Current convention followed for BJTs?

[JHGMA]

Hi all again,

In the attached file the collector current is positive, while the emitter current is negative.

I would like to ask about the convention that is used to determine the sign of the currents.

Thank you very much.

[derekdb]

Hi,

Did you set up your measurement correctly? I did the same thing but the currents were of the same sign. Please check the attachment. Hope that helps.

Regards,

[JHGMA]

Thank you for the fast answer.

I used the Analyses>>DCOperationPoint to measure the currents.

Using that analyses I obtained the different sign for the emitter and the collector current.

Do you get the same sign using the DCOperationPoint analysis?.

[johnsold]

I do not know about Multisim, but the convention in many cases is that all currents into a node are positive.  So for an NPN transistor normal (forward biased) base and collector currents would be positive and the emitter current negative, satisfying Kirchhoff's laws.

Lynn

[JHGMA]

As far as I can see, Multisim follows a different convention depending on the way the currents are measured.

If you use probes, the currents are all positive (as expected when using the probe in the proper direction). See the file attached.

But this si not the case when you use DC Operation Point Analysis, as I mentioned before.

That is why I am asking about the convention followed when using the DC Operation Point Analysis.

Thank you.

[derekdb]

The DC operation point analysis measures the the current directions and magnitutudes based on the inner build of the componenet. So in this case, unless you know exactly how the SPICE model of the BJT is can you tell the direction of the IE - the designer may have swapped the polarity inside. So to answer your question, I would say there's no convention in this case. Anyways, it's always easier to just insert the probes. Hope this helps.

Regards.

[JHGMA]

Ok.

Thank you for your help.