How do I test real jFET's for the value of rdson?

[Amptroll]

PRR is correct.  All the others are way overkill.  Good reading but not good answers.  I have also noticed that all the others
are mixing apples and oranges.  rds is not the same as Rd.  The convention in electrical engineering and for that matter
the scientific world community in general is:
lower case letters are reserved for small signal (dynamic values). 
UPPER CASE LETTERS ARE RESERVED FOR LARGE SIGNAL (STATIC VALUES).
1/gm is listed if you glean the DS.  If your math is good from the DS you can derive all you need. 
Also like one member has stated; two general cats.
Switching devices and amplification devices.  What do you need?
If its amp then as stated, rds is irrelavent.
If its switch the DS will have it.  Its that simple.
Thanks PRR.
Rob

[Rob Strand]

rds is not the same as Rd.  The convention in electrical engineering and for that matter
the scientific world community in general is:
lower case letters are reserved for small signal (dynamic values).
UPPER CASE LETTERS ARE RESERVED FOR LARGE SIGNAL (STATIC VALUES)

RD (or RS) has not been confused with rds.

rds is the small signal resistance which follows the common JFET equations.

RD is an internal *ohmic* resistance.   It's part of the SPICE model.  You can see it on page 151,

https://www.seas.upenn.edu/~jan/spice/PSpice_ReferenceguideOrCAD.pdf

[willienillie]

What do you need?...If its switch the DS will have it.  Its that simple.

Well not necessarily, if an amplifier FET has been used for switching, such as the 2SK30A in the TS9/808.

(And for the record, everyone, no I'm not going to go stick 330r resistors in my true-bypass TS clones.  It's just something I've wondered about.)

[savethewhales]

Okk! Thank you very much Rob! My multimeter has a internal impedance of 10Mohm so that's in the middle (more like the 100Mohm),and it seems to me that the values I got are fair enough.. Today I'm gonna do real measurements with the oscilloscope on my university amd maybe I can confirm/check the values I got from my FET'S.

Another thing, when I measured the FET's, I always tried to do it as fast as I could, knowing that it could damage them. Does this help not to affect Idss as you mentioned? If not then maybe yes Idss was affected.

[Rob Strand]

Another thing, when I measured the FET's, I always tried to do it as fast as I could, knowing that it could damage them. Does this help not to affect Idss as you mentioned? If not then maybe yes Idss was affected.

It's a good idea.

Some switching JFETs have very high IDSS and low rds_on.  You pretty much can't prevent the JFET from heating up.   For these cases you can do a low current measurements (but not too low) and fit the JFET equation to extract IDSS.   More than one measurement helps accuracy.

Manufacturers use pulse measurement techniques to prevent devices heating up but it requires more hardware to get accurate measurements.

[savethewhales]

It's a good idea.

Some switching JFETs have very high IDSS and low rds_on.  You pretty much can't prevent the JFET from heating up.   For these cases you can do a low current measurements (but not too low) and fit the JFET equation to extract IDSS.   More than one measurement helps accuracy.

Manufacturers use pulse measurement techniques to prevent devices heating up but it requires more hardware to get accurate measurements.
[/quote]

Ok I understand. I will try to discover the jFET caractheristics by real circuit testing (based on the calculations I've done), maybe testing the frequency response with diferent Gate Voltages.