MOSFET calculation help.

millwood said:

jfets in general and jfet switchers in particular have lower gm so the error in omitting 1/gm is bigger.

(vertical) mosfets in general have high gm (from low single digit s to low double digit s), measured at hundreds of ma and up, thanks to their parrallel construction: a large v-mosfet is essential a combination of hundreds of small v-mosfet.

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I already noted that high current rated FETs have high gm, and that taking Av to be Rd/Rs is appropriate under that circumstance.

millwood said:

some numbers, all measured at Id=0.5ma:

mosfets:
IRFP140: gm=100ms, 1/gm=10ohm;
IRF510: gm=75ms, 1/gm=13ohm;
BS170: gm=6.1ms, 1/gm=165ohm.

jfet switcher:
J112: gm=3.9ms, 1/gm=259ohm;

with a RS=3.3kohm, > 20x that of the highest 1/gm, the error from omitting 1/gm is minimal.

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We all agree that high current FETs have high gm, and that if gm is large, the gain of the OP's circuit is given with good accuracy by Rd/Rs.

millwood said:

kind of like we typically assume Vbe is constant in varying Ic for bjts, people typically do NOT factor in Vgs changes in gain calculation for mosfets.

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Then why did you give a formula "Av = delta Vd / delta Vgs", which explicitly factors in Vgs changes?

For BJTs gm is normally so high (38500 μS per mA of collector current) that a gain formula involving only load resistors (neglecting gm in other words) is appropriate. But, if a BJT is operated with very low Ic ("starved" mode), then gm must be taken into account.

millwood said:

and this example shows you why.

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People typically would NOT use high current FETs in the circuit under discussion in this thread; the drain current is only .5 mA after all. They would use something like the 3N157 I cited earlier, with a gm as low as 580 μS. Then 1/gm = 1724Ω, and omitting gm from the gain formula results in a substantial error.