Let's try using LTspice - Dot Commands (4): DC Analysis (.dc)

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This article is the fourth in a series on the dot commands available in SPICE. In this article, we will explain the commonly used DC analysis (.dc command).

If you answer the final survey, you will be able to download supplementary materials for the transient analysis (.dc) command in LTspice Help. The Help materials are netlist-based, but if you would like to obtain explanatory materials that actually perform simulations using the GUI, as in this article, please help us by answering the survey.

DC analysis ( .dc command)

DC analysis using the .dc command is useful for checking electrical characteristics such as the static characteristics of transistors and FETs, which can be measured with a parameter analyzer.

Normally, when performing DC analysis, you would set the analysis mode using the GUI on LTspice, but here we will explain the netlist notation after setting it up.
There are two types of LTspice syntax. < > is a required item when setting. [ ] is optional.

① .dc [<oct, dec, lin>] <srcnam> <start> <stop> <incr>

<srcnam> specifies the name of the voltage or current source to sweep. The value to sweep can be specified from <start> to <stop>. The increment of the value can be set with <incr>. It can be defined according to the table below using the keywords oct, dec, and lin. If omitted, lin is assumed. If the lin step interval is not specified, the interval between <start> and <stop> will automatically be 10 points plotted.

Type of sweep	Nsteps
oct	Analysis points per Octave (doubled)
dec	Analysis points per decade
lin	Number of analysis points between StartFreq and EndFreq

② .dc <srcnam> list <value1> [<value2> [<value3> [ ... ]]]

<srcnam> specifies the name of the voltage or current source to sweep. The values in the list specified by Value1, Value2, Value3... are assigned to the voltage or current source specified by srcnam.

③　.dc <srcnam> file <Filename>

The voltage or current sources will be assigned values read from the specified file. This file can contain comments, just like a normal netlist. Numbers must be separated by spaces or commas.

The Help explanatory materials provided by Macnica include the following DC analysis examples. If you would like to receive the explanatory materials, please fill out the survey.

DC analysis example

1. Check the .dc <sweep1> [<sweep2>][<oct, dec, lin>] <srcnam> <start> <stop> <incr> command
2. If Increment is omitted
3. Type of Sweep set to Octave and Decade
4. When Type of Sweep is set to List
5. Check the .dc <srcnam> file <Filename> command
6. Checking the BSIM simulation written in the netlist

Performing a DC Analysis

Here, we will introduce the section "Checking BSIM Simulation Written in a Netlist" from the explanatory materials. In LTspice Help, the contents written in the netlist are converted into a circuit diagram and a simulation is performed. First, draw a circuit diagram like the one shown in Figure 1.

Figure 1: FET static characteristics verification circuit

Copy the following from the LTspice help:
　
.dc Vds 3.5 0 -0.05 Vgs 0 3.5 0.5
.model nbsim NMOS Level=8

Select Edit → Spice Directive from the toolbar. This will open Edit Text on the Schematic, so paste the copied content and click OK. You can also check the contents set in the .dc by right-clicking on ".dc Vds 3.5 0 -0.05 Vgs 0 3.5 0.5".

Finally, we named the voltage source V1 Vgs and V2 Vds and set the voltage to 1 V. We also named the FET Nbism.

On the toolbarClick RUN to run the simulation.

The waveform window will open. Right-click on the waveform window and select Add Trace.

The Add Traces to Plot dialog box will open, so select I(Vds) from the list to display the FET drain-source current (static characteristics).

The static characteristics of the FET are displayed.

Download supplementary materials for DC analysis (.dc) commands

The explanation of the transient analysis (.dc) command in the LTspice Help is netlist-based, but if you fill out the survey, you will be able to download a GUI-based version of the explanation, similar to this article. After completing the survey, we will send you a URL to download the supplementary material by email.

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