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Old 16th June 2003, 06:53 AM   #1
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Default transistor matching

What is the typical variance of transistor parameters?

For example, if I want to have X number of matched output transistors, how many should I purchase in order to ensure that I get X that are "close enough"?

Thanks and regards,

Rob Weinstock
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Old 17th June 2003, 10:31 AM   #2
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Hi Rob,

hfe is one of the prime factors

I've found 2N3055's with hfe's from 60 - 200 all having the same batch number....

I went trough this batch of 100 and ended with 7 having the same hfe-reading.

Matching to within 1% is not too bad. Note the accuracy of the measuring devices, probably these limit the accuracy you can get.

I would suggest you buy (at least) 5 - 10 times the number you need, and you will get some sets. If you need to meet some specs like hfe > Y you;ll need more.
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Old 17th June 2003, 11:11 AM   #3
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I have considered exactly the same problem for a while, though
in my case it be rather expensive MOSFETs and JFETs and more
about finding matched complementary pairs, which is likely even
harder. So, well it is a bit different since the pairs need not be
matched to each other (or maybe, I'd need that too in some
cases). One could just approach it purely statistically, but since
all transistors of each type will most likely be from the same
production batch the statistics gets quite different. I don't
know, but I have assumed that devices from the same batch
are likely to have a smaller variation than the same number
of devices picked randomly. On the other hand, that would
mean that I may end up a bunch of similar N devices and
a number of similar P devices, but no matched pairs at all.

What is peoples experience and wisdom about this? Maybe
it is better to pay the considerably higher price of buying
matched devices from people who provde this service??
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Old 17th June 2003, 12:53 PM   #4
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I have looked at this -- if using Excel make sure you have the Pro version with the statistics package -- I guess I am revealing the fact that I was a lab rat at one time.

for MOSFET's (like IRFP240 + IRFP9240) -- within a "tube" you will find pretty close Vgs(t) -- if you buy them loose it's not the case. I would say that if you have a couple hundred of P and N channel devices you can match complementary pairs to some tens of millivolts. The transconductances match up pretty closely when the Vgs(t)'s match. (I have done the regression analysis, and the "eyeball" analysis via scatter plotting or histogram.)

for small signal BJT's used as current mirror's -- sometimes in a bunch I will find a bimodal distribution of beta's -- and sometimes a batch of a few hundred will have one mean and a standard deviation of only a few percent.

Jack
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Old 17th June 2003, 01:00 PM   #5
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For the MOSFETS (lateral ones) I was planning on buying them
directly from Exicon, so I would most certainly get devices from
the same batch, so maybe it isn't a very good idea, as I
suspected.

BTW, Jack, I know you sell matched pairs of the Hitachis, so now
you don't have to tell that yourself and get flamed for it.
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Old 17th June 2003, 01:07 PM   #6
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i sell matched everything, ties and socks
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Old 17th June 2003, 02:57 PM   #7
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I bet you dont sell a matched pair of socks like this Jack!!!hahaha

and the amp below has 12 fets per channel and that was after going through ALOT...but I was buying them by the thousands at the time..LOL

Mods feel free to delete this...my Irish humour gets carried away

DIRT®
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Old 17th June 2003, 02:59 PM   #8
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Quote:
Originally posted by JOE DIRT®
I bet you dont sell a matched pair of socks like this Jack!!!hahaha
But I wanted a matched complementary pair.
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Old 17th June 2003, 03:13 PM   #9
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on a serious note Christer...its better to buy matched pairs because you will end up with a large amount of fets that are no good to you

DIRT®
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Old 17th June 2003, 03:20 PM   #10
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I've found 2N3055's with hfe's from 60 - 200 all having the same batch number....

I went trough this batch of 100 and ended with 7 having the same hfe-reading.

-------------------------------------------------
If you want good match, go for Japanese transistors in the same gain group. Much easier to match, even dynamically and in complementary pairs.

US makers seem to sell the matched ones to trade customers first, then the remainder for retail.
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