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?

enter image description here

  • \$\begingroup\$ Use a current source to inject base current and note that not all sims are equal and neither do they have similar default transistors. \$\endgroup\$
    – Andy aka
    Commented Apr 3, 2020 at 13:37
  • \$\begingroup\$ Why do I need a current source when I already provide a voltage supply? Plus I do not have the component or knowledge to physically build a current source to learn from simulation but with voltage I can just hookup a DC supply and set a voltage. \$\endgroup\$
    – KMC
    Commented Apr 3, 2020 at 13:42
  • \$\begingroup\$ LTSpice has current sources. If you're going to argue then I'll bid you good day sir!!! \$\endgroup\$
    – Andy aka
    Commented Apr 3, 2020 at 13:43
  • \$\begingroup\$ Change the transistor type to something like a 2N4401. \$\endgroup\$ Commented Apr 3, 2020 at 13:44
  • 1
    \$\begingroup\$ Nobody actually drives a common emitter amplifier like that with a voltage source because it's asking for trouble (simA, simB or reality). The base emitter junction is a forward biased diode and that will conduct vary degrees of current (almost uncontrollably) over a small range of base-emitter voltages. Do you understand why a resistor is used in series with an LED? I was just messing around with my other comment, remembering Gene Wilder.... \$\endgroup\$
    – Andy aka
    Commented Apr 3, 2020 at 13:53

1 Answer 1


Try this setup:


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.

  • \$\begingroup\$ Falstad had 3.9 mA Ib and 392 mA Ic. This LTSpice result has 3.9 uA Ib and 0.39 mA Ic. They're different by a factor of 1000. I don't consider this "(very) similar". \$\endgroup\$
    – The Photon
    Commented Apr 3, 2020 at 14:58
  • \$\begingroup\$ @ThePhoton You're right, I misread the numbers, I'll modify the response. \$\endgroup\$ Commented Apr 3, 2020 at 15:25

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