J112 JFET I_DSS - datasheet inconsistent? - Page 1
Forum section: electronics / beginners

author: aneevuser
author_rank: Frequent Contributor
timestamp: January 24, 2020, 03:38:52 pm
content: I bought some J112s from RS. The datasheet is here:https://docs.rs-online.com/1b2b/0900766b813d387a.pdf.In the "On characteristics", it seems to suggest that I_DSS will be a min. of 5 mA (or 2mA ?) so I have tried to measure it for a couple of pieces assuming that it won't be too far off this value. However, I got no constant current behaviour with I_DS all the way up to about 50 mA, with a V_DS of 5 V, when the device began noticeably hot to the touch - it's dissipating 250 mW of a quoted max. of 350 mW.Having taken another look at the datasheet, I note that Note 2 on page 4 states that "The electrical characteristics table indicates that an J112has an IDSS range of 25 to 75 mA". Well, that may explain my observations, but it seems to contradict the "On characteristics" info, even assuming say a x5 variation in I_DSS between parts.So the question is: WTF is going on here? Is the datasheet contradictory, or am I misreading it and confused? And what value should I typically expect for the I_DSS of a J112?

author: schmitt trigger
author_rank: Super Contributor
timestamp: January 24, 2020, 04:00:23 pm
content: Well..... I checked a datasheet from another vendor (Vishay), and it has the same contradictory values between the table of electrical characteristics and the Id vs Vds curves at different Vgs.

author: exe
author_rank: Unknown
timestamp: January 24, 2020, 04:23:30 pm
content: Well, 50mA is definetely more than 2mA or 5mA. So, I don't see a contradiction here since they only specify min value. They also don't provide typical value, so I wouldn't put any expectations based on "on characteristics".J-fets are notorious for large parameter scattering. Even though modern production seems to be more consistent than datasheets often suggest.What to trust? I'd say measurements. One can also put a hope that jfets from the same reel have close characteristic, but that's not guaranteed.

author: schmitt trigger
author_rank: Super Contributor
timestamp: January 24, 2020, 04:32:13 pm
content: You are correct. The datasheet only lists a minimum value. Maximum can be infinity and beyond.

author: MasterT
author_rank: Frequent Contributor
timestamp: January 24, 2020, 04:46:15 pm
content: The doc is edited version for J111, J112, J113. They missed to remove 2.0mA line, specification for J113.And minimum 5.0 is o'k with "typical" 25-75.

author: aneevuser
author_rank: Frequent Contributor
timestamp: January 25, 2020, 09:09:04 am
content: Quote from: exe on January 24, 2020, 04:23:30 pmIt's not a logical contradiction, that's true, but AoE quotes a typical I_DSS spread of x5, and these have a spread of x15, if we take the lower limit of 5 mA and the upper limit of 75 mA. Surely that's crazy, even by JFET standards?QuoteYes, I guess that's all you can do.

author: aneevuser
author_rank: Frequent Contributor
timestamp: January 25, 2020, 09:11:28 am
content: Quote from: MasterT on January 24, 2020, 04:46:15 pmThanks. That clarifies that particular point.QuoteWhere are you getting this information from? I don't think the datasheet states explicitly that the 25-75 spread is typical?

author: aneevuser
author_rank: Frequent Contributor
timestamp: January 25, 2020, 09:16:52 am
content: Quote from: schmitt trigger on January 24, 2020, 04:32:13 pmMaybe they're aimed at the Buzz Lightyear toy repair market?

author: exe
author_rank: Unknown
timestamp: January 25, 2020, 11:07:35 am
content: Quote from: aneevuser on January 25, 2020, 09:09:04 amI think the is a slight confusion between typical values and min/max values. The real parameter scattering might be not that bad (though I have only one datapoint:https://www.eevblog.com/forum/metrology/building-your-own-voltage-reference-the-jvr/msg773359/#msg773359), but it is not guaranteed.In my opinion, large min/max variation in datasheets is to avoid liability, make tests easier and have less rejections. Big variation certainly not helping with designing and mass-production. I'm not an experienced designer, but I don't see how I'd use jfets without manually characterizing and binning each one of them. I'd really prefer to pay extra to have tighter specs, but... I'm glad jfets are still around, I think they are extincting.PS I use jfets as current sources and certainly there will be a trim-pot or something in final design to accommodate for such a large variation.