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jarthel 26th June 2007 12:21 AM

the need for matched fets
Well if the fets are matched but the resistors (and possibly caps) aren't matched, why is there a need to matched fets?

non-matching resistors will have different voltages across it.

Maybe using matched fets somehow balances the whole circuit even if the resistors/caps aren't matched?

MRupp 26th June 2007 09:42 AM

Very simple math. Resistors have usually 1% tolerance, often 0.1 %, and this is the maximum (garanteed) tolerance. Some caps have 1% nominal tolerance caps but I measured caps with 5% nominal tolerance that actually had tolerances well below 1%.

Fets by comparison have much higher tolerances for some of their critical parameters, more like 10 % and for some parameters even more (Idss of jfets more than 100 %). Go figure.

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