LTSpice Help

[fourierpwn]

Hi,

I am a little confused with some basics in LTSpice.

I was looking at this photo of some MOSFETs and I don't understand what setting the value of VSD and VSG (5 and 0 respectively) does when you're sweeping them in the .dc command anyway.



i.e., for the ID_VSD curve, does setting VSD to 5V even do anything? The source is being swept in the .dc command from 0 to 5V in 1mV steps. 

Confirmation on this would be most appreciated 

[fourierpwn]

I probably should have done this before I made the thread, but I have investigated this myself and it appears that there is no difference between setting the VSD source as it is overwritten by the .dc command.

I still don't see the point in bothering to set the source to a specific value though...

[Ed.Kloonk]

I'm no spice expert.

But I see no node connected to GND.

[danadak]

There are 3 grounds, at top of schematic.


Regards, Dana.

[Ed.Kloonk]

There are 3 grounds, at top of schematic.


Regards, Dana.

Ah. Right. Gotcha.

 

[Jay_Diddy_B]

Hi,

I am a little confused with some basics in LTSpice.

I was looking at this photo of some MOSFETs and I don't understand what setting the value of VSD and VSG (5 and 0 respectively) does when you're sweeping them in the .dc command anyway.



i.e., for the ID_VSD curve, does setting VSD to 5V even do anything? The source is being swept in the .dc command from 0 to 5V in 1mV steps. 

Confirmation on this would be most appreciated 

In the first two models you have VSD is not over written by the .dc command. Only the VSG source is swept. VSD is kept constant at 5V. This sets the operating conditions for the MOSFET.

In the third model VSD is swept from 0-5V in 1mV steps, that is why you have 0-5V on the horizontal axis.
This is sweep of VSD is repeated 6 times with VSG being stepped from 0V to 5V in 1V steps. This is why there are multiple curves.

Does this help?

Regards,

Jay_Diddy_B

[Ian.M]

You set all the sources to the default values you want then override one or more sources with a .dc sweep or a .step command.   This makes it easy to keep settings constant between multiple analyses.  e.g. you'd put all three .dc commands on the same schematic.

LTSPICE only allows you to have a single analysis command active.  If multiple analysis commands are present, LTSPICE lets you pick one from a list when you run the analysis, and auto-comments all the other ones by replacing the . with a ;
Once that's happened, to change to a different one simply uncomment it by replacing the ; with . then when you run again its back on the pick-list.

N.B. the d() function can be used on a plottable expression in the waveform viewer to plot the derivative of that expression. e.g. to plot gm directly.

[fourierpwn]

Thanks for the help everyone.

Re: the d() function. Yep, I understood this. Quite a neat feature!