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Can anybody explain about how bjt works??

Discussion in 'General Electronics Discussion' started by shantanu singh, Dec 28, 2016.

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  1. shantanu singh

    shantanu singh

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    Dec 28, 2016
    Basically tell me about difeerent regions
     
  2. Minder

    Minder

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    Apr 24, 2015
    Here is a start.
    M.
     

    Attached Files:

  3. Audioguru

    Audioguru

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    Sep 24, 2016
    A bjt is a bipolar junction transistor, not a Mosfet.
    It has 3 regions, cutoff, linear and saturated.
    When it is cutoff then it is turned off and does nothing.
    When it is linear then its output can swing up and down and it can be an amplifier.
    When it is saturated then it is a turned on DC switch.
     
  4. Herschel Peeler

    Herschel Peeler

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    Feb 21, 2016
    As you pull current through the emitter - base the resistance between the emitter and collector changes.
    Gain = collector current divided by base current.
     
  5. Ratch

    Ratch

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    Mar 10, 2013
    A BJT is a transconductance (voltage controls current) device. In the active region, the base current is an indicator of the collector current, but does not control it. The base current is an unavoidable waste that does not contribute to transistor operation.

    Ratch
     
  6. Herschel Peeler

    Herschel Peeler

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    Feb 21, 2016
    And there is nothing incorrect about that explanation at all. I find the current story easier to demonstrate, changing current to show gain. You can't explain gain by discussing base voltage.
     
  7. Ratch

    Ratch

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    You cannot show current gain without driving the base with a current source, or adding a lot of resistance to the base and/or emitter. From then on, you are demonstrating a current amplifying circuit using a transconductance device, not a BJT alone. A BJT by itself is not an inherent current amplifier.

    Ratch
     
  8. Herschel Peeler

    Herschel Peeler

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    Feb 21, 2016
    Your description of the transistor is correct. What formula for gain do you use that uses base voltage? when I read a datasheet for transistors it references collector current and base current to describe gain. We can't use base voltage to indicate gain because the same base voltage gives different gain at different collector currents.
    Is there a graph of base voltage versus collector current on a data sheet for a 2N3904?

    You are correct about how the transistor works but nobody uses this description on a data sheet ... unless you can show one. It is technically true, but can't be applied.
    .
     
  9. Ratch

    Ratch

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    What you see on a data sheet is a test circuit using the device. The test circuit contains current sources. I never said that a BJT should be designed as a voltage driven device. It is too nonlinear for that. However, it is not a current amplifier by itself like a lot of books and magazines say it is.

    Ratch
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    The appropriate answer is the one which satisfies @shantanu singh's question.

    He may have meant:
    1. How does a bjt work internally? The answer involves a discussion of doping and charge flow.
    2. How does a bjt work electrically? Here is where the discussion of it as a three terminal device goes.
    3. How does a bjt work practically? The answer involves it's use in various circuit topologies.
    A really big hint was provided in the body of his post.

    One person seems to have read it and assumed it was about modes of operation, but it could also have been asking about the regions within the transistor.

    Surely we can ask for clarification as to what he meant by "regions" before starting the normal "it's voltage. No, it's current" debate? It's especially funny because no matter what he answer might have been, that debate would be largely irrelevant.

    I'm sure he has already handed his homework in for assessment, but the basic answer is either

    Emitter, base, collector

    Or

    Cutoff, saturation, active, reverse-active
     
  11. Herschel Peeler

    Herschel Peeler

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    Feb 21, 2016
    You speak as though it must be one or the other. Both approaches are acceptable.
     
  12. Ratch

    Ratch

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    I am not clear on what you mean by "one or the other" and what the approaches are.

    Ratch
     
  13. Herschel Peeler

    Herschel Peeler

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    Yes, we can look at the transistor from the standpoint of a base voltage controlling the collector current or we can view it as base current controlling collector current. Most data sheets refer to base current. It takes an engineer's view to understand it is base voltage.
    For me, I can't calculate gain using base voltage. My input signal is base current. I am unconcerned with base voltage. Base voltage just falls where it may.

    On an LED, I control current. Voltage across the LED falls where it may.
     
  14. Ratch

    Ratch

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    Didn't I say in post #9 that one cannot design using the base-emitter voltage because it is too nonlinear? That is why you have to add lots of resistance to overcome the nonlinearity. Nevertheless, the Vbe, not the Ib controls the Ic in the active region.

    Ratch
     
  15. davenn

    davenn Moderator

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    the opposite to way most do it

    you have a LED of say 2.5V and 25mA, you want to run it off 12V, you choose a resistor that will drop the other 9.5V across it when 25mA is flowing

    The voltage across the LED cannot be random or arbitrary as you suggest it must be correct cuz if you have a higher V then more current will flow
    and it will have a short and brilliant life
     
  16. Herschel Peeler

    Herschel Peeler

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    ??? Elaborate on this, please. At what point does the nonlinearity change? Yes, it is nonlinear. The linearity changes based on current. The voltage is incidental. The voltage is the result of the current. The causal force at work is the current. Nowhere in the data sheet does it relate base voltage to collector voltage or current. The correlation is incidental.
    Explain how you imagine base voltage is significant?
    From outside then transistor I change resistance to change the current. I need not consider base voltage.
    Inside the transistor operation is describe in terms of current carriers, not voltages.
    Do you have another description?
    Yes, in a specific circuit we can relate base voltage to collector current but the observation cannot be applied to a different circuit. There is no correlation between a specific base voltage and some collector voltage or current.
     
  17. Herschel Peeler

    Herschel Peeler

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    Feb 21, 2016
    :)
    I use a constant current source and let the voltage fall where it may.
     
    Arouse1973 likes this.
  18. Ratch

    Ratch

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    By making the emitter resistor high enough, most of the base volt will drop across the emitter resistor and determine the collector current. The more most of the voltage is dropped across the emitter resistor, the more linearly the transistor will amplifier the current.

    Ratch
     
  19. Herschel Peeler

    Herschel Peeler

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    Feb 21, 2016
    Hmmm. Does that work for the base resistor also. The emitter resistor configuration isn't used very often. I just don't see that having a very profound effect. I'll have to play with the idea some.
     
  20. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    What about a current mirror. Doesn't this use a voltage to program a current?
    Thanks
    Adam
     
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