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ridikas 8th April 2013 03:02 AM

MOSFET gate resistors, carbon?
Are there any benefits and is it preferred to use "carbon" MOSFET gate resistors?

ticknpop 8th April 2013 03:42 AM

You don't want wire wound , but I've used metal film and bulk metal foil without a problem over the years

Jay 8th April 2013 04:07 AM

On the input of Gainclone I have tried different resistors and I found Carbon (Allen Bradley) to be my favorite. I have compared also small value resistor in front of line level opamp, and I found I preferred a resistor instead of no resistor.

Then I consistently used Carbon AB for gate resistor (in Pass class-A amps) and I found they are suitable for the job (better than a few others that I had time to compare).

I cannot see theoretical reason to restrict my favorite resistor (e.g. the input current is too low that Johnson noise shouldn't be an issue) so I continue with my practice (I bought a lot of 220R Allen Bradley for this purpose). The best position for this kind of resistor imo.

Ian Finch 8th April 2013 04:44 AM

The only benefit is very low inductance but if the amplifier shows no sign of instability or parasitic oscillation, it's a moot point whether you go to any lengths here when the simple expedient of an RC Zobel across the Mosfets will better control any problems (see Bob Cordell's book and comments in the long thread). AFAIK, nobody needs to revisit carbon composition resistors in audio now, with their expensive and long, fault-prone history. Boutique parts resellers would like you to, however :rolleyes:.

ridikas 8th April 2013 04:50 AM

Inductance was going to be my next question. If low inductance is desirable in a MOSFET gate resistor, then what about carbon film? Do carbon film resistors have a lower inductance than metal film, or do they still cut the same helical groove and are identical to metal film inductance wise?

Ian Finch 8th April 2013 05:02 AM

There is little or no difference in the principle of trimming of MO, MF or CF resistors so the inductance is comparable but the question remains as to whether it is detrimental anyway.
A general case may be that removing a small proportion of the total gate lead inductance makes no difference but there is a more effective solution using standard parts which obviates this.

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