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Old 16th January 2002, 09:54 PM   #11
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I am a little confused.

Is there a significant advantage using a TL431 or LM337 over using a resistor/stabelized powersupply to match FET's.
(The later method as explained in the PASS A75 manual).

I can not see an advantage from a temperature point of view. I noticed significant temp drift when matching my IRFP9610's and IRFP150's for an ALPEH3.

Or am I missing something here?

Regards
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Old 17th January 2002, 09:59 AM   #12
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Perhaps the problem with the A75 matching circuit is the mosfets have a Vdg (voltage drain to gate)of zero? The circuits I showed may have a similiar problem of low Vdg and low power dissipation. Maybe some Vdg is better than none?
Right now I do not have the power supply built for the preamp. But I do have the mosfets.
I was thinking matching at a lower power level could translate into a higher power level match or at least get them close(like the A75 matching circuit). Also without the hassle of heatsinking, soldering, unsoldering parts. Like I said I'm unsure about high curents and high powers. I should present the ideas in a better context. Saying its just an idea, simple or complex, useful or not(like the 377 circuit).
I apologize for not being clear or thinking things out.
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Old 17th January 2002, 08:16 PM   #13
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Of course we are simply trying to match the
devices, not characterize them particularly.
The Vdg at zero is not a problem as the actual
voltages of interest are the Vds and Vgs which are
identical. Practical experience shows that the
simple technique with a power supply, resistor
and DC voltmeter gives you the results you want
without getting much more elaborate.
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Old 17th January 2002, 10:49 PM   #14
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TomT, Mr. Pass, thank you both. My confusion is gone. I'll stick with the A75 matching method.
I try to compensate the temp drift with taking the measurements in almost identical conditions
- All FET's measured for exactly the same time. I use 60 seconds per FET (give or take a second or so).
- Prevent any airflow or cooling during the measurement.

I do not know if these precautions are essential, but they worked for me. I have no problems with DC on the output (+/- 20 and 15 mV on each channel) , or temperature drift in my ALEPH 3.

Regards
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Old 18th January 2002, 05:42 PM   #15
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I only test them for about 5 seconds, but I do make
sure that they all have reached room temperature
before I start.
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Old 19th October 2003, 04:17 PM   #16
Asen is offline Asen  Bulgaria
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I have succeeded in matching 4 sets of 3 IRFP240s for an Aleph 30 project out of 20 MOSFETs.
Obviously it's far more difficult to match 2 pairs for the input. I've measured 10 devices (IRF9610) at different currents. Here are the results:

15mA 13mA 7.5mA
1. 3.35 3.32 3.25
2. 3.57 3.55 3.54
3. 3.63 3.61 3.60
4. 3.69 3.66 3.59
5. 3.72 3.71 3.71
6. 3.75 3.71 3.61
7. 3.90 3.88 3.87
8. 3.93 3.92 3.93
9. 3.97 3.95 3.93
10. 4.03 4.02 4.98

Do you see 2 pairs (properly matched) between these devices or I should better go buy 10 more?

Thanks!
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Old 19th October 2003, 05:09 PM   #17
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Obviously you can leave that 4.98 home, but otherwise, you
have lots of matches.
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Old 23rd January 2004, 02:16 PM   #18
Megabox is offline Megabox  Hong Kong
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Default Matching Quantity

Hi all,

I am going to build an Aleph-P 1.7 and have to match 8 x IRF610 and 4 x IRF9610. How many MOFETs should I need to buy if the difference among themself is 0.01v or less? Please advise.

Thanks.
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Old 23rd January 2004, 08:29 PM   #19
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I'm not sure about IRF610, but I bought some 9610's for an Aleph 3 I'm building and was surprised to find 4 of them matched within 0.02V. Of course I need only need two pairs of two, but if you need four, 10 may be enough, maybe a few more won't hurt :-).
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