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Old 2nd October 2012, 11:43 AM   #1
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Default F5 output MOSFETS matched or not?

Sorry guys, I'm sure most of you regulars here are thinking "oh no! another question on this, isn't there enough info here?" but please, I have searched and read and cannot get a straight answer.

Q: Does the output MOSFETS IRFP240/IRFP9240 have to be complementary matched (N to P)?

Mr Pass has articles mentioning the inputs should be matched, and the outputs should be matched if an amp has N's in parallel and P's in parallel. The F5 output is one complementary pair only as you know.

I am sure in a perfect world they would be matched, but what can I expect if they are not?

Thanks
Willem
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Old 2nd October 2012, 11:47 AM   #2
Zen Mod is offline Zen Mod  Serbia
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just chill

make it and enjoy
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Old 2nd October 2012, 11:53 AM   #3
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:-) thanks but I am finding it hard to chill if I don't know or understand why.
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Old 2nd October 2012, 12:36 PM   #4
Itsmee is offline Itsmee  England
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If using a single pair, the N & P do not need to be matched.
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Old 2nd October 2012, 12:45 PM   #5
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Thank you Itsmee, that is the definitive answer I needed. I have also received a pm with a response where the person measured the Vgs on the P at 3.82V and the Vgs on the N at 4.30 and the outcome was a THD+N of 0.0022% at 1W. So I guess it need not be matched.
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Old 2nd October 2012, 12:59 PM   #6
Zen Mod is offline Zen Mod  Serbia
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didn't I told you to chill ?

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Old 2nd October 2012, 01:55 PM   #7
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yes you did thanks ZM. This is me chilling....look how cool I am

Last edited by valvesound; 2nd October 2012 at 01:59 PM.
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Old 2nd October 2012, 10:13 PM   #8
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I just responded to a fellow who asked the question via email -- I just built a couple more F5's and the last batch were F5-R1's -- the Vgs of the opposing pairs were 4.30 and 3.82 -- with the 200 ohm trim-pot and experimenting with the output source resistors i was able to get the THD% down to 0.0022@1W 1Khz. I think that the range which NP described was 0.001% to 0.004%

fwiw, i start with source resistors of 0.68 ohms and parallel in values to determine the optimal value -- this pair of amps used 0.68 ohms on one leg and 0.34 ohms on the other.
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Old 3rd October 2012, 05:56 AM   #9
h_a is offline h_a  Europe
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Due to the very different transconductances of the IRFP240 and IRFP9240 (and most other vertical fets) it is even in theory pretty pointless to attempt N-to-P matchings.

These complimentary fets are never the same, no matter if VGS is matched or not.

VGS matching in the output stage is anyway only done to ensure equal turn-on and not for any sort of distortion cancellation. For current sharing there are the source resistors.

Just keep reading Nelson's articles - everything practically needed is in there
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Old 3rd October 2012, 06:17 AM   #10
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Thanks guys, info much appreciated!
Hannes if I knew about you selling F5 kits it would have made my life much simpler and cheaper as I paid a lot for matched jfets.

Willem
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