NMOS IV characteristic completely different with 2 similar setup

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kintaro

Joined Jun 1, 2022
7
I am plotting the IV characteristics of the NMOS of 2 similar setups.

Here I sweep the Vg from -13 to 13V and vdd from 0 to 50.

note that the only difference is that vdd is positive for one but negative for the other. both grounded at the other end.

But why I am getting 2 different IV characteristic results? should't they be the same? Shouldn't that Id curve should be only related to difference in drain and source voltages when nmos is turned on with same Vg?
 

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