Most frequently asked questions for beginners
Q: What are the different LTspice file types?
Schematic: name.asc the drawing with your circuit Symbol: name.asy symbols for the schematic Logfile: name.log infos and results from .four, .measure, .op Netlist: name.net you don't care about Result: name.raw usually a binary file Result: name.fft result of an FFT, usually a binary file Plot settings: name.plt Model file: abc.xyz contains the models, any file name Symbol: name.cir if you have a netlist input file
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: 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: 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 as 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 after a new invocation of LTspice.