look for.Just my 2 cents :/

author: Cerebus
author_rank: Super Contributor
timestamp: September 20, 2020, 01:48:07 pm
content: Quote from: Käsebrot on September 20, 2020, 12:56:05 pmFunny, from here you seem to be the one on your high horse. (If youarea sensible audiophile who understands actual engineering rather than cargo cults of magic capacitors, transistors, op-amps and the like, but are going to exhibit high dudgeon every time the word audiophool is mentioned, this probably isn't the forum for you, and you definitely don't want to look around the rest of this place.)Ever actuallytriedbuying Linear Systems parts from a Distie rather than bleating about it? Didn't cause me any problems. You have until the end of the year to get them from micross.com, the UK distributor who sell online and in small quantities (and whom I found by following links from Linear Systems own site), after which all bets are off because of the impending doom of Brexit.

author: Käsebrot
author_rank: Newbie
timestamp: September 20, 2020, 02:52:31 pm
content: The distributor for Germany makes it abundantly clear that they sell exclusively to businesses and require minimum orders. The ones in the link I posted are fairly expensive, buying NOS from dodgy eBay sellers is not my cup of tea and I don't care enough to mail every Linear Systems distributor if he might sell me a bunch. My conclusion was to just stay away from circuits requiring matched JFETs.I am not going to respond to your other remarks in detail, you seem to be desperately searching for someone you can lecture on something and I am not interested, sorry to disappoint.

author: Cerebus
author_rank: Super Contributor
timestamp: September 20, 2020, 02:56:13 pm
content: Quote from: Käsebrot on September 20, 2020, 02:52:31 pmAh, so "too hard" and onlyyouare allowed to lecture me, OK, I think we've gotyournumber.