Easily modifying BJT parameters in LTSpice?

[crono1009]

I've created a multistage amplifier in LTSpice IV and want to use actual measured hfe (Beta) values for each of the transistors, though I can't find an easy way to modify them. I could modify their library values in notepad, but I am using the same transistor model for a few of the stages and that would make both of their hfe values the same (when I've measured them to be substantially different).

In the past I've used B2Spice and modifying each of my components parameters was as easy as double clicking on the component itself and plugging in values. Is this possible in LTSpice?

Any help would be greatly appreciated, Thanks!

 

[vk6kro]

You can do this, although it is a little messy.

Look for a file called standard.BJT in the following directory:
c:\Program Files\LTC\LTspiceIV\lib\cmp\

If your transistor is in there, you can copy it to another position in the list (probably at the top) and then modify the "BF=" figure to your measured value.
You can do this with each of your measured values and then give each transistor a modified name.
For example, you could give a 2N2222 the name 2N2222-100 if you had given it a Hfe of 100.
Then save the file.

To use it, select the generic NPN transistor. Put it on the schematic page. Right click and choose "pick another transistor". Then look for your modified version.

 

[crono1009]

Thanks! That worked perfectly.

 

[crono1009]

Actually vk6kro I have another question about LTspice if you don't mind. Let's say I created put a generic npn transistor in my circuit. Then I created a .model for that transistor for example, .model 2N3904-95 NPN(Bf=95). Will LTspice insert its own default values in for all of the other BJT parameters (junction capacitances and such)?

Also as a rule of thumb while creating .model BJTs are there any values (Bf, VAF, RX etc.) I should always input while leaving the rest of the values as LTspice defaults? I'm designing pretty simple multistage amplifiers at the moment and don't need super accurate results.

Thanks again!

 

[vk6kro]

I haven't really tried just leaving parameters out, but LTSpice does have default parameters so I expect it might fill in the gaps if you left something out.

You could probably get a better result by picking a similar transistor and just changing the parameters you wanted to change. That way, the other things like internal capacitances might be closer than the default values.

You can see an interesting article on this. Go to HELP on LTSpice. Search for "parameters" then select "Q", bipolar transistor.

 

[Benylob]

Nice info

woww, this thread is very helpful

 

[es1]

I know this is a somewhat old thread but, one can also just put the model right into the drawing. This removes the need to make extra files.

http://denethor.wlu.ca/ltspice/#models