What these JFETs do here? See Page 9-3.

http://www.ko4bb.com/Manuals/Fluke/Fluke_8840A/Fluke_-_8840A.pdf

Looked into several books and Wiki and of course Google.
All I can think of is these are wired to work as diode. Any idea?

Thank you.

 

Big download, 25 megabytes. It would help if you told us to which of the 180 pages you refer.

 

Oops 25Meg! Thanks for telling me. I made the page into a 150KB jpeg file.
http://www.flickr.com/photos/83142688@N05/8282249271/
This is part of Fluke 8840A Instruction Manual.
Out of 13 JFETs, 7 JFET's Drain and Source are shorted.

 

Go Advanced, manage attachments, browse, click on it, open, upload.
and don't forget to name a page number.

 

Out of 13 JFETs, 7 JFET's Drain and Source are shorted.

Some Jfets are normally 'On' and are biased to turn the 'Off"

 

Some Jfets are normally 'On' and are biased to turn the 'Off"

Thanks for the respond.
Honestly though, as a beginner, I don't quietly understand what you said.
Could you point to a tutorial or example web page explaining in detail? Any webpage that has JFET's Source and Drain shorted circuit.
Thanks again.

 

Could you point to a page in the instruction manual that has these jfets on it?

 

Your image is hard to read. No, thanks.

 

Page 9-3.
I attached the pdf page.
Thank you.

 

The FETs are used as diodes for over voltage protection.

 

That's my implied question. Why do they use JFET instead of diode? Did the Fluke engineers think it is easier to their eye or did they think it is cheaper? There must be some advantage instead of using the diode.

 

Somewhere deep in the filing cabinet of my mind, these are labeled, "low leakage" diodes.

ps, I find your pdf to be very good quality. I was able to zoom it 500% and it did not get fuzzy.

 

Thanks for the pdf.

If you were talking about the 7 fets wired as diodes I think the schematic shows why they test shorted D-S in-circuit.

Never mind ... I think I mis-understood your original statement

This will help explain Jfets:

http://www.allaboutcircuits.com/worksheets/jfetamp.html

 

Thanks #12 for kind words. I learned how to upload the file and next when I need a solution, I have to sit and look aorund a little longer than just shooting the help.
Thanks tubeguy for the reading suggestion, I am sure I will learn more about this good article.

 

@tubeguy: Thanks for the reference. I would like to point out that a jfet is an excellent constant current device, as demonstrated in the second oscilloscope view. They are actually born as constant current devices, and there is a method, using only one resistor, that cancels their temperature drift. If nado wishes to know this method, I will demonstrate how to do it.

 

@#12,
Please so. I am sure it will help all of us as well as me.

Regards,

 

I agree that the ones with drain and source connected are used as low leakage diodes.
The ones with gates connected to LM339 outputs are analog switches.
CL301 is a current sink.
QQ314 is a differential amplifier.

 

In the graphs provided by the manufacturer, there is one called, "Transfer Characteristics". I can't pull one out of a PDF so I drew one. You can see that for all 3 temperature curves, there is a point where they all cross each other. On this graph it is about -6.4 volts. If you place a resistor on the source lead and connect the gate to the other end of the resistor, the jfet will pass the same current, no matter what the temperature is, as long as there is enough voltage to keep the jfet in its operating range.

A hair dryer is a valuable tool to find out whether you have the correct resistance in the source circuit.

 

Yes, I think I have a data sheet with a chart that shows the zero temperature coefficient point.
I will post it when I find it. I actually used that feature in my undergrad final year project.

 

I don't mean to be rude, but why are we discussing zero TC current sources?