I have a question about NPN and PNP transistors.

Discussion in 'General Electronics Chat' started by CIVEDM, Jul 19, 2014.


    Thread Starter New Member

    Jan 28, 2014
    I understand to a point how they work. I saw many amazing tutorials on youtube. First question,... basic difference between NPN and PNP is that you need a current source to close the NPN and a ground or no current to close a pnp to allow to allow a current to flow right? So if NPN has no current at the gate current will not flow where in the same scenario a PNP will flow current?

    Also, of the 3 modes; open/ closed(switch); triode(variable resistor); and saturation-- I dont understand saturation. I mean I do but I dont. I know that if the voltage from the drain to the source is greater than the overdrive voltage the transistor will be in saturation mode where it acts as an amplifier. What is being amplified?!?! the voltage from the gate to the source?

  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    hi C,
    The main difference between PNP and NPN transistors are the pin voltage polarities.

    The NPN require positive voltages with respect to their emitters, [Use NPN to remind you]
    The PNP require negative voltages with respect to their emitters, [Use PNP to remind you]

    So for NPN a positive Base current is required to control the Emitter to Collector current. For the PNP a negative Base current is required.

    Open is when there is no Emitter to Collector current.

    Class A, is when the Emitter to Collector current is controlled in a Linear way by the Base current.

    Saturation is when the transistor is fully switched on and the Collector voltage is very low, approx 0.1V to say 0.3V for general purpose transistors.

    The above is a very simple description.

    Do you follow OK.?

    Thread Starter New Member

    Jan 28, 2014
    So does that mean if the PNP has no voltage from the base to emitter that current will flow from the collector to the emitter?

    So in saturation mode the collector voltage is much much lower than what you are trying to flow?

    Oh and sorry about the late reply.
  4. Brevor

    Active Member

    Apr 9, 2011
    No the base must be negative with respect to the emitter for current to flow between the emitter and collector. In other words if there is no voltage between the emitter and base no current will flow between emitter and collector.
  5. shakilabanu


    Jul 8, 2014
    To understand these first you should try not to mix up your terminologies like base-gate?? NPN, PNP transistors dont have gates !! they have base.

    I hope you know the difference between FETs and BJTs.
  6. Jony130

    AAC Fanatic!

    Feb 17, 2009
  7. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    NPN is a Normal transistor.

    PNP is just the same thing but upside down. :)