Fet Follower Regulator On Resistance?

[macka]

I am interested to know if anyone has tried mosfets other than the IRF610 and the 9610 for the simple Passlabs supply follower regulator featured in the preamps and Zen amps.

Looking at the fet on resistance specifications the above do not have particularly low values while other varieties like the 9540 have lower on resistance and 044 has very low values.

In cases where the shunted capacitor from the source to ground is not an large value say 220-1000 uf and the regulator is directly connected to the load, from my limited understanding mostfets with low on resistance would appear to offer a better performance .

I recall Neslon discussing this in the Zen Vers 3 article but that was specific to high current loads. Would there be an advantage for a pre amp or perhaps active crossover regulator.


Can anyone comment or verify this?

Macka

[mirlo]

... is what you should be looking at if you are trying to lower the output impedance of the regulator.

If the FET is turned on so hard that its on resistance comes into play, it is acting as a switch, not as a follower, and not performing any regulation.

When there is enough difference between input and output voltage so that the regulator is actually regulating, the output impedance is the reciprocal of the transconductance.

-- mirlo.

[GRollins]

I'm pressing my luck...Santa is coming down the chimney even as we speak...
mirlo is correct. When you're using a device in follower configuration, the Zout drops way, way down. Use most anything you've got on hand with sufficient voltage/current/wattage ratings. I needed a follower this afternoon and grabbed the first thing that came to hand (an IRF610, as it happened). That's not to say that I won't change the part once I flog a little more gain out of the phono circuit (less than .1dB deviation from RIAA...hot dawg!), but on the other hand, I may just leave it, you never know. Followers aren't all that big a problem. People act like they're mysterious, but I've only ever had problems with one follower, and that was a circuit where I was trying to pull off a clever stunt. They're pretty tame beasts, usually. Just bias the dickens out of the thing and put a heat sink on it. In other words, pretend it's an output device--which in a sense, it is--and run it hard. You'll be fine.
Oops...busted...the guy in the red suit just stepped onto the hearth...

Grey