Gm is Operation Point parameter. U can get one value of Gm for given Vgs value when u do DC Operation Point Analysis. Such way u can't sweep Vgs, so using CADENCE/SPECTRE u can't plot gm-vs-vgs drawing. If only u've got familiar with OCEAN/SKILL.
Another way to plot gm-vs-vgs is to do AC sweep. U need to place 2 vdc sources with dc voltage VGS, one between gate and source, next one between drain and source. Set ac voltage of the first source equals 1V. Set AC sweep analysis relatively Vgs at certain frequency. Gm value will be Gm=Iac(drain)/1V => Gm=Iac(drain) [A/V]. So u can plot gm-vs-vgs.
The necessity of gm-vs-vgs drawing looks stranger, i hope u know what u need.
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