| Electronics > Beginners |
| Transistor, i hate you! |
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| rfeecs:
--- Quote from: phiza on October 12, 2019, 02:03:12 pm --- But why 27mV? I set the simulation temperature to 20°C... I think it's the 2N3904 model parameters on multisim, I'll try to check it. --- End quote --- The Ebers-Moll model equation on the pages you referenced is not quite right. The more accurate equation for collector current similar to the Shockley diode equation: Notice it has an n in the exponent. That's the ideality factor. It varies between 1 and 2 depending on the material and even depending on the current. For the SPICE model, I think this parameter is Ne or NF. Note the default LTSPICE BJT model value for Ne is 1.5. This gives a change of 27mV per octave. Look up "Gummel Plot". |
| phiza:
--- Quote from: rfeecs on October 12, 2019, 06:02:42 pm --- Look up "Gummel Plot". --- End quote --- yep! This is the right name for that thing. I investigated the model a bit and there are two more options about temperature, i set everything to 20°C. --- Quote from: rfeecs on October 12, 2019, 06:02:42 pm --- For the SPICE model, I think this parameter is Ne or NF. Note the default LTSPICE BJT model value for Ne is 1.5. This gives a change of 27mV per octave. --- End quote --- I also found NF that is the "forward current emission coefficient". I set it to 1 and now Ic doubles every 17.5mV at lower voltages. Looking at the formula we see that a lower value of "NF" or "n" increases Ic for the same amount of Vbe. Maybe with some math it's possible to get the best value for the scope, i mean some kind of approximation about the optimal ranges of Vbe or things like that. Again thanks, i go back to study now, turning off simulations for a while. ^-^ |
| Jan Audio:
You hate the software, not the transistors, lets not kill him. |
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