Most frequently asked questions for beginners
Q: What are the different LTspice file types?
Schematic: name.asc the drawing with your circuit (a text file) Symbol: name.asy symbols for the schematic (a text file) Logfile: name.log infos and results from .four, .measure, .op (text file viewable from within LTspice) Netlist: name.net intermediate text file netlist (usable by other SPICE engines) Result: name.raw binary output data file (text format may be optionally specified) Result: name.fft binary result of an FFT Plot settings: name.plt text file used to save and restore Waveform Viewer plot settings Model file: abc.xyz text file containing model(s), any file name Symbol: name.cir frequently used extension for an external netlist input file (text)
Q: How do you copy and paste between schematics?
A: Click on "Copy" in the tool bar. Select what you want to copy. Make the other schematic active and then click on "paste" in the tool bar.
Q: How do I copy and paste between symbols in the symbol editor?
A: It is not possible to use copy and paste in the symbol editor. Symbol files are ASCII-text. Merge the text as described in message 7201 in the Yahoo Group. 
Q: How can I add intrinsic device models (BJTs, FETs, etc.) to LTspice?
A: Please take a look to the many examples in the section "Files > Lib".
Q: How can I add subcircuits to LTspice?
A: You WILL find many answers when you search the messages for words like library, symbol or FAQ.
Please read first the programs help: Help ->Schematic Capture -> Editing Components -> Creating New Symbols Help -> Help Topics ->FAQs -> Third party models Help -> Help Topics ->FAQs -> Mosfet You will find also help in the linked documents from our LTspice Yahoo group. Links Links > Spice Courseware And Tutorials There is another document about symbols and models in our Files section. Files > Tut > Symbol Types For Subcircuits
Q: I have a pulse source in my schematic with zero transition times. LTspice only shows slow transition times of 2ns. What's going on here?
PULSE(0 5 0 0 0 20n 100n)
A: LTspice automatically will use a default value for <trise> and <tfall> if these parameters are set to zero. Default value: 10% of Ton or 10% of Tperiod-Ton whatever is smaller. You have to specify Trise and Tfall if you want a certain value.
PULSE(0 5 0 100p 100p 20n 100n)
Don't use steeper transitions than required by your application.
Q: I have used a pure sine source in my schematic, but the output signal of my circuit looks slightly different from cycle to cycle.
A: LTspice has waveform compression enabled as the default setting. This compression reduces the amount of saved data during the simulation. It's a lossy compression and thus it can distort the saved signals. You can switch it off with the following command line in your schematic.
It could be switched off in the "Control Panel -> compression" pane as well, but this setting will be lost upon closing the current session of LTspice.