How to match a set of JFETs

[quarks]

Hello all,

right now I am working on a calibrator and found two defect JFETs out of a matched set of 4. Besides the problem to still get the obsolete J108 JFETs, I wonder what else, besides the ON resistance (for a switch application), should be checked/matched and how to do it. Any hint is welcome.

The Picture below shows the circuit I am working on.

Thanks
quarks

[Psi]

I wonder what else, besides the ON resistance, should be checked/matched

Voltages would be the first thing. (drain-source, drain-gate)

Gate-source voltage vs drain current

Input capacitance

[quarks]

Voltages would be the first thing. (drain-source, drain-gate)

Gate-source voltage vs drain current

Input capacitance

thanks for your reply.
How can I practically meassure the Input capacitance?

In my application the JFETs are used as DC Switches (I added this to my post now). Therefore I guess the ON resistance is still the main influence. At least the appl. wants the ON resistance of each N-channel JFET to be 3-5 Ohms.   

[Psi]

If you don't have the datasheet that's a good question.

The gate capacitance is important for switching speed and drive current.
If your application is a simple on/off switch and is not oscillating or switching fast then you can ignore it.
(ie, if switch-on/off lag doesn't matter)

[fpliuzzi]

Hello,
Maybe the simple test circuit shown in the article at edn.com will help you match your JFETs.

www.edn.com/design/other/4370552/Simple-circuit-lets-you-characterize-JFETs

Also, Erno Borbely has a nice document about JFETs that contains some useful info about matching them. Just google "JFETS: THE NEW FRONTIER, PART 1" & "JFETS: THE NEW FRONTIER, PART 2".

Regards,
Frank

[quarks]

www.edn.com/design/other/4370552/Simple-circuit-lets-you-characterize-JFETs

google "JFETS: THE NEW FRONTIER, PART 1" & "JFETS: THE NEW FRONTIER, PART 2".

Hello Frank,

thanks a lot. I had a first look at it and will later read through for the details.

I also just had a look at the Peak Atlas DCA 75 Pro tester, which seems to be very good for this job.
http://www.peakelec.co.uk/acatalog/dca75-dca-pro.html

bye
quarks

[fpliuzzi]

www.edn.com/design/other/4370552/Simple-circuit-lets-you-characterize-JFETs

google "JFETS: THE NEW FRONTIER, PART 1" & "JFETS: THE NEW FRONTIER, PART 2".

Hello Frank,

thanks a lot. I had a first look at it and will later read through for the details.

I also just had a look at the Peak Atlas DCA 75 Pro tester, which seems to be very good for this job.
http://www.peakelec.co.uk/acatalog/dca75-dca-pro.html

bye
quarks

Hi quarks,
The Peak Atlas DCA 75 Pro tester does look like a very useful device to have around. I've gotten a lot of use out of my M-Cubed Electronix Semiconductor Analyzer (no longer available) and will probably upgrade to the DCA 75 Pro at some point myself. A couple of months ago I watched a few youtube videos demonstrating its capabilities and was impressed.

One possible minus for me is that it seems to be limited to testing lower voltage components and components that only need a low level of test current for them to be checked out properly. It looks like I would still need to hang onto my curve tracer and my collection of home-brewed  test fixtures that I've put together over the years to test the devices that the DCA 75 Pro couldn't handle (and that my M3 Semiconductor Analyzer can't handle either).

However, because of the vast number of things that the Peak Atlas DCA 75 Pro looks like it can do so well, it would make a great addition to my test bench.

Regards,
Frank

[fpliuzzi]

Hi quarks,
I just noticed your post about measuring the input capacitance of a JFET. The pdf that I have attached is a list of the available JFET tests included in a 1994 book by John Lenk that I've had for many years (I've had this pdf file on my hard drive for a long time).

If it looks useful to you, I saw some used copies of the hardback book (as low as $5.98) at

www.amazon.com/McGraw-Hill-Electronic-Testing-Handbook-Procedures/dp/0070376026

Regards,
Frank

EDIT: I added a text file version of the pdf's contents for anyone who cannot open the pdf.

[sorin]

I think that the bigger problem should be the leakage current

[vlf3]

You could build a simple curve tracer, for a scope... not much in the design, see URL:

http://electronicdesign.com/displays/simple-circuit-turns-scope-function-generator-jfet-curve-tracer

Circuit:

[quarks]

Thanks for the replies.

I have ordered the DCA75 Pro and will check the other suggestions while waiting for it.

BTW I could not open Franks pdf. Has anyone else had this problem too?

[vlf3]

Hi quarks... if your referring to the EDN web-page as below ! it opens in my browser ? otherwise I can't see any pdf file.

http://www.edn.com/design/other/4370552/Simple-circuit-lets-you-characterize-JFETs

[quarks]

@ vlf3,
no I had trouble opening the pdf in Franks (fpliuzzi) post. But now I can open it.

@ all,
I received the atlas DCA Pro today and it is quite useful.

Thanks for all replies
quarks