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I'm using LTspice on Mac, and trying to only use netlists, no GUI. I'm trying to follow berkeley ee240. They use a model that I don't think is available online so I'm just using default mosfet models.

So I need to sweep length, and plot the resulting Vth parameter. I cannot plot V(vout) without manually selecting the trace, it will not show up using the '.plot DC V(vout)' command.

I cannot access the Vth parameter. I have tried various combinations of lv9 and vth but can't get anything to appear on the plot. V(Vout) only appears on the plot if it is selected from the GUI. Vth is not an option in the GUI. Netlist:

.model nm1 nmos(level=14)
.param length=1u

* netlist

Vgs vgs 0 1.2
Vds vds 0 1.2

M1 vout vgs gnd gnd nm1 L={length} W=1e-6
R1 vds vout 1k

* analysis
.op
.DC lin param length 90nm 500nm 50nm
.plot DC V(vout)

.end
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1 Answer 1

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I'm not familiar with LTSpice, I use a different simulator (Cadence Spectre) but there I can also not plot Vt directly. The reason for this is that Vt is an internal model parameter. In Spectre I can get a value for Vt at a certain operating point, for that I need to do a DC operating point simulation so not a DC sweep ! If I do multiple of these operating point simulations I could draw a curve from that.

A simpler way is to emulate the way that you would measure Vt on a bench. What you do is force a very small current (1 uA) for Id and measure Vgs. Since Id is very small Vgs will be almost equal to Vt. A simple circuit for this is here:

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Thank you for your answer. However, the work for the course I'm trying to follow really requires the access of internal parameters (not just vth which I used as an example here). They use Hspice, but the instructor states other simulators should work also. I am aware of the other ways to get Vth and also I can see an indivual Vth for an operating point in the log, but I really need to be able to plot the value vs the sweep. \$\endgroup\$
    – Gswffye
    Commented Sep 14, 2015 at 18:21
  • \$\begingroup\$ OK, then use the first method I described: do a parametric sweep over the parameter that you want on the X-axis, for example the length, just like in your DC sweep. In essence that's the same simulation you have described in your netlist only for each point in the sweep an operating point is calculated. This does not happen when you do a standard DC sweep. I think you need to use the .OP statement for the analysis and the .PARAM statement for sweeping the variable, see web.ece.ucdavis.edu/~hurst/EEC210/hspice_quick_reference.pdf It has been almost 18 years since I used Hspice ;-) \$\endgroup\$ Commented Sep 14, 2015 at 18:46

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