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Old 10th January 2013, 01:49 PM   #311
CeeVee is offline CeeVee  Portugal
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Quote:
Originally Posted by buzzforb View Post
Ceevee,
I have found the same thing, but when testing in reverse, (meaning I test already matched by battery method), the tracer gives an excellent view of very well matched traces. In truth, I don't think the curve tracer does much for us with these small signal jfets other than finding perfectly aligned curves, which seem to be easily found. I think it speaks to the quality of the Toshiba parts. Try heating up teh jfets and then tracing them and see if you get seperation of curves.
Hi Buzzford,

That is what i use it extensively for; comparative Transfer curve matching after DMM matching,
That way i have a reliable Idss value and use the curve tracer for Rdgen fine tuning for example with 2SJ74BLs and 2SK170BLs.

I just brought this up because it would be nice if we could rely on it's measured Idss instead of having to use DMM matching beforehand.
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Old 10th January 2013, 01:56 PM   #312
NicMac is offline NicMac  Italy
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Quote:
Originally Posted by CeeVee View Post
Locky_z,

I have an issue with the curve tracer and it seems others have as well.

When measuring JFETs the IDss measured by curve tracer is
on average 1mA at least above what we get by the DMM test with a 9V battery for example.

So PC curve tracer is useless this way, can we calibrate it so it gives reliable Idss values ?
Yes I find the same Idss "offset", but no - the tracer is not useless because of this.
Rather, I would say that it works very nicely if you want to match curves and/or estimate transconductance. In relative terms it is very accurate and deliver very reproducible measurements. Perfect for the hobbyist - and useless for professionals
A simple way of absolute calibration would of course be very nice.

Quote:
Originally Posted by buzzforb View Post
In truth, I don't think the curve tracer does much for us with these small signal jfets other than finding perfectly aligned curves, which seem to be easily found.
Are you talking about SK170/SJ74 curve matching? While NN and PP matches are rather easy to find I can assure you that this is not the case for NP. The tracer comes in very handy here
I would rather say that it could be much improved in the high current functionality...
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Old 10th January 2013, 02:09 PM   #313
NicMac is offline NicMac  Italy
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Quote:
Originally Posted by CeeVee View Post
Hi Buzzford,

That is what i use it extensively for; comparative Transfer curve matching after DMM matching,
That way i have a reliable Idss value and use the curve tracer for Rdgen fine tuning for example with 2SJ74BLs and 2SK170BLs.

I just brought this up because it would be nice if we could rely on it's measured Idss instead of having to use DMM matching beforehand.
My suggestion:

First trace and match the devices based on these data (for example using virtual in silico degeneration of the J74).

Then sort the matched sets by measuring Idss at equilibrium.

To convince yourself that this really works try to measure Idss "the old way", but now with degeneration.

I am very, very pleased with the result
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Old 10th January 2013, 02:12 PM   #314
locky_z is offline locky_z  China
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Quote:
Originally Posted by CeeVee View Post
Locky_z,

I have an issue with the curve tracer and it seems others have as well.

When measuring JFETs the IDss measured by curve tracer is
on average 1mA at least above what we get by the DMM test with a 9V battery for example.
Yes, your are right.The reason is that Curve Tracer measured tiny voltage not precision.

When you get the Vgs-Id curve , from the curve ,The X-axis(Vgs)=0,The y-axis is Idss.
Although the X-axis = 0, but actually Vgs may be plus/minus a few millivolts.


The 'Offset test' button functin is to show the tiny voltage measure error,
When the button click,system will short the B-E / C-E / RC /RB by relay J6 J7. This mean Vbe=Vce=Vrc=Vrb is ture equal 0V, but the measure value may be not zero.
Because the four voltages will be affected by the impact of the 'Ve' voltage and the temperature. So system will draw four curve 'Ve->Vbe'/'Ve->Vce'/'Ve->Vrb'/'Ve->Vrc' curve.



To reduce the error, you can check the 'Calibration before measure' checkBox,
When It is checked, The system will calibration first and then do the curve measure.
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Old 10th January 2013, 02:16 PM   #315
NicMac is offline NicMac  Italy
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Quote:
Originally Posted by locky_z View Post
When It is checked, The system will calibration first and then do the curve measure.
I always check this box.
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Old 10th January 2013, 02:35 PM   #316
CeeVee is offline CeeVee  Portugal
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Quote:
Originally Posted by NicMac View Post
My suggestion:

First trace and match the devices based on these data (for example using virtual in silico degeneration of the J74).

