Plot Device Operating Point Parameters

[jansto-noob]

Hi,

I would like to perform a DC analysis and plot operating point parameters of an NMOS. So far I have achieved to get them in a table, but the table is numbered in binary and I cannot plot the results in a cartesian graph. I am totally new to this tool.

Thanks in advance to anyone helping out.

[cmat21]

Hi, I can't find the command used for the DC sweep analysis statement in the Nutmeg script. The basic syntax is dc <srcnam> <Vstart> <Vstop> <Vinc>. I don't think you need an additional sweep parameter block.

[jansto-noob]

Thx a lot for your answer! If I understand you correctly, you suggest the setup below. I get no Errors, but there is no data.

Using ngspice directly, without Qucs-S, I have achieved plotting gm for a DC sweep using this ngspice code:

* MOSFET operating point (Ngspice 45)

.INCLUDE "C:\Users\cmosedu_models.txt"

* Supply voltages
VDD VDD 0 1
VG VG 0 1

* MOSFET instance
M1 VDD VG 0 0 N_50n W=1e-6 L=50e-9

* DC sweep for input voltage 
.DC VG 0 1 0.1

* Control block
.CONTROL
  SAVE ALL @M1[GM]
  RUN
  * Save the operating point parameters for each step
  
  * Plot all parameters
  PLOT @M1[GM] 
.ENDC

Since I also couldn't get a DC Parameter Sweep of my MOSFET width or length running (probably needs some exhaustive Spice code, since ngspice manual seems to indicate, that there is no .step directive anymore, but several lines of code including a loop have to be implemented) I will probably move to a different tool, neither Qucs-S nor ngspice.

I am basically looking for a tool for eductaion with a nice GUI, which lets users sweep transistor parameters and plot the derived operating point parameters (gm, vth, vdsat, etc.) :(

[tomhajjar]

Always best to post your project.

Some plotting problems are caused by choosing the wrong vector to plot.

Here are some projects done by @dwarning

qucs - S.zip

[jansto-noob]

I only just saw your answer. Thanks a lot! I will definitely take a closer look and adjust this for my intend.

Obviously, I haven't got the hang of the ngspice nutmeg syntax yet. Unfortunately, it seems to be a little overkill for the average user like me. I didn't find any (easily) accessible examples or introduction to the syntax in the time frame I set myself for this project. Sweeping MOSFET W and L or looking at OP parameters is quite common in analog IC design, so for this purpose it would be great to readily document this approach.

In the meantime, I had solved my problem by just writing a python script, which performed simple ngspice simulations in a loop and gathered the op parameters for postprocessing in python. That seemed a lot easier to me than figuring out the, for me, unintuitive ngspice syntax.

[tomhajjar]

https://github.com/ra3xdh/qucs_s/discussions?discussions_q=is%3Aopen+label%3Aexamples

[jansto-noob]

Thx. I don't get, why in @dwarning example he is using operators like set = "$&points" etc. Is there a good source explaining this syntax?

I have found out, that both these versions are equivalent, while the second looks much nicer.

[tomhajjar]

For more detailed info on Nutmeg scripting for ngspice, you can ask over at ngspice Discussion.

https://sourceforge.net/p/ngspice/discussion/

@dwarning (Dietmar Warning) maintains a Verilog-A repository that I grab osdi files from. He also posts on the ngspice and Qucs-S sites.

https://github.com/dwarning/VA-Models