(NPN/PNP Transistor

Discussion in 'General Electronics Chat' started by sharanbr123, Oct 9, 2014.

  1. sharanbr123

    Thread Starter Member

    Sep 29, 2014
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    Hello All,

    I think I am a little confused with the operation of a transistor. Here is my doubt.

    In NPN transistors, is it true that current flows into emitter node from the collector node?

    Also, in case of NPN transistor, the overall effect is two diodes with Anodes connected, when both tha diodes are in active state (saturation region), in which direction does the current flow?

    Again, when transistor is in acive region, BE diode is forward biased and BC diode is reverse biased.
    In this case, does current flow from base to Emitter?
    If so, I am assuming that emitter is the source of current but it is actually sinking current?

    As you can see, I am a lot confused. Can I get comments please ...
     
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
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    the "two diodes " are not seperate, the base to emittter current controls the collector to emitter current. just tieing two diodes together wont work. the materials are in molecular contact, usually difused on eachother. the base region is very thin, and allows carriers through when he b to e is foreward biased.
     
  3. wayneh

    Expert

    Sep 9, 2010
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    The convention is current flow from base and collector towards emitter, higher positive voltage toward lower. (Electrons go the opposite direction but it is not conventional to equate that with current flow.)
     
  4. sharanbr123

    Thread Starter Member

    Sep 29, 2014
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    Thanks, everyone. I did get some clarity ... I adding a follow-up questions below ...

    In case of NPN transistor working in Saturation mode, base-collector junction is forward biased. Similarly, base-emitter junction is forward biased.
    This means that current flows from base to collector and from base-emitter. Is this correct?

    If above is correct, then for the Vce supply, current is flowing both into the -ve terminal and into +ve terminal.
    How is this possible? Capture.PNG

    Please see the snapshot that I am referring to ...

    Thanks in advance ...
     
  5. wayneh

    Expert

    Sep 9, 2010
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    The collector is normally at a higher voltage than the base and the emitter, so current does not flow uphill. I believe transistors usually have a spec for how far the base voltage can exceed the collector voltage. I suppose current can flow and damage the transistor in that scenario.
     
  6. simo_x

    Member

    Dec 23, 2010
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  7. sharanbr123

    Thread Starter Member

    Sep 29, 2014
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    Thank you very much. A couple of questions on the same topic ...

    Why the transistor symbol shown as current flow from base to emitter or emitter to base even though current flow from base to collector and vice-versa (when transistor is in saturation mode)?

    For base to collector junction to be forward biased (or base to emitter), is it necessary that there is positive voltage at base terminal and negative voltage at collector terminal. Typical trasistor schemes show positive voltage connected to collector terminal
     
    Last edited: Oct 11, 2014
  8. ScottWang

    Moderator

    Aug 23, 2012
    4,858
    767
    In the practical circuit, the B needs a Rb resistor to limiting the Ib current, and the C needs a Rc or Load to limiting the Ic current.

    Using the voltage to explain how the bjt working is more easily to understand.

    When transistor is in liner mode:

    C voltage level is the highest pole, it will be Vc>0.2V and some more, lets say Vc=1.5V
    B voltage level is the middle pole, lets say Vb=0.7V
    E voltage level is the lowest pole, and it is connecting to ground, lets say Ve=0V.

    So
    Vbe = 0.7V,
    Vce = Vc-Ve = 1.5V,
    Vbc = Vb-Vc = 0.7-1.5V = -0.8V, the Vb(0.7V) is less than Vb(0.7V) -0.8V, so the B and C is reverse, there is no current flows through B to C.

    When transistor is in saturation mode:

    C voltage level is the middle pole, it will be about Vc=0.2V.
    B voltage level is the highest pole, lets say Vb=0.7V
    E voltage level is the lowest pole, and it is connecting to ground, lets say Ve=0V.

    So
    Vbe = 0.7V,
    Vce = 0.2V,
    Vbc = Vb-Vc = 0.7V-0.2V = 0.5V, the Vb(0.7V) is great than Vb(0.7V) 0.5V, so the B and C is forward, but the diode of Vbc needs 0.7V to active and the current start flows through B to C, but 0.5V can't active the diode of Vbc, so still there is no current flows through B to C.

    I hope there is nothing wrong.
     
  9. sharanbr123

    Thread Starter Member

    Sep 29, 2014
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    0
    I would like to get some more clarification from members here ...

    I am using NPN transistor operating in saturation mode as an example ...

    1) Base-Emitter junction is forward biased
    2) Base-Collector junction is forward biased
    In this case, I am making the following observations,

    a) Current flows from Base to Emitter
    b) Current cannot flow from Base to Collector since Collector is connected to positive source of voltage. This terminal can source current but cannot sink it
    c) Current cannot flow from collector to emitter since Base-Collector junction is forward biased and current cannot flow from N (collector) to P (Base).
    In other words, current cannot flow from Cathode to Anode.

    In many articles describing transistor operation, collector to emitter current is shown but somehow I am unable to appreciate how this is possible either in active region or in saturation region.
    Hoping for comments ...
     
  10. alfacliff

    Well-Known Member

    Dec 13, 2013
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    since the base to collector voltage or current does nothing, why are you worrying about it? the current through the transistor is controlled by the base to emitter voltage or current.
     
  11. Papabravo

    Expert

    Feb 24, 2006
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    1. One of your assumptions is WRONG. In an NPN transistor the base collector junction is normally reverse biased.
    2. Ie + Ib + Ic = 0
     
  12. sharanbr123

    Thread Starter Member

    Sep 29, 2014
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    @alfacliff
    no, actually, the fundamental doubt I am having is how current is flowing across a reverse biased junction.
    That is from, emitter to base (forward biased) and from base to collector (reverse biased) ...

    Also, for a given voltage Vce, how Ic increases with increasing Ib?
     
  13. sharanbr123

    Thread Starter Member

    Sep 29, 2014
    49
    0
    Hello, actually, my question was based on the assumption that transistor is operating in saturation mode ...
     
  14. Jony130

    AAC Fanatic!

    Feb 17, 2009
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