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design folded cascode op amp using gm/id method

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pbs681

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gm/id methodology

Hi all,

Currently i m designing folded cascode amp by using gm/id method. My problem is that, i don't see how are we going to bias drain voltage by using this method. This method only describe the gate biasing (Inversion region) and W/L ratio.

Also, the paper mentioned that only EKV model has modelled the gm/id behaviour properly. Does it mean that if I use BSIM3 model, I cannot use this method to design my circuit?

Thanks
 

folded cascode op

So you are rigth, the gm/Id methode in that paper does not take VDS in account, so far I can remember it use the Transistor in Diod conection to produce the Gm/Id plots. I'm using the gm/Id methode in Bsim3 and it still works fine. To solve the vds problem, creat plots with different VDS Voltages, that's what I'm doing
 

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    pbs681

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gm-id methodology

edge_tv said:
So you are rigth, the gm/Id methode in that paper does not take VDS in account, so far I can remember it use the Transistor in Diod conection to produce the Gm/Id plots. I'm using the gm/Id methode in Bsim3 and it still works fine. To solve the vds problem, creat plots with different VDS Voltages, that's what I'm doing
Click to expand...

edge_tv, thanks a lot... but in diode connection, the drain is always shorted to gate. How do you create plots with different Vds voltage?

I have another question. I have problem to plot gm/id vs Id/(W/L). I am using cadence environment to run my simulation. After simulation completed, I will plot gm by take derivative of Id against Vg. Then I will divide the plot with Id to get the Y axis. Can somebody tell me how to set the X-axis to Id/(W/L) in cadence environment?
 

gm/id methodik

Set the VDS Voltage with a Voltage source to tha value your are interestet (like 300mV), and than sweep Vgs. I would recomment, not using Vg/Id (it is not correct, even (Vgs-Vth)/Id is not correct). You can plot directly gm, becaus it is an output paramter in spectre. But this is a littel bit complicated, the esays way is to use ocean. (Save the Drain current of your divecs, and than save the simulation to an ocean skript. Than you can see the correct name of your Transistor, change Id to gm. Than save the data to an txt.file and than you can bring it to Matlab ) The ocean reference is a good help
 

gm/id method

>>I have problem to plot gm/id vs Id/(W/L).
video gm/Id vs Vov similarly and gm/id vs Id/(W/L)
 

how to generate graphs + gm/id method + cadence

in gm/Id method use following approximation for VDS
VDS = 2 * inverse of (Gm/Id);

Added after 9 minutes:

To plot Gm/Id Vs (Id/(W/L)), it is better to use commands instead of just using buttons on the screen. u can make plot of gm, Id using commands. Once u got Gm and Id plot. Divide it, u will get Gm/Id plot. Then Id/(W/L) plot can be obtained by deviding Id by W/L. One u obtained both plots. Paste one plot on another. So that there will be two plots on one graph sheet. Select one plot, then by right clicking on it, select option 'Set as X axis'. This will give Gm/Id Vs Id/(W/L) plot.
 
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