JFET Questions - Page 1
Forum section: electronics / beginners

author: eev_carl
author_rank: Regular Contributor
timestamp: January 26, 2019, 03:06:08 pm
content: Hi,I'm trying to reproduce the load lines in the JFET J211 datasheet. When my Vgs is 0 and Vds is 5, I get 8.2mA of current and not the 20mA in the datasheet. I was wondering if there was something wrong with my test setup or if my JFET was bad (I just replicated my results with a new one though).The attached schematic shows the JFET with a positive Vds and a negative Vgs. In the simulation, Vgs is 0. I'm also unsure as to why the simulation shows only .4mA of current. (Differently spec'd JFET?)https://www.mouser.com/ds/2/149/J211-889304.pdfThanks,Carl

author: Wimberleytech
author_rank: Super Contributor
timestamp: January 26, 2019, 03:33:40 pm
content: You need to supply the model for the transistor you are simulating. That model needs to be the model of the device in the datasheet.I found this but cannot vouch for it:.model J211 NJF(Beta=1.808m Betatce=-.5 Rd=1 Rs=1 Lambda=10m Vto=-2.9+ Vtotc=-2.5m Is=101.9f Isr=977.5f N=1 Nr=2 Xti=3 Alpha=13.92u+ Vk=88.13 Cgd=3.913p M=.3448 Pb=1 Fc=.5 Cgs=3.496p Kf=701.5E-18+ Af=1)

author: Benta
author_rank: Super Contributor
timestamp: January 26, 2019, 03:55:34 pm
content: According to the data sheet, drain current can be anywhere between 7.0 and 20 mA at zero gate voltage, so your results are within spec.

author: TimFox
author_rank: Super Contributor
timestamp: January 26, 2019, 03:55:58 pm
content: The data sheet value for Idss (drain current at zero gate-source voltage) ranges from 7 to 20 mA across production, so your 8.2 mA value is within specification.

author: Ian.M
author_rank: Super Contributor
timestamp: January 26, 2019, 04:17:06 pm
content: I've set up a .dc sweep with Vgs stepped to attempt to reproduce the datasheet curves Carl posted, using Wimberleytech's J211 model. It seems to be somewhat better than the default NJF one, but it still doesn't look much like the curves in the datasheet. It has a sharper knee, and much lower output resistance in the saturation region.

author: w2aew
author_rank: Super Contributor
timestamp: January 26, 2019, 07:51:51 pm
content: Quote from: Ian.M on January 26, 2019, 04:17:06 pmYour simulation sweeps Vgs to 50V, while the plot shared by the OP goes to 5V. If you reduce the sweep on Vgs in your sim, the results are pretty close to the mfr curves.

author: Ian.M
author_rank: Super Contributor
timestamp: January 26, 2019, 08:44:28 pm
content: Aagh!! I didn't see those wretched decimal points. TIFU: by not reading the original datasheet which with abs. max. Vdg of 25Vm would have clued me in.However its still not *quite* right:The Idss curve (Vgs=0) is clearly lower. However I think the inconsistency is in the datasheet as that lists the max. Idss at Vds=15V as 20mA, then later on fig.4 shows it as approx 22.5mA at Vds=5V. I don't expect a hump in a JFET characteristic, so how is it going to get down to 20mA with a 10V increase in Vds?

author: Benta
author_rank: Super Contributor
timestamp: January 26, 2019, 08:58:41 pm
content: It's just an old, not updated data sheet. No one bothers. The important thing is, that the device is within specs. The curves are always beautified.