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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