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Old 25th March 2013, 05:13 AM   #11
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If you're taking the time to match parts and charging a premium for it (which is certainly fair), perhaps you could accommodate different builders and applications by matching some at one Vds, then another separate batch at a different Vds. That way, buyers could identify which sets can be used with confidence at or near the Vds they require, perhaps 10 and 15, which seem to be pretty common. Sort of a semi-custom specification with meaningful flexibility yet simplified, and less expensive, work on your part.

Peace,
Tom E
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Old 25th March 2013, 10:02 AM   #12
tony399 is offline tony399  United Kingdom
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I will take a close up of the backside and post it tonight if I get a chance, there is a round indent which has no letter in it.

So far I have remeasured around 40 to 50 of each type which is a range from minimum Idss up to 8mA. This plus the initial range sorting measuring has taken a couple of days work.

The first ten or so I did a measurement at 10Vds and then at 20Vds and again I will post the results so people can see how matching at the correct Vds is important.

If further Vds points are necessary to prove transconductance curve I'm happy to do this on a few samples.
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Old 25th March 2013, 10:19 AM   #13
tony399 is offline tony399  United Kingdom
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By the way has anyone investigated diytrade.com, they are showing J74s which look ok to me..
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Old 25th March 2013, 12:02 PM   #14
tony399 is offline tony399  United Kingdom
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Funnily enough I had a picture of the backs already for this very reason bear. I hadn't posted it because it was taken on the works camera, my computer at home doesn't take SD cards and I didn't have a usb cable to fit the camera.
CIMG0724.JPG
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Old 25th March 2013, 07:21 PM   #15
tony399 is offline tony399  United Kingdom
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Example one;
K170 @10Vds = 6.65mA, @20Vds = 7.10mA
J74 @10Vds = 6.65mA, @20Vds = 6.99mA

Example two;
K170 @10Vds = 6.82mA, @20Vds = 7.28mA
J74 @10Vds = 6.81mA, @20Vds = 7.14mA

Example three;
K170 @10Vds = 6.88mA, @20Vds = 7.34mA
J74 @10Vds = 6.88mA, @20Vds = 7.23mA

This is the level of matching I can achieve on some pairs, seems a pity not to do it at the ideal Vds..
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Old 26th March 2013, 11:48 PM   #16
tony399 is offline tony399  United Kingdom
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Due to the amount of counterfeit devices around at the moment I am going to work with another diyaudio member to get my devices measured on a curve tracer. Apparently this a fairly good method of spotting some of the better fakes.
Please no more PMs for the time being.
Thanks,
Tony
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Old 27th March 2013, 12:05 AM   #17
NicMac is offline NicMac  Italy
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Your measurements nicely indicate that Idss does not change dramatically with Vds
Personally, I would prefer knowing both Idss and Yfs at a single Vds rather than Idss at multiple Vds.
Tracing will show you that your J74 are genuine - and that they will never trace K170 well unless you degenerate
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Old 27th March 2013, 11:40 AM   #18
tony399 is offline tony399  United Kingdom
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Default curve tracer

I have been told I can get a curve tracer from ebay to attach to a scope for $20, the only one I can find is;

Power Transistor Curve Tracer adapter XY Oscilloscopes | eBay

Obviously this is designed for power transistors not fets, does anyone know of one for fets that is cheap and works well?

I've had a quick look at octopus but need to do some more research.

Also, on the question of matching, the circuits that I have seen are designed to cope with a relatively loose tolerance.

This would help me as I could match more sets and not worry about custom Vds, in fact all the ones I have can be done at 10Vds with 5% tolerance at worse.

Anyway, need to get them verified first.
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Old 27th March 2013, 11:47 AM   #19
Salas is offline Salas  Greece
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Get a Peak DCA75 made in the UK.
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Old 27th March 2013, 11:52 AM   #20
tony399 is offline tony399  United Kingdom
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Quote:
Originally Posted by Salas View Post
Get a Peak DCA75 made in the UK.
Nice bit of kit but not cheap at 115!
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