Jellybean, Modern, Small, JFET, Complementary Pair Recommendations? - Page 1
Forum section: electronics / beginners

author: t1d
author_rank: Super Contributor
timestamp: May 08, 2024, 02:15:44 pm
content: Jellybean, Modern, Small, JFET, Complementary Pair Recommendations?I did search the Web and Mouser. However, I decided I needed recommendations. I am wanting discrete individual packages. And, in a modern design. These are for learning and general stock.Recommendations for part numbers?Thank you for your help.

author: edavid
author_rank: Super Contributor
timestamp: May 08, 2024, 03:00:20 pm
content: I have never seen any JFET complementary pair in a databook. Are you sure they exist? What would be the purpose?

author: Benta
author_rank: Super Contributor
timestamp: May 08, 2024, 04:58:24 pm
content: ^ This.

author: BrokenYugo
author_rank: Super Contributor
timestamp: May 08, 2024, 06:02:59 pm
content: Modern design? I think the jellybean discrete jfet is about dead, MPF102 ain't coming back. I think most of the SMD parts are just the same old designs in a newer package.Any JFET still in production and under $5 for a strip of 10 probably qualifies as useful for learning and general stock. IIRC that list is pretty short.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 08, 2024, 06:29:02 pm
content: There are still quite a lot of JFETs in SOT23 case available, though many of them are very similar parts (e.g. different manufacturer and testing levels).P-jfets are more an exotic thing with not many more to choose. Hardly and need for a complementary part - if at all for special switching, but here CMOS chips have largely taken over.Dual n JFETs may still be a thing for an amplfier. Here the SK2145 is a classic, though relatively small to solder for a beginner. At least it is cheap and usually easy to get.

author: David Hess
author_rank: Super Contributor
timestamp: May 09, 2024, 12:37:31 am
content: The selection for jellybean p-channel JFETs is small unless you want to buy from Linear Integrated Systems or similar, leaving the J175/J176/J177 series of p-channel choppers, which are roughly comparable to the J111/J112/J113 series of n-channel choppers.

author: MathWizard
author_rank: Super Contributor
timestamp: May 11, 2024, 01:15:46 am
content: If they can make CPU's with billions of transistor's for a few hundred dollars, why can't they make through-hole JFET's that are well matched and predictable, and maybe for less than $1 each, before shipping.I think I've asked that before, and I think the answer was no one besides old hobbyists care about JFET's, especially not through hole ones.I got a new batch of them last year, I need to try them in some radio projects. I got some fairly fast ones too, to try making VHF-UHF probes.Now what if you call up a JFET maker, and money is no issue. What's the best they could do ? How similar could they make 1,000 JFET's, how about 1,000,000 ?

author: Kleinstein
author_rank: Super Contributor
timestamp: May 11, 2024, 04:02:05 am
content: JFETs are naturally scattering quite a lot with the threshold voltage and the related parameters. For a single wafer the matching can still be OK though and dual parts can have reasonable matching. Otherwise it needs binning to sort out afterwards.There are a few afordable dual JFETs and the SK2145 (and related in smaller case) supposedly have reasonable good matching.There are still a few uses for JFETs, especially with low noise amplifiers, electrete microphones and RF. RF and microphones naturally prefers SMD over THT.Designers have learned to work around the relatively large scattering in the threshold, though slightly finer binning would be a nice thing, though hard to get for some reason.

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: May 11, 2024, 04:50:20 pm
content: These are matched non-jelly-bean pairs Interfet IF3602. All FETs are n-chan.At least you don't get outliers in the same package. $100 a dual.And these transistors are on the same process / design.And Interfet has a selection service. But you are not the one who getsthe selected duals.In hi-end audio circles they praise the p-chan 2SK74 + its n-chan partner,but Toshiba (?) does not make them anymore since eons.

author: Zero999
author_rank: Super Contributor
timestamp: May 11, 2024, 04:58:13 pm
content: Quote from: BrokenYugo on May 08, 2024, 06:02:59 pmI don't think so. They're widely used in electrect microphone modules, which are widely available. I can see why matched pairs, P-channel and through hole parts are less common, but there are plenty of SMT N-channel J-FETs to choose from.

