Hello all, I have been trying to sweep through the width of a transistor to see the effects if has o...

Thomas Figura

11/03/2022, 11:36 PM

Hello all, I have been trying to sweep through the width of a transistor to see the effects if has on Id, I read in the ngspice manual, section 17.5.58 that a reset command is needed after any alterparam command, but when I run this simulation I get the error below. I am just a little unsure of how to fix this as it says w is undefined, but I thought I defined W properly in the code section.

🌍 1

Stefan Schippers

11/04/2022, 1:51 PM

Use

alterparam W=$&w_act

Also add a .

options savecurrents

outside the .control block and a

save all

inside the control block, before the run statement

Stefan Schippers

11/04/2022, 2:18 PM

also remove the 'u' suffix from start_w, stop_w, delta_w, since sky130 scales all numbers by 1e-6. Also add a remzerovec before the write command to remove vectors with no values (I can't be more precise on this, ask ngspice developers why this is needed). Here the test schematic. Sending also the .sch file, save it as

test_sweep_mos_w.sch

The commands:

Copy code

* ngspice commands
.param W=1
.options savecurrents
.dc v2 0 1.8 0.01
.control
  let start_w = 1
  let stop_w = 90
  let delta_w = 5
  let w_act = start_w
  while w_act le stop_w
    alterparam W = $&w_act
    reset
    save all
    run
    remzerovec
    write test_sweep_mos_w.raw
    let w_act = w_act + delta_w
    set appendwrite
  end
.endc

Stefan Schippers

11/04/2022, 2:19 PM

test_sweep_mos_w.sch

Thomas Figura

11/04/2022, 2:39 PM

Thank you so much Stefan

👍 1

Stefan Schippers

11/04/2022, 2:45 PM

@Thomas Figura the above example is now part of the

xschem_sky130

test schematics. It is a good starting point if one needs to sweep a device parameter. Much better than starting from scratch, since as you have seen there are some quirks (the remzerovec, the $& prefix ...) I also have spent some time figuring out all these things some time ago.

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