aaron_do
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Hi all,
if you take a transistor is saturation, the gm is approximated as,
2IDS/(Vgs-VT)
and in subthreshold it is approximately
IDS/nVt
For the input referred channel noise in the saturation region we have,
Vn2=4kTγ/gm = 2kTγ(Vgs-VT)/IDS
and in subthreshold,
Vn2=2kTn/gm = 2kT(n2)Vt/IDS
From these equations, it seems that subthreshold biasing offers better gm per IDS, better noise per IDS, and comparable noise per gm. Only drawback is subthreshold has higher input capacitance per gm (resulting in lower fT). Just want to double check if these manipulations seem strange to anyone?
BTW i understand the difficulty in defining VT, however, i'm trying to confirm that weak-inversion (in between subthreshold and saturation) is basically the best biasing point. I also understand that linearity is poorer in weak-inversion/subthreshold...
thanks, Aaron
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