EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: spicey on November 06, 2021, 10:00:55 am
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Hello,
I am testing LTSPICE and, although at the moment I am a little rusty, out of curiosity I have drawn the schematic of a simple common-emitter amplifier based on a NPN transistor, 2N3904 measured for hfe (256) with a multimeter. I have tested the simulation and, surprise, it does not match the results I get directly on the scope. I also tried to define a new model for 2N2904 with a 256 hfe but the simulation results in LTSPICE were the same.
Here are some images with the schematic,
(https://i.postimg.cc/brDY2Nyd/ltspice-schematic.png)
the result of the simulation with LTSPICE with an obvious top and bottom clipping,
...
in the real amplifier the input is connected to a 5kHz 5V signal generator and the aim is to convert/amplify it to a 5kHz 18V signal. Here is a screenshot of the scope with the amplifier and there isn't clipping at all,
(https://i.postimg.cc/bwb1DzMd/scope.jpg)
the LTSPICE peak-to-peak voltage for V(out) is over 23.2V while in the scope is 17.6V
Would anyone be so kind to tell me what I'm doing wrong or what I'm missing? Any help will be appreciated, thank you.
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On the scope shot it seems that the input signal is 5V peak to peak and the output is 18V peak to peak. Isn't that exactly what you would expect?
Your LTSpice input signal is 5V amplitude, which is 10V peak to peak.
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Usually the difference between real world and the simulation is from not using the same parts, like more parasitic capacitance, or wire inductance.
Here the obvious difference is the amplitude. Another likely difference is the output impedance of the source.
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Thanks a lot.
Your LTSpice input signal is 5V amplitude, which is 10V peak to peak.
That's it! Thank you, guymo.
Edit: Just to give context to the problem: I had a hard time when I resumed using LTSPICE after a few years, I did not remember how to handle the GUI, how to place components and I even came to resort to edit the text files by hand to copy and paste symbols from old schematics. The sine wave source was copy-pasted borrow from an old project and by not editing it, after hours of fighting hard with LTSPICE, I did not realize that it had twice the amplitude. On the other hand, the V(in) trace in the LTSPICE plotter was there just to do it the same as with the scope, but I didn't really even watch it.