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Old 20th December 2003, 01:52 AM   #1
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Default transistor matching?

one of my multimeters has a hfe testing feature. so I decided to run a few 2n5551 by it. Well, I did about twenty of them, hfe came to be anywhere between 95 - 102. surprisingly close. the manual said the hfe is measured at 1ma so that's pretty realistic.

has anyone tried this? if this is true, do we really need transistor matching?
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Old 20th December 2003, 03:04 AM   #2
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Default Re: transistor matching?

Quote:
Originally posted by millwood

I think that if the matching is repeatable it is okay as a start. The best matching IMHO is in the circuit when possible. That way they see exact voltage and current.
If you match equal reading parts, put them in and see if they match in the actual circuit. This is not possible in every case. Sometimes you cannot tell if they are matching or not.

George



one of my multimeters has a hfe testing feature. so I decided to run a few 2n5551 by it. Well, I did about twenty of them, hfe came to be anywhere between 95 - 102. surprisingly close. the manual said the hfe is measured at 1ma so that's pretty realistic.

has anyone tried this? if this is true, do we really need transistor matching?
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Old 20th December 2003, 03:15 AM   #3
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i buy these in the "ammo box" style -- 2k at a crack -- you would think that the hfe would be tighter (ie. a lower std deviation) when you buy them this way or on a reel -- many of the parameters will change just by handling the devices !

when you batch the transistors together, you shouldn't be surprised that hfe and Vbe are often lumped together -- just as with HEXFETs or Lateral MOSFETS in which Vgs and transconductance wind up overlapping on a histogram. As I said in the previous paragraph, however, temperature does have an effect on the measurements.
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Old 20th December 2003, 03:30 AM   #4
sam9 is offline sam9  United States
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Same experience -- sort of.

If I buy a dozen or so from Digikey the lot numbers printed on them are often the same and the hfe measurement range is pretty small. BUT, if I go down to the local surplus shop where one literally scoops them out of a bin (I'm not making this up), you need two dozen to find a single match.

Conclusion, yes you need to match them because you CANNOT assume that a bunch of them will always measure in a narrow range. However, if you know you are going to use them in current mirrors or differential amps, buy a bunch of them from some source where they are likely to all come from the same lot. Then match up the hfe, that may also help narrow the range of parameters you can't match.
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Old 20th December 2003, 09:37 AM   #5
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millwood,

yes, we do need matching but it depends on the circuit.

Last I had to replace 4 Transistors of 2 differential amps, one pnp, the other npn.
These dual diff amp took besides other things, both care of the feedback, hence the output offset of the amplifier.
Selecting the transistors with my multimeter HFE didn't work out:
i had selected 4 transistors with the same hfe, but in the circuit it didn't work, large output offset.

So i made a test jig, a copy of the diff amp, and selected again for equal collector voltage(=equal collector current with the same resistor load).

This turned out to work fine.

Dick.
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Old 20th December 2003, 10:03 AM   #6
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Question Matching

Quote:
Originally posted by djmiddelkoop
millwood,

yes, we do need matching but it depends on the circuit.

Last I had to replace 4 Transistors of 2 differential amps, one pnp, the other npn.
These dual diff amp took besides other things, both care of the feedback, hence the output offset of the amplifier.
Selecting the transistors with my multimeter HFE didn't work out:
i had selected 4 transistors with the same hfe, but in the circuit it didn't work, large output offset.

So i made a test jig, a copy of the diff amp, and selected again for equal collector voltage(=equal collector current with the same resistor load).

This turned out to work fine.

Dick.
Hello Dick,
Not my experience. I matched all four input transistors (two differential amps with PNP and NPN’s) of my Ampzilla and it worked out great. Sound was MUCH better. There was also an article in the Audio Amateur looong ago about the matching issue. Yes they builded a rig. If you don't have a full complementary design I believe it is easier and better using a dual monolithic pair like 2SC3381, MAT02, LM394 to name a few. Right Hugh?
The differential pair works by virtue of the equality of the two halves. See also:
http://db.audioasylum.com/cgi/m.pl?f...+john&session=
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Old 20th December 2003, 10:34 AM   #7
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Goedemiddag Elso,

mayby there is something wrong with my multimeter HFE measurement ?

Yes, it was a Ampzilla circuit look-a-like, a Metaxas amp.

Using a dual monolithic pair is indeed much better, as you say :
the differential pair works by virtue of the equality of the two halves.

thanks for your reply,
Dick.
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Old 20th December 2003, 10:56 AM   #8
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Smile Matching

Quote:
Originally posted by djmiddelkoop
Goedemiddag Elso,

mayby there is something wrong with my multimeter HFE measurement ?

Yes, it was a Ampzilla circuit look-a-like, a Metaxas amp.

Using a dual monolithic pair is indeed much better, as you say :
the differential pair works by virtue of the equality of the two halves.

thanks for your reply,
Dick.
Hallo/hello Dick,
Back in time when I did the matching I did it with an analog Pantec transistor tester. Later I got that fancy multimeter with hFE tester. I got a totally different value on the latter meter but two transitors that proved equal on the Pantec were very close on the multimeter too. Beware the reading depends on the battery voltage. A fresh battery gives higher readings.
BTW do you have a copy of the Metaxas circuit? (I hardly dare to ask)

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Old 20th December 2003, 04:41 PM   #9
jcx is offline jcx  United States
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Vbe matching is often more important than Hfe in diff pairs and current mirrors, although emitter degeneration can reduce sensitivity to mismatch

Pease warns that digital noise from multimeters can confuse these measurements due to noise rectification, he put C across his meter and a used series R to reduce noise rectification in his diode slope measurements at the end of his book

any idea on how much Early Voltage varies? - again circuit techniques such as cascode/super-pair can help
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