Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Voltage divider bias

  1. Jan 12, 2010 #1
    In Voltage divider bias ,
    we are not drawing any electrons from the base.
    Then how can the electron current pass through the base and go to the collector and reach the positive terminal.
    Only if You draw electrons from the base by maintaining the base at positive potential,
    then only the electrons from Vcc can flow through the base and to the collector and reach the positive terminal of Vcc.

    The need for removal of electrons from the base is that.
    In the first circuit shown,
    when the base emitter is forward biased, the holes in the P region diffuse into the N region and the holes in the base are now filled by electrons as a result the base becomes the negatively charged. Now the electrons from Vcc negative terminal cannot pass through the transistor as they have to cross the base which is now negatively charged . But since the base is made positive the electrons that filled up the holes are attracted as a result the base now consists of holes and the electrons from - Vcc can now flow through the transistor .
    Since in Voltage divider bias this provision( Vbb) is not there , then how can electrons be withdrawn from the base and how can current flow through the transistor.
     

    Attached Files:

  2. jcsd
  3. Jan 12, 2010 #2

    Averagesupernova

    User Avatar
    Science Advisor
    Gold Member

    Why do you think an amplifier set up with voltage divider bias will have no base current?
     
  4. Jan 12, 2010 #3
    I am speaking only in terms of electron movement and not conventional current.
    The first thing should happen in any transistor is that , we have to draw electrons from the base . Then only collector electron current begins to flow from Vcc (-) to Vcc +.
    This does not happen.
    If you feel this happens , then please explain it.
     
  5. Jan 13, 2010 #4

    Averagesupernova

    User Avatar
    Science Advisor
    Gold Member

    I still refer you to my original post. Electron movement and conventional current are the same thing ignoring direction. Analyzing the circuit you have posted is simple enough with either electron flow or conventional current flow. So again I ask you why you think there is no current (or no electrons moving) into/out of the base of the transistor?
     
  6. Jan 13, 2010 #5
    Actually conventional flow is not reality,
    only electron flow is reality.
    I have clearly stated in the beginning the need for considering electrons.They are the one who sit in the holes of base and block the Vcc electrons from crossing the base and going into the collector.
    If you consider conventional current (flow of positive charges)
    you cannot give an explanation like the one i have given.
    Anyways where is the answer to my question.
    you are giving a one line answer,
    show the proof behind your explanation.If possible mark it on the diagram.
     
  7. Jan 13, 2010 #6

    Averagesupernova

    User Avatar
    Science Advisor
    Gold Member

    Ok fine. We'll go with electron flow. NOW WHY DO YOU THINK THERE IS NO CURRENT LEAVING THE BASE OF THE TRANSISTOR? I keep giving you a one liner because you refuse to answer the question that I have asked. Maybe I'm misunderstanding you. Is this question specific toward the way the transistor is configured in your first post or are you refering to transistors in general?
     
  8. Jan 13, 2010 #7
    See attached image of NPN transistor. Electrons flow into emitter, and out through the base (into voltage divider) and out through the collector (through collector resistor) to Vcc.
    Bob S
     

    Attached Files:

    Last edited: Jan 13, 2010
  9. Jan 13, 2010 #8
    BOB S How are you? Long time no see.

    I'll start from transistor basics,
    Firstly there are holes in base and electrons in emitter.When the base emitter junction is forward biased, the holes from base diffuse into the emitter (i.e electrons in the valence band of the emitter fill the holes in the base.)
    As a result the base which had earlier holes and was electrically neutral now due the entry of new electrons has now become negatively charged.
    The electrons from the -terminal of Vcc cant pass the transistor as the negatively charged base is blocking it.
    Since the base is connected to positive terminal.It will attract the electrons from the base and as a result holes are once again generated in base.
    Now the electrons form -terminal of Vcc can pass through the transistor and go back to the +positive terminal of Vcc. Thus only if you draw the electrons from base it allows the collector electron current to flow. i.e the base current controls collector current .This is the principle of amplification.

    But you have said that the Electrons flow into emitter, and out through the base (into voltage divider) and out through the collector (through collector resistor) to Vcc.
    But what i am asking is that the base electron current controls the other current.
    If that is the case then electrons should start to flow only from base.
    For that you require a positive voltage applied to base so as to attract electrons from the base. But we dont have a positive voltage applied at the base.
     
  10. Jan 13, 2010 #9
    Ok according to you let electrons leave the base . I accept it .
    Now show it how can it flow . By referring my doubt in the post no:8
     
  11. Jan 13, 2010 #10

    Averagesupernova

    User Avatar
    Science Advisor
    Gold Member

    Sudar I don't understand anything you are saying at all. I get the sub-atomic structure of the transistor and the flow of electrons. That's nothing new to me.
    How can you say there is not a positive voltage applied at the base? Here is what happens. Vcc is +15 volts referenced to ground. Please don't say -Vcc. There is no negative Vcc. The voltage divider is set up to get a voltage on the base that is somewhere between Vcc and ground. Let's pick a value of 5 volts referenced to ground. The voltage on the emitter will be about 4.3 volts since the emitter is never more than about .7 volts lower than the base with an NPN transistor. (Forward biased P/N junction) So we now have an emitter current that is 4.3/Re. This same current minus the base current will flow in the collector also. I don't understand how you can say that there will be no current leaving the base. The voltage divider forms a voltage on the base that is MORE POSITIVE than the emitter and as long as the voltage across the base and emitter turns on the base/emitter junction, you will have current flowing in the base.
     
