Is the following a correct understanding of what makes a NPN/PNP transistor turn on or off?

NPN (more common)

  • Must have voltage > ~0.7V on base
  • Must have (conventional) current flowing into collector (top)


  • Must have zero voltage on base
  • Must have (conventional) current flowing into emitter (top)

Are the above two conditions accurate for each? And Are there any other 'requirements' to turn on a NPN/PNP transistor?


1 Answer 1


No. The amount of conventional current (not electron current) entering the base and flowing out through the emitter determines whether an NPN turns on (i.e. how much current passes from the collector to the emitter). For a PNP, it is the amount of conventional current flowing into the emitter and out of the base that determines how much current passes from the emitter to the collector). The base-emitter junction behaves like a diode which is where the typical 0.7V value across the base-emitter comes from.

Note that things like "the voltage on the base" have no meaning for a transistor. The voltage relative to what? Relative to ground? What if neither of the other two pins are at ground? Voltage is relative and the transistor does not know what the voltage is anywhere except between two of its pins.

Saying "the voltage at X pin" is like saying "the distance at your house". It doesn't mean anything.

  • \$\begingroup\$ thanks for this, I would mean like this: everycircuit.com/circuit/5229445137563648. Where it says "813mv" around here: gyazo.com/43138386ad50901dcc60b2320db09e7b. \$\endgroup\$
    – David542
    Jan 18, 2020 at 5:00
  • \$\begingroup\$ @David542 The simulation is saying that is 813mV measured relative to GND. But since you don't know where GND is (at least if the image you showed me is all there is since I can't access anything else) then it's meaningless with regards to determining transistor behaviour. Typical language when someone says "this node is at 1V" implies "1V relative to GND" and you have to make sure that's what you're actually interested in which you won't be if neither of the two terminals you are interested in are connected to ground. \$\endgroup\$
    – DKNguyen
    Jan 18, 2020 at 5:02
  • \$\begingroup\$ you can view the circuit here, I've just made it public: everycircuit.com/circuit/5229445137563648 \$\endgroup\$
    – David542
    Jan 18, 2020 at 5:04
  • \$\begingroup\$ @David542 I still can't view it because it requires Chrome. But no matter what it is, it doesn't change what I said. \$\endgroup\$
    – DKNguyen
    Jan 18, 2020 at 5:05
  • \$\begingroup\$ I see, here's the full circuit then: gyazo.com/c425ee9df9394433f60cedc6618bc487 \$\endgroup\$
    – David542
    Jan 18, 2020 at 5:07

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