Then sort the matched sets by measuring Idss at equilibrium.

To convince yourself that this really works try to measure Idss "the old way", but now with degeneration.

I am very, very pleased with the result
Glad to get your feedback on that Nic, thanks
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Old 10th January 2013, 02:46 PM   #317
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Quote:
Originally Posted by NicMac View Post
Yes I find the same Idss "offset", but no - the tracer is not useless because of this.
Rather, I would say that it works very nicely if you want to match curves and/or estimate transconductance. In relative terms it is very accurate and deliver very reproducible measurements. Perfect for the hobbyist - and useless for professionals
A simple way of absolute calibration would of course be very nice.


Are you talking about SK170/SJ74 curve matching? While NN and PP matches are rather easy to find I can assure you that this is not the case for NP. The tracer comes in very handy here
I would rather say that it could be much improved in the high current functionality...
I was speaking of NN and PP. Of course NP is a whole different ball game. I love the tracer, just pointing out that it is not optimal for Idss measurement and the battery test works great. I have used it to sort matched Idss fets fro better curve alignment. In this regard it excels. I think it might be one of my most valuable tools in electronics.
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Old 10th January 2013, 03:01 PM   #318
CeeVee is offline CeeVee  Portugal
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Quote:
Originally Posted by buzzforb View Post
I was speaking of NN and PP. Of course NP is a whole different ball game. I love the tracer, just pointing out that it is not optimal for Idss measurement and the battery test works great. I have used it to sort matched Idss fets fro better curve alignment. In this regard it excels. I think it might be one of my most valuable tools in electronics.
Guess i was just aiming for a little too much i use it exactly like you guys and it is indeed very handy at comparative transfer curve matching once i have done the DMM matching on a little PCB i have built where i plug in the P and N fets.

I found it particularly nice at dialing in the Rdgen on the 2SJ74s and comparing the curve interactively with the matching 2sk170s.

i had some doubts about the reliability of what i was seeing on the laptop but your validation as well as Nics give me the reassurance i needed.
thanks.
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Old 10th January 2013, 03:46 PM   #319
NicMac is offline NicMac  Italy
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Quote:
Originally Posted by CeeVee View Post
I found it particularly nice at dialing in the Rdgen on the 2SJ74s and comparing the curve interactively with the matching 2sk170s.
True.
But even easier:
1) Trace all the FETS without any kind of degeneration.
2) From the traces sort and record Id @ 0V and 0.1V
3) Copy to excel (and fix all the stupid decimal and errors)
4) Calculate Yfs = (Id@0V-Id@0.1V)/0.1V
5) Recalculate Idss and Yfs for the J74 using selected Rdeg values (I use 4R3, 4R7, 5R1, 5R6, 6R2, 6R8 and 7R5).

Congratulations! - you now have 7 times as many J74 devices to choose from

6) Match you K170 and degenerated J74 devices for both Idss and Yfs.
7) Remeasure (or trace) the pairs/sets you have identified, but now with degeneration and be ready for a pleasant surprise
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Old 10th January 2013, 03:52 PM   #320
CeeVee is offline CeeVee  Portugal
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Quote:
Originally Posted by NicMac View Post
True.
But even easier:
1) Trace all the FETS without any kind of degeneration.
2) From the traces sort and record Id @ 0V and 0.1V
3) Copy to excel (and fix all the stupid decimal and errors)
4) Calculate Yfs = (Id@0V-Id@0.1V)/0.1V
5) Recalculate Idss and Yfs for the J74 using selected Rdeg values (I use 4R3, 4R7, 5R1, 5R6, 6R2, 6R8 and 7R5).

Congratulations! - you now have 7 times as many J74 devices to choose from

6) Match you K170 and degenerated J74 devices for both Idss and Yfs.
7) Remeasure (or trace) the pairs/sets you have identified, but now with degeneration and be ready for a pleasant surprise

Thanks Nic,

But...Rdegen trick comes at a price....slightly higher distortion, read this somewhere !, and according to Patrick ( EUVL ) deviating from 5R1 is to be avoided.
I fail to understand this but then that will be because i don't know enough of the subject.

My reasoning was in line with what you just laid out, the lower the Rdegen needed the better i thought. 5R1 being an average value.

I'd love to get the facts on this.
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