Help Matching JFET's

DualTriode · 29th January 2011, 11:46 PM

Hello,
I am looking for help selecting a 0 to 19.99 ma digital panel meter for matching JFET Idss.
I have a meter that read to tenths of a ma I want one that reads to 0.00, Hundredths of a milliamp.
I realize that it would cost less to by matched items. As it is I bought a gob of 2SK170’s and a tenth of a ma is as close as it gets with the meter on hand.
DT
All just for fun!

DualTriode · 31st January 2011, 04:47 AM

Hello,
This
DCA-20PC-3-DC4-RL-C Murata Power Solutions Digital Panel Meters
Is the solution I picked, not cheap! It is no wonder matched JFETS go for a large mark up.
DT
All just for fun!

wrenchone · 31st January 2011, 04:53 AM

I built my FET matching jig so that a standard DVM could plug in for both voltage and current measurements (one each). Since it's the relative measurement that really counts in matching, a fairly humble DVM can serve as long as all the FETs are matched in the same session with the same meter. It's worked for me for a number of years, even in fiddly applications like jfet SRPP preamps.

DualTriode · 31st January 2011, 05:39 AM

Hello,
This may be in the range of too many toys.
I wanted that extra digit. I am not finished yet. The jig is getting a LM317 or 1085 voltage regulator set to the standard 10 volts DC. Using a 9 volt battery and watching the Idss drift as the voltage drooped under load drove me nuts.
This is turning into a project I guess.
Wrenchone I have followed your JFET threads. It looks like you have it working well.
DT
All just for fun!

wrenchone · 31st January 2011, 05:19 PM

I have a setting on my jig for IDSS, but what I generally test for is Vgs using fixed 1, 2, 5, and 10ma current sources and sinks. - just a bunch of separate "ring of 2" sources.

DualTriode · 1st February 2011, 02:50 PM

Quote:

Originally Posted by wrenchone

I built my FET matching jig so that a standard DVM could plug in for both voltage and current measurements (one each). Since it's the relative measurement that really counts in matching, a fairly humble DVM can serve as long as all the FETs are matched in the same session with the same meter. It's worked for me for a number of years, even in fiddly applications like jfet SRPP preamps.

Hello,
I am new to JFETs. As far as matching Idss it seemed like the place to start. Wrenchone you remind me that there are also other parameters that come into play. The test jig is being built into a surplus instrument chassis with ample room to add other features.
If you will tell us more about your preference to measure voltage relative to current as the preferred matching method.
DT
All just for fun!

wrenchone · 1st February 2011, 05:36 PM

Look at post #49 in my JFET SRPP thread for a circuit for testing Vgs at a given drain current.

wrenchone · 2nd February 2011, 05:13 AM

I've sorted jfets using either IDSS or VGS at specified drain current. If using low power sources like 9V batteries, the IDSS method gets a little awkward at higher currents, and you may risk crisping an especially lively fet. This is especially true for low RDS/high IDSS devices like the PN4391. The Vgs/IDS method is easier on the fets (and the batteries), and gives you guidance as to how to set appropriate source resistors for a desired drain current.

Calvin · 06:55 AM

Hi,

could look like this:

jauu
Calvin

wrenchone · 2nd February 2011, 07:48 AM

Could look different, too. Maybe a board for sale to follow, who knows?

29th January 2011, 11:46 PM	#1
DualTriode diyAudio Member Join Date: Aug 2009 Location: Sacramento	Help Matching JFET's Hello, I am looking for help selecting a 0 to 19.99 ma digital panel meter for matching JFET Idss. I have a meter that read to tenths of a ma I want one that reads to 0.00, Hundredths of a milliamp. I realize that it would cost less to by matched items. As it is I bought a gob of 2SK170’s and a tenth of a ma is as close as it gets with the meter on hand. DT All just for fun!

2nd February 2011, 06:48 AM	#9
Calvin diyAudio Member Join Date: Nov 2004 Location: close to Basel	Hi, could look like this: jauu Calvin Last edited by Calvin; 2nd February 2011 at 06:55 AM.

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31st January 2011, 04:47 AM	#2
DualTriode diyAudio Member Join Date: Aug 2009 Location: Sacramento	Hello, This DCA-20PC-3-DC4-RL-C Murata Power Solutions Digital Panel Meters Is the solution I picked, not cheap! It is no wonder matched JFETS go for a large mark up. DT All just for fun!

31st January 2011, 04:53 AM	#3
wrenchone diyAudio Member Join Date: Feb 2004 Location: Silicon Valley	I built my FET matching jig so that a standard DVM could plug in for both voltage and current measurements (one each). Since it's the relative measurement that really counts in matching, a fairly humble DVM can serve as long as all the FETs are matched in the same session with the same meter. It's worked for me for a number of years, even in fiddly applications like jfet SRPP preamps.

31st January 2011, 05:39 AM	#4
DualTriode diyAudio Member Join Date: Aug 2009 Location: Sacramento	Hello, This may be in the range of too many toys. I wanted that extra digit. I am not finished yet. The jig is getting a LM317 or 1085 voltage regulator set to the standard 10 volts DC. Using a 9 volt battery and watching the Idss drift as the voltage drooped under load drove me nuts. This is turning into a project I guess. Wrenchone I have followed your JFET threads. It looks like you have it working well. DT All just for fun!

31st January 2011, 05:19 PM	#5
wrenchone diyAudio Member Join Date: Feb 2004 Location: Silicon Valley	I have a setting on my jig for IDSS, but what I generally test for is Vgs using fixed 1, 2, 5, and 10ma current sources and sinks. - just a bunch of separate "ring of 2" sources.

1st February 2011, 05:36 PM	#7
wrenchone diyAudio Member Join Date: Feb 2004 Location: Silicon Valley	Look at post #49 in my JFET SRPP thread for a circuit for testing Vgs at a given drain current.

2nd February 2011, 05:13 AM	#8
wrenchone diyAudio Member Join Date: Feb 2004 Location: Silicon Valley	I've sorted jfets using either IDSS or VGS at specified drain current. If using low power sources like 9V batteries, the IDSS method gets a little awkward at higher currents, and you may risk crisping an especially lively fet. This is especially true for low RDS/high IDSS devices like the PN4391. The Vgs/IDS method is easier on the fets (and the batteries), and gives you guidance as to how to set appropriate source resistors for a desired drain current.

2nd February 2011, 07:48 AM	#10
wrenchone diyAudio Member Join Date: Feb 2004 Location: Silicon Valley	Could look different, too. Maybe a board for sale to follow, who knows?