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Current direction?

  1. Nov 16, 2009 #1
    current direction???

    in transistor (npn) saturation, we know (in CE) Vbe=~0.75 V. and Vce is around 0.2 V.
    In such a case according to characteristic plots, we know Ic flows to emitter (via base)

    But current always flows from higher to lower potentials.
    Why in this case it is not???

    please solve this...
     
  2. jcsd
  3. Nov 16, 2009 #2

    vk6kro

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    Re: current direction???

    You would expect the base current to flow from base to collector wouldn't you?

    However this is still a diode and it needs 0.6 volts to make it conduct. So the base has to be 0.6 volts more positive than the collector before it would make the B - C diode conduct.

    This would only happen if the collector was at zero volts which will not happen if the transistor is powered.
     
  4. Nov 16, 2009 #3
    Re: current direction???

    if it is like a diode, then it is operating in reverse bias, which allows only small currents (usually in micros) .
    Another factor is such that , how can current pass comfortably in reverse bias (C-B), where it could not in forward potiential barrier (B-C)?

    Thank you
     
  5. Nov 16, 2009 #4
    Re: current direction???

    hey for the CE junction it is reverse biased which means holes in p type can move to n type and vice versa .But the minority charge carriers in p type namely electrons can move to
    n type in reverse bias
    since electrons which have come from emitter and are now in the base they are attraced due the positive voltage applied at the n type so they flow easily
    (Hey dont confuse with minority in p type and electrons from emitter both are independent but both can flow easily to the n (collector) since + field is applied at the collector.
     
  6. Nov 16, 2009 #5

    vk6kro

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    Re: current direction???

    If you test any NPN transistor with a multimeter, you will see that it tests like two diodes with their anodes joined at the base.

    So, if the collector voltage is less than the base voltage, the junction is forward biassed, but not enough for it to conduct unless the collector is actually at zero volts.
     
  7. Nov 16, 2009 #6
    Re: current direction???

    """"since electrons which have come from emitter and are now in the base they are attraced due the positive voltage applied at the n type so they flow easily """"

    as you said above, due to 0.2 V they get attracted easily;
    but a much higher 0.65 V is pulling them to base region....
     
  8. Nov 16, 2009 #7
    Re: current direction???

    i agree with you that it can't be forward...
    what about when transistor is in saturation? where Vbe is around 0.8 V...


    also why will electrons (presently in p material) go to a attracting 0.2 V leaving 0.65-0.75 attraction towards base?

    thank you
     
  9. Nov 16, 2009 #8

    vk6kro

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    Re: current direction???

    what about when transistor is in saturation? where Vbe is around 0.8 V...

    If current does start to flow into the collector from the base, then this will drop the normal base current so the collector voltage will not drop any further.
     
  10. Nov 17, 2009 #9
    Re: current direction???

    mr vk6kro
    leave alone our battle with potentiometer if u think about what i am asking u can answer it
    u cant understand what i'm asking
    ok lets go further
    could u explain how base current can be kept constant for output characteristics
    there u vary Vce.but if u vary Vce base current should decrease as the +voltage of collector has increased as a result it will attract more electrons towards collector as a result base current should decrease but its said that base current is constant how is that

    p.s-how to become a science advisor like u i.e how to get that tag that u have it in ur name
     
  11. Nov 17, 2009 #10
    Re: current direction???

    ok lets have another approach.......

    suppose all values Vbe, Vce, Ib, Ic, Ie....... are Constant.....i,e.now we are at any one single point in characteristic curves.

    now if an electron, starting from earth(emitter ground) , travels all it's way to Vce;

    can any one say how the potiential changes for that electron, to all its journey to Vce?
    initial potiential of earth is obviously zero.

    lets talk as if a pure science; in terms of voltages, attractions,collisions(if) etc....
    ...as if someone asked you to plot the 'potiential Difference' graph.
    value of PD on Y-axis and distance from emitter earth to Vce on X-axis.

    you can also tell in terms of force(NET)-distance if you are comfortable..

    remember that Kinetic energy of electron is not considered when we talk on electron movements. it is considered constant.
    also we know force inside any matertial (irrespective of concentrations of any charges inside) is zero. (vectors cancel each other)

    only at junctions(or ends) that any electrons get into troubles.
     
  12. Nov 17, 2009 #11
    Re: current direction???

    what i think is that since Vce is higher tha Vbe
    the electric field is larger
    hence according to work energy theorm change in K.E = work
    i.e the field of Vce has increased its energy
    since voltage is nothing but energy the voltage of the electron has increased
     
  13. Nov 18, 2009 #12
    Re: current direction???

    """since Vce is higher tha Vbe """

    not always is Vce is higher than Vbe...

    i already mentioned in starting that Vbe=0.7-0.8 in saturation;Vce=0.2-0.25;
     
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