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Old 25th November 2005, 01:25 PM   #1
Skorpio is offline Skorpio  Denmark
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Default MOS-FET matching

Is it possible to match the source resistors instead of the output MOS-FETs when building an Aleph 3?

The resistors are typically +/-5%. If you in you test circuit test for at certain current (2A) and adjust the resistor until this is reach....? Starting with 1 ohms and paralleling those with small 10 ohms until current is equal....

Drawbacks?
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Old 25th November 2005, 01:53 PM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
I asked this question many months ago, but it fell on deaf ears.

I matched and managed to get 8 quads out of my 16 pairs.
But one quad had one errant FET. I built it up with unmatched source resistors and sure enough the source currents appear to be different. But I suspect most of the voltage error is due to the resistor variation and not Vgs variation. I cannot hear a problem.

I have learned my lesson. I am currently matching the emitter resistors for the BJT amps that fill in the remaining gaps in the arsenel of bi-amped speakers for the 5.1 surround sound that is still under construction.

My recommendation;- match the source resistors and match the Vgs at the operating quiescent current.
Then measure the variation. Trimming, if you consider it necessary, can follow later but the maths is more complicated if you then take account of the different source resistance.

Good luck and let's hope someone enlightens us.
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Old 25th November 2005, 02:08 PM   #3
Skorpio is offline Skorpio  Denmark
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My idea is to match the FET and resistor as a pair at the actual application current.

Starting with ex. 0.68 ohm in a test circuit, and then adjust the resistor by parallel resistance until the needed current is there. The mount the complete set as it is....
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Old 25th November 2005, 02:37 PM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
I think your proposal will work.
But how do you know what the source currents are after you have completed the project? All the source resistors will be different and all the different voltages may mislead you into thinking the source currents, as installed, are also different.
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Old 25th November 2005, 03:09 PM   #5
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Hi

From my experience, a normal matching procedure (within 10mV and matched at the operating current and at another one to get a transcondutance) will raise no particular problems, even if the source resistors are not that matched.

The best and cheapest solution I've found to "match" the source resistors is to use paralleled small 0.6W metal film 1% resistors, to achieve the desired resistance and wattage. The resulting power resistors will be statistically matched. More soldering, but worth the effort Not any problem in my AX that's been continuously on (or in standby @ Ibias/4) for two years now...

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Old 25th November 2005, 03:15 PM   #6
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Quote:
Originally posted by CheffDeGaar
The best and cheapest solution I've found to "match" the source resistors is to use paralleled small 0.6W metal film 1% resistors, to achieve the desired resistance and wattage. The resulting power resistors will be statistically matched.
And you will also get a lower inductance with this solution, which is good. I think it was Upupa Epops who said he always do so in his power amps to get a lower inductance.
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Old 25th November 2005, 03:26 PM   #7
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Quote:
Originally posted by Christer
I think it was Upupa Epops who said he always do so in his power amps to get a lower inductance.
Rendons donc à César ce qui est à César

And, yes this is an appreciable collateral effect

Skorpio :
As a side note, I may add that the matching in an Aleph is less critical than in an X one. Match the upper fets together, the lower ones together, use a lot of small resistors, and you're done
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Old 25th November 2005, 03:45 PM   #8
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Paralleling of " standard " resistors have several advantages : accuracy, reliability, low iductance, low price, good Thc and only one disadvantage : higher labouriousity... And one advantage for me : nice design of PCB
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Old 25th November 2005, 07:42 PM   #9
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Quote:
Originally posted by CheffDeGaar
Rendons donc à César ce qui est à César
As far as i've gathered Pavel is more round than tall, walks around in a bathrobe all day, with a string of bayleaves on his head.
So you are dead on ! (Et tu, Brutus !)
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Old 28th November 2005, 06:27 PM   #10
Skorpio is offline Skorpio  Denmark
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Quote:
Originally posted by AndrewT
Hi,
I think your proposal will work.
But how do you know what the source currents are after you have completed the project? All the source resistors will be different and all the different voltages may mislead you into thinking the source currents, as installed, are also different.
I will monitor the current in each leg with a 0.1% resistor temporaly mounted in series with the drain...
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