I am having a homework question about transistors where I am asked to find the voltage drop between the base and the collector (Vbc). I drew the circuit in Proteus but I am not sure where to put the two dots which are going to represent the point B (for base) and point C (for collector). I am not sure if I should put the positive end of the DC voltmeter on the B point and the negative end on the C point. Or should it be vice-versa?

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So, I think that I should put the positive end of a voltmeter after the R10 resistor and the negative end before the R9 resistor therefore I get -9.97V but if it is vice-versa I get +9.97V. I am confused as to which one is the right one, if someone has an idea I would gladly listen to them.

  • \$\begingroup\$ There is no absolute answer. The results are actually the same. You would just have to explain how you took the measurement. It used to be that datasheets for PNP transistors would be "fastidious" in using signs a particular way. But today? Less so. You just need to keep a mental context and know what's meant. See What defines the sign of transistor rating values?, for some discussion. \$\endgroup\$
    – jonk
    Jun 20, 2020 at 20:53
  • 1
    \$\begingroup\$ Usually the convention is Vbc = Vb - Vc and the arrow that represents the potential difference goes from the last letter (C) to the first (B). (Unless you're German, that is) - oh, forgot the rest: in this case the plus is on B and the minus is on C. Look at Vbe, if you are unsure. How would you measure it? \$\endgroup\$ Jun 21, 2020 at 4:00

1 Answer 1


I tend to think of these types of transistors as a faucet. Water (current) goes from the tap (base) to the drain (emitter). Your faucet has a valve on it that changes the flow of water (current). You use that resistor in the circuit to control how much current goes into the emitter, just like you would use a valve to control the tap.

If you think of it that way, then when Q1 is biased properly, the valve is open and water (current) is flowing relatively freely to ground. BJT's are current controlled devices. Since there is not much difference between ground and the base, then I would put my negative lead there. The positive lead would go on the collector.


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