Hi Experts,

    I have done some spectre simulations (using MDL feature) on simple basic circuit as shown in figure attached ,
my intension is to find DC voltage gain and Gm of NMOS using Resistive load connected to POWER supply. when i compared the results say gm i.e by using buil in feature X1.M0:gm and actual calculated one i.e ( X1.Mo:ids/V(IN) )
by varying the V(IN) i.e Gate Voltage , i observed there is diference between these two values and i wanted to know what this built in gm stands for and the one explicitely i am calculating, similarly for 3 types of Av 's that i am calculating  below is the MDL deck

alias measurement DC_Exp {                                                                                

   run DC_op
export real GM = X1.M0:gm    
export real GM_cal = Idrive/VGS    
export real GM_cal_in = Idrive/V(IN)    
export real VGS = X1.M0:vgs    
export real VTH = X1.M0:vth    
export real IDS = X1.M0:ids    
export real Idrive = I(X1.R0)    
export real Av   = GM * 99.9999
export real Av_cal   = GM_cal * 99.9999
export real Av_volt   = V(OUT)/V(IN)

}

run DC_Exp
   
printing each above values at each step of variable input voltage

So let me know if i am wrong in doing above kind of measurement , since i am doing DC i have not done any Av = dVout/ dVin  kind results

And also teh attached figure shows the simulation graph between Vds Vs Ids , in my simulations the Ids not becoming constant but keep increasing w.r.t Vgs where as Vds is constant through out my experiment

It would be really great help if you can give me idea on how to actually calculate those above
 
Thanks & Best Regards
Kiran