  12. Jan 14, 2010 #11
    Please explain the concept of voltage divider .
    I cant understand how a resistance can provide bias. i.e how can it supply a positive voltage.
     
  13. Jan 14, 2010 #12

    MATLABdude

    User Avatar
    Science Advisor

    sudar_dhoni, based on your post history, I believe that many of the holes in your understanding of electrical concepts (and the reason many of us have trouble conveying things to you) stem from the fact that you seek in-depth understanding of advanced topics (e.g. transistors and amplifiers) without having learnt or understood the fundamental concepts which underlie them.

    What's worse (from my point of view, and presumably that of other posters) is that you use these complex topics (which, to me at least, presupposes a certain level of understanding / background) as a way to understand the simple topics.
     
  14. Jan 14, 2010 #13

    MATLABdude

    User Avatar
    Science Advisor

    To the point however, a voltage divider can be used to supply a bias to a transistor because it is at higher potential than ground (assuming the transistor is not connected to anything lower than ground.) And the only way the passive resistors can do this is because they are connected to a power supply or battery which can pump (positive) charge back up to a higher potential (or equivalently, push electrons down to ground and allow them to float back up to the positive terminal)
     
  15. Jan 14, 2010 #14
    If you feel this difficult to explain,then atleast can any one suggest a very good referrence book or webiste which explains my doubt very well.
     
  16. Jan 14, 2010 #15

    Averagesupernova

    User Avatar
    Science Advisor
    Gold Member

    I'm still not 100% sure what your doubt is sudar. I thought I explained it reasonably well to someone who should have the voltage divider basics down pretty well. If you do not fully understand a voltage divider then I am wondering how you have ended up analyzing a transistor circuit that uses voltage divider bias (or any transistor circuit for that matter). Here's a trick you may want to try. Remove the transistor from the circuit leaving everything else as is. Then tell us what you think the voltages would be when measured between the nodes that connected to the three transistor leads. In other words, what would the voltage be between Vb and Ve, between Vc and Ve, and between Vc and Vb? For the sake of this excercise, lets assume that R1 is 2.2K, R2 is 1.1K, Re is 220 ohms and Rc is 470 ohms.
     
  17. Jan 14, 2010 #16

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    I think you are trying to run before you can walk. If you can't apply simple electronic theory, relating to resistor networks then how can you expect to have a clue about more advanced electronics?
    You ask for a source - try Wikkers or your local bookstore or library. The shelves will have loads of college books which will take you through the subject in a conventional way - which has been shown to work pretty well for millions of students. Or are you 'above' such things?
     
  18. Jan 14, 2010 #17
    http://en.wikipedia.org/wiki/Ohm's_law
     
  19. Jan 14, 2010 #18
    I know how a voltage divider works without a transistor.
    But with a transistor i cant understand it.
    First of all you all cant understand my doubts.Since you cant understand it you tell that i dont know the basics.
    I know all the basics. Without knowing the basics i cant come all this way upto transistors.
    If you cant understand my doubt, please tell it. Dont tell that i dont know the basics and act smart. I know how a voltage divider , rheostat, potentiometer works in ordinary D.C circuits. But when we introduce transistor its different.Please read my first post carefully. I have clearly mentioned the working of transitor i.e how base current controls collector current.
    I myslef have found the answer which i will represent diagramatically

    In diagram E represents the electric field of the battery.
    Please confirm whether this is right or wrong.
    The eletric field from the positive terminal of the battery Vcc attracts the electrons from the base. As a result electrons leave the base and the rest of the process takes place.
     

    Attached Files:

    Last edited: Jan 14, 2010
  20. Jan 15, 2010 #19

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    "If you feel this difficult to explain"!!!
    I think it is you who find it hard to accept things.
    A number of us have tried to give an answer. Clearly we are totally incapable so I suggest you just read a book or look at Wikkers or just Google for a tutorial on simple electrical circuit theory. Anyone who reckons they understand solid state Physics to a level where they can question conventional theory must be more than capable of doing that.
     
  21. Jan 15, 2010 #20
    Ok i will follow conventional theory itself with water analogies.
    Could you please give me a water analogy for minority carriers, leakage current .
    If you give me this answer i'll surely follow conventional theory.
    I am not against conventional theory.In fact if i cant find a proper answer in terms of electron movement , i think the transistor as a valve as mentioned in this site
    http://www.satcure-focus.com/tutor/page4.htm
    Its a beautiful way of thinking a transistor.But it cant explain about the reverse leakage current, minority carriers ,etc.
    IF you give me a solution to that all my problems are over and i will start to follow the conventional way.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Voltage divider bias
  1. Voltage divider biasing (Replies: 13)

  2. Voltage divider (Replies: 15)

Loading...