Author Topic: Dual N-Channel JFET replacement  (Read 1094 times)

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Offline todorp

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Dual N-Channel JFET replacement
« on: September 16, 2020, 12:54:22 pm »
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The 2N3958 is obsolete and I am unable to find a suitable replacement (neither throu-hole nor SMD).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
 

Offline Kleinstein

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Re: Dual N-Channel JFET replacement
« Reply #1 on: September 16, 2020, 01:19:30 pm »
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.
 

Online TheMG

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Re: Dual N-Channel JFET replacement
« Reply #2 on: September 16, 2020, 01:21:44 pm »
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.
 

Online tggzzz

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Re: Dual N-Channel JFET replacement
« Reply #3 on: September 16, 2020, 01:27:12 pm »
Whether some of these http://www.linearsystems.com/product.html are suitable is left as an exercise for the student.

Finding known parts can be done using octopart or findchips.
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline todorp

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Re: Dual N-Channel JFET replacement
« Reply #4 on: September 16, 2020, 02:50:12 pm »
Ok, I will try to match single jfets as I am not prepared to spend $20 for the matched pair :-).

Whether some of these http://www.linearsystems.com/product.html are suitable is left as an exercise for the student.

tggzzz, does LinearSystems sell only through their site? Niether Mouser nor Digikey seem to stock any of their parts...
 

Offline RandallMcRee

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Re: Dual N-Channel JFET replacement
« Reply #5 on: September 16, 2020, 03:38:01 pm »
Ok, I will try to match single jfets as I am not prepared to spend $20 for the matched pair :-).

Whether some of these http://www.linearsystems.com/product.html are suitable is left as an exercise for the student.

tggzzz, does LinearSystems sell only through their site? Niether Mouser nor Digikey seem to stock any of their parts...

Hmm, LSK389 are available fromTrendSetter electronics for $5 per one dual unit. Just FYI.
https://www.trendsetter.com/discrete-semiconductors/transistors-jfet.html
 

Online tggzzz

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Re: Dual N-Channel JFET replacement
« Reply #6 on: September 16, 2020, 03:48:55 pm »
Ok, I will try to match single jfets as I am not prepared to spend $20 for the matched pair :-).

Whether some of these http://www.linearsystems.com/product.html are suitable is left as an exercise for the student.

tggzzz, does LinearSystems sell only through their site? Niether Mouser nor Digikey seem to stock any of their parts...

No idea. Use octopart.com.
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline David Hess

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Re: Dual N-Channel JFET replacement
« Reply #7 on: September 18, 2020, 04:42:53 am »
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.
 

Online bdunham7

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A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit DMM is good enough for most people.
 

Offline T3sl4co1l

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Re: Dual N-Channel JFET replacement
« Reply #9 on: September 18, 2020, 05:30:45 am »
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.)

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Offline todorp

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Re: Dual N-Channel JFET replacement
« Reply #10 on: September 18, 2020, 08:16:31 am »
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
 

Offline Kleinstein

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Re: Dual N-Channel JFET replacement
« Reply #11 on: September 18, 2020, 09:14:34 am »
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.
 

Offline T3sl4co1l

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Re: Dual N-Channel JFET replacement
« Reply #12 on: September 18, 2020, 02:13:37 pm »
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.

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Offline Cerebus

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Re: Dual N-Channel JFET replacement
« Reply #13 on: September 18, 2020, 06:21:14 pm »
Whether some of these http://www.linearsystems.com/product.html are suitable is left as an exercise for the student.

Finding known parts can be done using octopart or findchips.

The 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.
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Offline Cerebus

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Re: Dual N-Channel JFET replacement
« Reply #14 on: September 18, 2020, 06:23:55 pm »
Ok, I will try to match single jfets as I am not prepared to spend $20 for the matched pair :-).

Whether some of these http://www.linearsystems.com/product.html are suitable is left as an exercise for the student.

tggzzz, does LinearSystems sell only through their site? Niether Mouser nor Digikey seem to stock any of their parts...

Italian distributor, from Linear System's website:

DETAILS

Schurter Electronics S.p.A.

Country: Italy

Phone:+011 39 02 30465320

Fax:

Email:info.it@schurter.com

Website:www.schurter.com
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Offline Alex Eisenhut

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Re: Dual N-Channel JFET replacement
« Reply #15 on: September 18, 2020, 06:40:34 pm »
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.
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Offline David Hess

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Re: Dual N-Channel JFET replacement
« Reply #16 on: September 18, 2020, 07:31:02 pm »
I was just curious: do people not use matched jfets at all since they are so difficult to source?

In 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.
 

Online graybeard

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Re: Dual N-Channel JFET replacement
« Reply #17 on: September 18, 2020, 08:13:12 pm »
« Last Edit: September 18, 2020, 08:44:35 pm by graybeard »
 

Online magic

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Re: Dual N-Channel JFET replacement
« Reply #18 on: September 19, 2020, 05:43:51 am »
No mention od matching in the datasheet ::)
 

Offline Kleinstein

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Re: Dual N-Channel JFET replacement
« Reply #19 on: September 19, 2020, 07:05:19 am »
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.
 

Offline Alex Eisenhut

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Re: Dual N-Channel JFET replacement
« Reply #20 on: September 19, 2020, 12:37:32 pm »
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
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Offline Käsebrot

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Re: Dual N-Channel JFET replacement
« Reply #21 on: September 20, 2020, 11:48:58 am »
You can buy matched Linear Systems JFETs here, but they aren't exactly cheap: https://diyaudiostore.com/collections/jfets/products/linear-systems-matched-jfets
 

Offline Cerebus

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Re: Dual N-Channel JFET replacement
« Reply #22 on: September 20, 2020, 12:47:35 pm »
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.
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Offline Käsebrot

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Re: Dual N-Channel JFET replacement
« Reply #23 on: September 20, 2020, 12:56:05 pm »
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. :palm:

 



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