transistor biasing

143

02:01:50

Why isn't the transistor in saturation mode, if the base current is larger than 1 fA? 

published 4 years ago

592azy2circuitdude

4 years ago

This is because the base emitter voltage on the transistor is less than 0.7 volts. You have it set to 0.5 volts which is not enough to forward bias the base emitter PN junction. This is the cut-off region for the transistor (Vbe < 0.7V).

592azy2circuitdude

4 years ago

As for the 1fA setting that Every Circuit has for a BJT, I'm not sure. It doesn't have anything to do with the base emitter current that causes the transistor to saturate. I believe it has to do with the way the circuit simulator models a transistor.

592azy2circuitdude

4 years ago

This is my best guess. Good luck.

Robert_Kidd

4 years ago

Wind the 1V supply up to 5V. EC using 0.8V rather than 0.7V for base emitter so increase to 0.8V and it works! No idea where you get the 1 fA from, base current is a few nA with your circuit.

boriscerar

4 years ago

592azycircuitdude: Thank you very much for your reply! Have a nice day, sir.

boriscerar

4 years ago

Robert_Kidd: The 1fA setting is under BJT transistor settings listed as saturation current; based on this, the tranny should be fully open with these settings.

592azy2circuitdude

4 years ago

Swell! 👍

boriscerar

4 years ago

Robert_Kidd: But, based on 592azycircuitdude's answer, the setting does not have anything to do with the transistor's base emitter current; anyways, if you play around with the setting, it does differ the saturation level. (again, based on the 592azycircuitdude's answer, it might be something related to the modeling of the tranny) Have a very nice weekend, everyone!

592azy2circuitdude

4 years ago

Yes, upon doing some research on this, I discovered that the 1fA saturation current is a parameter in the BJT equation (likely the Ebers-Moll model). It was somewhat beyond my level of understanding, but I gathered that the transistor is modeled using 2 diodes and 2 current sources. The Is current is the reverse saturation current of the charge carriers within the PN diodes.

592azy2circuitdude

4 years ago

Not sure about everything there, but this was a good topic to bring up. Thank you for posting.

add comment in editor

EveryCircuit is an easy to use, highly interactive circuit simulator and schematic capture tool. Real-time circuit simulation, interactivity, and dynamic visualization make it a must have application for professionals and academia. EveryCircuit user community has collaboratively created the largest searchable library of circuit designs. EveryCircuit app runs online in popular browsers and on mobile phones and tablets, enabling you to capture design ideas and learn electronics on the go.