Electronic – How to determine mosfet capacitances (Cgs, Cds, Cgd, …) in LTSPICE

designltspicemosfetspice

I am simulating a 2 stage miller opamp in LTSspice. I'm using the standard models provided by my uni. I can simulate the operating point, but in the operating point data, all capacitances are 0. I assume this is because LTSpice just sets all capacitances as open in the operating point analysis but I'm not sure. However, since I need to have an idea about these capacitances, I was wondering how I can get to them after doing the simulation. In the uni labs we use a tool (JMOSCal, designed by someone in our department a few years ago) that will give all these values, but it requires hSpice, which I don't have access to at home.

LTSpice clearly does calculate these values at some point, since my opamp doesn't have infinite bandwidth.

EDIT:
I would like to mention the following: The reason I'm using LTSpice is because it's the best I have at home. I don't have access to the fancy Cadence or Mentor tools. LT spice is a good simulator in terms of accuracy. However, it's license does not permit the use of technology models from fabs. Predictive technology models do not fall under this clause, and hence can be used.

I would also like to point out that this is not a standard simple model, but a high acuraccy and advanced predictive technology model. We are talking about 200+ parameters per device type. They are available from this website: Arizona State PTM models page The one I am using is the "PTM High performance 45nm metal gate, High-K, strained-silicon".

I need to do high-accuracy simulations. The accuracy I can achieve with hand-calculations is not enough. Even if I could manually calculate them to any decent level of accuracy, I would not be able to just "add them" – LTSpice clearly does model them internally, because I can see poles and zeros that would not be there if no capacitance were modeled.

How do I find these values?

Best Answer

Though it has been more than one year since you ask this question, I think it might be helpful to leave a comment here in case someone has the same problem (like myself in twenty minutes ago). I find these values could be find in the log files named "yourschematicname.log" in the folder storing your schematics. This holds true for both BJT and CMOS. And I am sure that they vary when you modify the DC bias point.

Parasitic capacitances of BJT:

Parasitic capacitances of NMOS:

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Electronic – LTSpice, MAC97A8 TRIAC model, Vgs error

From the LTSPice IV manual:

X. Subcircuit 
Syntax: Xxxx n1 n2 n3... <subckt name> 
[<parameter>=<expression>] 
Subcircuits allow circuitry to be defined and stored in a 
library for later retrieval by name. Below is an example of 
defining and calling a voltage divider and invoking it in a 
circuit. 
* calling a subcircuit 
* 
* This is the circuit 
X1 in out 0 divider top=9K bot=1K 
V1 in 0 pulse(0 1 0 .5m .5m 0 1m) 
* This is the subcircuit 
.subckt divider A B C 
R1 A B {top} 
R2 B C {bot} 
.ends divider 
.tran 3m 
.end

Notice that params: never appears in the LTSpice syntax for a subcircuit call.

I am guessing that including this token in your X card has confused LTSpice about how the subcircuit should be called.

Electronic – Help Converting a PSPICE MOSFET .LIB to an LTSPICE .SUB

If I simply save that PSpice file to a ntk3139p.lib file and import it in LTspice, it all works fine:

The Rds(on) looks in line with the datasheet for that part.

What you need to keep in mind is to change the PMOS statement line to X (because it's a subcircuit) and match the name to the subckt name declared in that lib. Ctrl-click to access the advanced properties page for the FET:

This is actually explained in detail at http://www.linear.com/solutions/1083

And if you actually want to import that model into LTspice so that you don't have to use an .include statement, what you need to do is

Copy the aforementioned ntk3139p.lib into LTspiceIV\lib\sub; this directory can [and does] contain both .lib and .sub files.
Create a ntk3139p.asy in LTspiceIV\lib\sym (or in one of its subfolders, in which case the component will show up in the corresponing category in the F2 select component dialog). This .asy file is initially a copy of pmos.asy that comes with LTspice in this case. Now you need to edit this ntk3139p.asy file either in a text editor or using LTspice itself (via Edit->Attributes or Ctrl+A) so that it reads:

Now you can add the new component, but since we're using X as type it automatically get labelled as an IC (U) rather than MOSFET as before. But we don't need an .inc line anymore for the simulation to work:

Honestly this procedure is usually not worth the hassle for me... and if you reinstall LTspice or load your schematic on a different machine, you have to do it all over again, never mind that it becomes less clear in the schematic what components you need[ed] extra libraries for. Furthermore, you can no longer change the MOSFET by right-clicking on it an picking a new model. If you try that with your custom asy file, you get:

Which for me is the most annoying part. So I don't recommend doing this import procedure for MOSFETS; I think it's only worth the hassle for ICs.

I honestly don't know exactly what the .sub files are restricted to contain in LTspice, but the ones that come with the program are all binary files containing LT's proprietary models, some of which also make use of LT's extensions like steady and so forth. I don't think the .sub binary format that LTspice uses is publicly documented anywhere.

Best Answer

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