author: floobydust
author_rank: Super Contributor
timestamp: May 11, 2024, 05:52:17 pm
content: For electret condenser mic's the IDSSvariances don't cause a problem, the signal is mV and only ~1.6:1 variation at the low 0.25mA range. Ref.2SK596SJFET's - it's always been their wide variations that made them very difficult in a production environment, in most circuits. They became avoided.Hobbyists, Tektronix could hand-select the parts. I checked and 5:1 typical, even worse was common not including pinch-off voltage variations.P.S> Heard a top mega-corp EE interview question "what are JFET's used for?"... the young EE crash and burned lol did not get the job.

author: Zero999
author_rank: Super Contributor
timestamp: May 12, 2024, 07:59:30 pm
content: The original post was just about individual J-FETs, not matched pairs.There really isn't much need for discrete matched J-FET pairs any more, since they exist on the input stage of many op-amps, which is what they're normally used for.

author: t1d
author_rank: Super Contributor
timestamp: May 12, 2024, 11:57:16 pm
content: Wow, what great contributions = Thanks! And, it is a much better discussion than I had anticipated. All very interesting, educational and helpful.Some folks have included the consideration of matched pairs. That is a natural extension of the the original question, of course. I surely would have added that as a follow-up query. So, I am glad that it is being included.With much appreciation! Cheers!

author: David Hess
author_rank: Super Contributor
timestamp: May 13, 2024, 05:40:38 am
content: Quote from: MathWizard on May 11, 2024, 01:15:46 amThey can, but not for $1 unless you want to buy huge quantities. Companies like Linear Integrated Systems make matched monolithic dual N and P JFETs, and MOSFETs and bipolars, for different applications.QuoteThey are still used in precision analog and audio applications.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 13, 2024, 06:49:32 am
content: SMD JFETs are still available. A common types (for switching and low noise amplification) are mmbf4393, mmbf4392 (different threshold). Essentially the same is also avialable as mmbfJ113 / 112 and BSR58/57.At least the SOT23 parts essentially all have the same pinout (gate at one side and the drain and source exchangable on the other), so the exact type often does not matter.For the more RF use there are a few other types (e.g. mmbfJ309, mmbf4416).The types for microphones are a bit special - mainly a thing for the manufactureres of the electrete microphones and nothing one would need for a hobby.The threshold seems to vary quite a bit, even for modern processing. Still the parts from the same batch are usually reasonable matched and not scattering all over the place.Some parts also come with additional binning for a bit better predictibility. Usually JFET circuits are build to allow for the variations. This is part of the reason they are not that popular to use.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: May 13, 2024, 12:16:14 pm
content: For last 50 years or so JFETs are probably my favorite devices to design with. About 48 years ago I've used JFETs to build the front end for my first oscilloscope (the voltage amplifier was made on BJTs + valves). I've used JFETs extensively since, both for home projects and in my professional electronics designs. I even made anice voltage reference from JFETs, with <10ppm/C tempco and better than 10ppm/year stability. My current contract work design uses JFETs (moreover, there is no chance not to use them and get the performance required). So there is a room and certainly a good cause for discrete JFET designs. Fortunately, the AoE book still dedicate tens of pages to JFET circuits, performance and nuances, and rightly so. One of the most worn out books in my lab is the "Low Power Discretes Data Book" from Siliconix, 1989 edition (see photos below). I use it often and it was extremely useful for my current project. The vast majority of JFETs made today are described in that book, on many occasions it will provide much more extensive and valuable information than manufacturers datasheets.To answer the question from the first post: there is no such thing as a complementary JFET pair, at least not from currently made devices. The Japanese manufacturers did make an effort at some point to produce these and had a number of devices which could be considered complementary, including differential pairs. What still in production is a number of "standard" JFETs, mostly N-channel, however some P-channel devices are still made and available from stock. The variability of JFET parameters can be countered by a good circuit design and a selection as the last resort. Which devices are useful to have at hand would largely depend on the application, you could start with J113, J112, J211 for N-channel and J176 for P-ch, from ON Semi. I wouldn't bother with InterFET devices, if only for the price reason. The choice would be considerably larger if you consider SOT-23 SMD packaged devices.CheersAlex