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Old 3rd December 2002, 11:45 AM   #1
naula is offline naula  Finland
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Default Matching the MOSFETs

I'm going to build Aussieamplifiers 200W@8ohms symmetrical amp. Construction notes says MOSFETs are recommended to match. I understand why but I would like to know how big can the difference be in current sharing without matching. In other words what will happen if I don't match the output devices.

I'm going to build stereo power amp. So I have to buy eight 2SK1530 and eight 2sj201. Is it enough if I match with those eight devices, taking four as similar as possible ? I don't have money to buy extra FETs

Link to the schematic:
http://www.aussieamplifiers.com/sym-sch.htm
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Old 3rd December 2002, 12:38 PM   #2
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Naula,

it depends a bit on how (un)lucky you are If not very lucky (different batches) you can have differences of 20-50%. What you can do is build the thing and then switch fets between left and right channel (this is what I did with my crescendo) until you get a reasonable spread.

The other possibility is to measure Vgs for a given bias (see "matching" in numerous threads) for the 16 fets, match them as good as possible, put them in the circuit and then decide if you have to buy a few more.......

I think current differences of 20% (highest to lowest) are acceptable.

william
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Old 5th December 2002, 10:45 PM   #3
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Insofar as Anthony is using low values (.22 ohm) of
Source resistance, matching becomes more critical to
insure that the devices share current in such a powerful
circuit.

Best to get some extras and match them as close as
possible; I would suggest .1 volt Vgs variation or smaller.
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Old 6th December 2002, 02:33 AM   #4
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Default if you purchase a "sleeve"

of Toshiba, IRF or ST Micro devices, they usually match very closely. For Toshiba (lateral MOSFET's) there are 30 in a sleeve. For ST Micro there are 50.
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Old 6th December 2002, 07:47 AM   #5
UrSv is offline UrSv  Sweden
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You might also check my posts here:

Substitute Lateral Mosfets

/UrSv
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Old 6th December 2002, 01:50 PM   #6
djdan is offline djdan  Romania
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Lateral MOS-FET need maching ?
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Old 6th December 2002, 02:24 PM   #7
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Default The Vgs(t) can vary

by a few hundred millivolts
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