gm simulation using spectre

[Julian18]

spectre gm simulation

Hi, there.
I am simulating using spectre and running into a small problem.
I sweep the bias voltage of a MOS and see the change in drain current, now i can get a graph of ID vs. Vg, take derivative of this graph to get gm vs. Vg. while i can also get gm vs. Vg directly in Result Browser, but the two results are different. What 's going on here?



Thanks

 

[Julian18]

use cadence to simulate a single CS MOS, plot gm vs. vg and d(Id)/d(vg) vs. vg, get different results (similar but different plots) , why?

Thanks

Added after 1 hours 21 minutes:

here is my simulation result.

 

[Julian18]

any ideas?

 

[yxo]

use cadence to simulate a single CS MOS, plot gm vs. vg and d(Id)/d(vg) vs. vg, get different results (similar but different plots) , why?

Thanks

Added after 1 hours 21 minutes:

here is my simulation result.

At the first, gm = 2*Id/(Vgs-Vt)
Try this equation