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Old 23rd October 2008, 02:06 PM   #1
AndrewT is offline AndrewT  Scotland
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Default High Idss jFETs

Hi,
lsk 170c jFETs are testing out at 10mA to 30mA.
I've got about 40 between 20mA and 30mA Idss.
What would these be best used for?
Can I find the Yfs easily to see if they are close to a 2sk369v?

Any thoughts?
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Old 25th October 2008, 01:20 PM   #2
Mooly is offline Mooly  United Kingdom
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Hi Andrew,
That question got me thinking, does this help
http://books.google.co.uk/books?id=0...esult#PPA35,M1
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Old 25th October 2008, 01:31 PM   #3
lineup is offline lineup  Sweden
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here is datasheet from manufacturer www.linearsystems.com
http://www.linearsystems.com/datasheets/LSK170.pdf
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Old 25th October 2008, 01:34 PM   #4
lineup is offline lineup  Sweden
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here is spice models
for the LSK170 family
http://www.linearsystems.com/downloa...l#JFETs320_170
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Old 25th October 2008, 02:18 PM   #5
AndrewT is offline AndrewT  Scotland
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thanks,
Mooly,
can the Yfs test measurement be done at DC? instead of at RF?

When testing Idss, the Vgs =0V and Vds=10V with RL=100r.
Then Yfs=infinity, which is plainly ridiculous.
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Old 25th October 2008, 02:56 PM   #6
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Quote:
Originally posted by AndrewT
thanks,
Mooly,
can the Yfs test measurement be done at DC? instead of at RF?

When testing Idss, the Vgs =0V and Vds=10V with RL=100r.
Then Yfs=infinity, which is plainly ridiculous.
I have referenced Erno's 1999 article on JFET's with his testing methodology -- nice test setup:

http://www.borbelyaudio.com/adobe/ae599bor.pdf
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Old 25th October 2008, 04:01 PM   #7
Mooly is offline Mooly  United Kingdom
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Andrew, this is something I have never measured. I don't know is the honest answer ! I suspect there is no one correct answer in that the result depends on the particular application, and that application depends on the correct choice of FET. Sort of chicken and egg thing. Sorry.
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Old 25th October 2008, 04:30 PM   #8
Elvee is offline Elvee  Belgium
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Hi,

For a given technology, there is a direct correlation between gfs, Idss and Vgs off.
Here is an example taken from the BF256 datasheet.
It's just a matter of finding the right parameters to put into the equation of the device. It can probably be found on the web.
Otherwise, you can measure gfs at low frequencies; a static, DC measurment makes no sense however.
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Old 25th October 2008, 05:14 PM   #9
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by jackinnj


I have referenced Erno's 1999 article on JFET's with his testing methodology -- nice test setup:

http://www.borbelyaudio.com/adobe/ae599bor.pdf
Thanks Jack.
If mhos is the inverse of ohms then R=V/I becomes Mho=I/V or if the curve is not straight then Mho=deltaI/deltaV.

However Borbely states Mho=2*Idss/Pinch off voltage.
Why is there a factor of 2 times in Borbely's formula?

and if Borbely is close to right and both terms in the formula are DC derived, then why is Yfs @ DC a nonsense?
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Old 25th October 2008, 06:02 PM   #10
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Quote:
Originally posted by AndrewT

Thanks Jack.
If mhos is the inverse of ohms then R=V/I becomes Mho=I/V or if the curve is not straight then Mho=deltaI/deltaV.

However Borbely states Mho=2*Idss/Pinch off voltage.
Why is there a factor of 2 times in Borbely's formula?

and if Borbely is close to right and both terms in the formula are DC derived, then why is Yfs @ DC a nonsense?
Id = Idss * [1-Vgs/Vp]^2

gm = d I (d)/ d V (gs)

so when you differentiate, the 2 goes on the outside.
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