carlgrace
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Aaron,
You're spot on, mate.
It is a very "academic" result, let me put it that way. You're exactly right that the noise is reduced by reducing rds, but only to the extent you can get the lost gain back by increase Rload without bound. Also, you're correct about the input match. The 1/gm rule-of-thumb for input resistance is only accurate if rds is big. When rds is the same order of magnitude as 1/gm, the actual input match is closer to 1/gm in parallel with rds, which means the power consumption is going to shoot the moon here. Also, the paper ignores a very practical consideration, linearity. It is quite impossible to increase Rload without bound because with reduced rds, rds with dominate the output resistance. When rds is small, its value changes significantly with output swing. A changing output impedance equals distortion, which is not a good thing.
There is a reason people don't design common-gate LNAs like this.
Regards, Carl
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