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Old 27th January 2010, 01:54 PM   #1
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Default Measuring Hfe

Is this a good jig to find matching transistors? Is hfe what I need to match (2SK170) for the BOZ preamp? What about for differential pairs in the amp section to limit offset?
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Old 27th January 2010, 02:29 PM   #2
AndrewT is offline AndrewT  Scotland
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no.
FETs are measured differently from BJTs.

Don't try to measure absolute values using our amateurish set up jigs.
It is far better to compare DUT to REF when trying to find matches.

BTW,
that jig uses constant base current rather than constant collector current for measuring hFE. This will give very misleading results.
Many DMM have a similar circuit for their hFE function because it is cheap. It is only good for grouping similar devices. It is absolutely no good for selecting matched pairs.
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Last edited by AndrewT; 27th January 2010 at 02:33 PM.
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Old 27th January 2010, 02:50 PM   #3
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Erno Borbely shows a JFET measurement jig on his website --
http://www.borbelyaudio.com/adobe/ae599bor.pdf
http://www.borbelyaudio.com/adobe/ae699bor.pdf
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Old 27th January 2010, 11:24 PM   #4
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Quote:
Originally Posted by jackinnj View Post
The figure 5 jig looks easy enough to build, but how is the jig used?

And if the bjt testing jig I posted not adequate, any suggestions for a better one?
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Old 27th January 2010, 11:37 PM   #5
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Why not just get Idss like this?

Transistor matching
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Old 28th January 2010, 12:06 AM   #6
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As Andrew pointed out, the setup you posted works for small signal bipolar transistors, not JFETs.

For JFET's it's helpful to know I(dss) and gm. Idss is measured when the switch next to the voltmeter is depressed and the Source of the JFET is grounded.

The pinch-off voltage V(p) is measured by connecting the source to -10V through a 1Meg resistor -- gm can then be calculated

gm =~ 2x Idss / V(p)
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Old 28th January 2010, 10:02 PM   #7
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So this is the simplest, correct way to measure Idss:

Transistor matching
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Old 29th January 2010, 10:15 AM   #8
AndrewT is offline AndrewT  Scotland
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yes.

To give an enhanced capability, combine the two jigs into one by joining the two sources together. This will automatically join the two gates together and ensure Vgs = 0V.

Replace the mAmmeters with a pair of accurately matched ~100r resistors.
Feed +9V, or if you have it +10.2V to +11.0V, to the two drain resistors.
Measure the voltage drop across the drain resistors and/or compare the voltage between the two drains.

This can be fitted to an 8pin dip socket so each FET can be face to face with it's partner. A spring clip, a bulldog clip (for paper printoffs) is perfect, allows the two FETs to be equal Tc if they are of equal Id. Temperature is the biggest problem for us. We cannot hold Tc at 25degC easily.
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