Electronics > Beginners
Transistor, i hate you!
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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