"flipper" wrote in message
...
Anyone know if this is an 'inherent' relationship: that as Gfs
increases so does gate capacitance sort of correspondingly so we're
fighting a loosing battle trying to trade one for the other?

1 / (2*pi*R*C) is the cutoff frequency. Put in R = 1/Gfs and you get
something like fT. This isn't so much a physically realizable frequency as
it is a quality factor (tubes were similarly rated in terms of Gm vs. C).

1 mho is an awful lot of transconductance. At 170pF, that's 936MHz. The
equivalent transresistance is quite small (= 1 ohm), which suggests actual
resistances (like gate spreading resistance) or inductances (figure 5nH
typical source inductance) will dominate. (Ls is 29 ohms at that frequency,
way past cutoff, and Rg might be 1 ohm or so. Cg resonates with Ls at
173MHz, which would be series resonant with the input, but tweaked by drain
current, giving rise to funny phase shifts as seen in the s-parameters.)

Tim

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