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How to know a transistor is carrying out its function ?

  1. Nov 21, 2012 #1
    For a npn transistor, Vb= +0.7V ,Vc=+1.0V , Ve=0
    Then my teacher says it is not operating, why is that so?

    Thank you.
  2. jcsd
  3. Nov 21, 2012 #2
    I see no reason to believe the transistor as specified is not operating.

    Let us suppose a 5 volt supply and a 3k9 collector resistor, then with the figures stated the current through the collector resistor Rc is (5-1)/3.9 milliamps which is a litle over 1 milliamp.
    This is also the collector current and perfectly acceptable.

    I wonder if there is not a typo with the figures, for example is there a missing negative sign or has someone mixed up Vc and Ve?
  4. Nov 21, 2012 #3
    Why (5-1) instead of 5 ?
    No typo, my teacher said Vb is +0.7V
    My teacher said although this is in active mode, the transistor is now off( not carrying out its function as in active mode) as Vb-Vc is less than 0.5V
    I can't understand why is that so?
  5. Nov 21, 2012 #4
    Attached is the simplest arrangement to setup your conditions.

    Say the collector resistor is 3k9 (as before) and the beta of the transistor is 100 then the base will require 1/100 milliamps current (10 microamps) as in the diagram there is 4.3 volts across the base bias resistor so Rb = 4.3 / (1/100) k ohms = 430 k say 390k as nearest preferred value.

    Your teacher is somehow mistaken. There is nothing whatsoever wrong with the collector being only 0.3 volts above the base. In fact in some circuits in saturation the collector could be taken below the base (ie negative with respect to the base). This condition is called saturation and is what is required in a switching transistor.

    The collector - emitter voltage of the type of transistor I talk about here will be about 0.2 volts ie the collector will be at +0.2volts. The base will still be at +0.7 volts.

    Attached Files:

    Last edited: Nov 21, 2012
  6. Nov 21, 2012 #5
    What if it is collector is 4V above the base? Or I should say the base-collector is highly reverse biased? Any thing will happen?

    When collector is negative with respect to base mean that we forward base-collector?

    So do you mean that 'Vb-Vc is less than 0.5V' does not mean anything in an active mode of transistor?
  7. Nov 21, 2012 #6
    What if it is?

    This is bound to happen in proper normal operation as an amplifier.

    One of the manufacturers published parameters is Maximum Collector-base voltage Vcb which is usually several tens of volts.

    Yes, but a transistor works differently in saturation from active mode.

    No it means that 'Vb-Vc is less than -0.5V' since the collector is at a higher voltage than the base.

    You have posted this in the electrical engineering section.

    Physicists are interested in how a transistor works from the point of view of junctions, cariers, holes elctrons etc.

    Electrical engineers are not really interested in this but want a model that allows them to predict quantities of interest to create useful circuit configurations.

    Which are you interested in?
    Last edited: Nov 21, 2012
  8. Nov 21, 2012 #7
    I mean Vb=+0.7V, Vc=+100V, and Ve=0
    In npn, Now the base-collector is highly reversed biased. So now the transistor is still carrying out active mode?

    Vb-Vc is less than +0.5V mean the collector is at a lower voltage than the base, correct?

    Physicist. Then where should I pose this thread?

    Thank you.
  9. Nov 21, 2012 #8


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    I think we need to know the configuration before we go farther. Schematic please?
  10. Nov 21, 2012 #9
    Given Vbe=0.7V

    Attached Files:

  11. Nov 22, 2012 #10


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    Staff: Mentor

    You appear to be saying that IC = β. IB

    If that's what you are saying, then the transistor is certainly in the active region. That's how the active region is defined: collector current is controlled by and directly proportional to base current. Yes, even if VC = 0.5V.

    (N.B. Even when a transistor is operating in saturation, we still say it is "working" and "carrying out its function", etc. That's the function of a transistor switch.)
  12. Nov 22, 2012 #11
    So do you mean that the transistor is operating in active mode for the question post #9 ?
  13. Nov 22, 2012 #12


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    Staff: Mentor

    I am, for the reason outlined. With VCE=5 V, it is almost a certainty that the transistor is not in the saturated region of operation.

    (Did you intend to type 5 V there, or did you mean 0.5V?)
  14. Nov 22, 2012 #13
    5V . because of I want to ask what will happen when it is highly reverse biased .
    Thank for replying.
  15. Nov 22, 2012 #14
    I do not believe that the circuit arrangement you showed in post#9 will lead to the voltage values etc you listed.
  16. Nov 24, 2012 #15
    Where do you think is wrong?
  17. Nov 24, 2012 #16
    Can you see the difference between your arrangement and my post#4?
  18. Nov 24, 2012 #17
    Voltage supplied?
  19. Nov 24, 2012 #18
    Well what you have drawn is a transistor with zero base emitter voltage, regardless of your list.
  20. Nov 24, 2012 #19
    There is voltage supplied
    Then change R1 to Vb
    R2 to Vc
    Will it look okay?
  21. Nov 24, 2012 #20

    Look again at my circuit configuration and compare it with yours.

    Where is the base resistor connected in each?
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