Electrical – NPN transistor in LTSpice

ltspicenpn

I just started learning NPN transistor and was trying to replicate Falstad – NPN sample in LTSpice. Somehow I get all base, collector and emitter current in the micro-Amp range while the sample shows current in the milli-Amp range. Since the circuit sample in Falstad does not resistor on either terminals (B, C, or E), I add a low resistance resistor to mimic a wire just for LTSpice to show current probe symbol and see the plot of current.

Could someone explain why am I getting this drastically different current even when I copy the same parameters in the sample, and explain what I did wrong?

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Best Answer

Try this setup:

test

Also, I don't know what you did before, but running a .dc op point with the command you have in the picture only brings up the error log just like in my screenshot. That's because, the way you set it up, you're running a "sweep" from 0.75 V to 0.75 V, in steps of 0.75 V, which translates to one point, only, which means LTspice will consider that as another way of spelling .op.

(edit) The reason why you see different numbers is because the default values differ, as mentioned in the comments, too. In LTspice's case, you can see which values are by looking in the help at LTspice > Circuit Elements > Q. Bipolar Transistor. I can't tell you which values are for Falstad, but, clearly, they are different. If you choose a model from LTspice's database (right-click on the transistor, Pick New Transistor) and choose 2SCR293P (for example), the values will come closer to what you see in the Falstad simulator. SInce that NPN has an Ic=1A, it means that the default values for the Falstad transistor are quite generous. Either that or the internal resistances are very small.

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