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Transistor cant understand even a bit

  1. Nov 13, 2009 #1
    i cant understand the transistor completely
    i can understand how it works but this stupid conventional current is confusing me completely
    plz can any one give a lengthy explanation on working of transistor
    CE CB CC types. i dont understand all these types of transistor i.e how current flows in all these
    also this transistor characteristics is making me mad
    but i am very anxious and curious to learn about it
    can any 1 help me
     

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  3. Nov 13, 2009 #2
    The NPN transistor bears a vague similarity to the triode vacuum tube.
    Electrons are emitted from the emitter (filament)
    The flow is controlled by the base (grid)
    The electrons reach the collector (plate).

    In vacuum tubes, the grid potential is several volts below the filament, while in the NPN transistor, the base is ~0.6 volts above the emitter. The grid controls space charge around the filament to limit plate current. Thermionic emission controls the electron charge density near the filament.

    Emitter follower (common collector) is similar to cathode follower. Common emitter is similar to common cathode, and grounded grid is similar to grounded (or common) base.

    Bob S
     
    Last edited: Nov 13, 2009
  4. Nov 13, 2009 #3
    in vacuum tubes electrons flow from the battery
    but in transistors current flow from the transistor
    also i cant understand the function of rheostat in the figure posted by me
    in fact i cant understand the whole of this circuit diagram
    what is the meaning of voltage between emitter and base , collector and base
    do they have potential differece
     
  5. Nov 13, 2009 #4
    In your transistor, current flows into the collector, and out the emitter. The base potential controls the amount of collector current. A higher base voltage increases the collector current. R1 controls the collector voltage, and R2 the base voltage.
    Bob S
     
  6. Nov 14, 2009 #5
    BOB S thanks a lot for comparing with the triode
    i understood triode
    plz say whether i am right or right
    the voltage between emitter and base is Vbe i.e supply voltage which is used to destroy depletion region
    the voltmeter measures the potential diff between emitter and base which is same as the supply voltage Vbe am i right
    i understood everything except for1 thing
    i cant understand y a rheostat is used
    my sir told me that it is used to vary the voltage
    i cant understand this because the supply voltage just establishes a driving force or electric field in the transistor and no current flows from supply to transistor then what is the purpose of rheostat there as no current flows from supply to transistor
    this 1 thing is confusing me? plz help me GENIUS BOBS
     
  7. Nov 14, 2009 #6

    Integral

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    Since you seem to be more interested in the circuitry around the transistor then the transistor itself I am moving this to the EE forum.
     
  8. Nov 14, 2009 #7
    There is a current flowing from the base to the emitter, called Ibe. There is another current flowing from the collector to emitter, called Ice. The ratio Ice/Ibe is called hfe, or gain. So the rheostat provides the Ibe current to turn the transistor on. See
    http://www.rason.org/Projects/bipolamp/bipolamp.htm
    Look up 2N3904 specs, incl hfe at
    http://www.datasheetarchive.com/pdf-datasheets/Datasheets-24/DSA-467653.pdf [Broken]
    Bob S
     
    Last edited by a moderator: May 4, 2017
  9. Nov 14, 2009 #8

    vk6kro

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    Conventional current vs electron current has nothing to do with how a transistor operates.

    Power still comes from the battery and any current that leaves the battery has to return to it.

    Transistor currents.PNG

    In this diagram, the current splits into base current and collector current. R just limits the base current.
    At the emitter, the currents combine again and return to the battery.

    transistor currents 2.PNG

    The base current controls the collector current. I have drawn these currents on the same scale to show that the collector current is a LOT bigger than the base current.
     
  10. Nov 14, 2009 #9
    If you search my posting history, I've contributed to threads where the transistor, bjt & FET, is thoroughly dissected. I'd recommend reading these threads, and if you have a specific question, it can then be addressed.

    Transistor operation is not rocket science, but far from trivial. At 1st, one must simply trust the semiconductor OEMs because they make the devices, and know more than anyone. After much time and study, it will make sense to you. But trust Fairchild, On Semi, Texas Instruments, Natl Semi, etc. more than non-peer-reviewed web sites. The web is filled with people who think they know more than the OEMs. I am a practicing EE in my 32nd year, with a year left to go on a doctorate and I still have much to learn. Trust me, there are only a small number of people who have a great understanding of the innards of a transistor. Use them as a reference. They are the ones who MAKE the transistor. Disregard contrarians who insist they know more than OEMs.

    Claude
     
    Last edited: Nov 14, 2009
  11. Nov 15, 2009 #10
    i want to know only one thing that why that rheostat is used
    if u say to vary the supply voltage
    how can it vary the supply voltage

    what i know is that the supply voltage is mainly used to provide the electric field to destroy the depletion region in the EB junction forward biased due to which electrons in emiiter move to base.Only minute of them are attraced towards the positive terminal of Vbe while the rest flows to the Collector and so on
    what i want to know is that to vary the supply voltage why rheostat is used
    how does it vary the voltage
    i thought that the current from the battery mainly flows through the volmeter which measures the voltage of the source and that for the transistor it justs provides electric field to destroy the depletion region
    since transistor is connected parallel to voltmeter and supply the voltage measured by voltmeter is the same applied for the transistor as it is connected parallel
    plz tell me why that rheostat
     
    Last edited: Nov 15, 2009
  12. Nov 15, 2009 #11

    vk6kro

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    what i want to know is that to vary the supply voltage why rheostat is used
    how does it vary the voltage
    i thought that the current from the battery mainly flows through the volmeter which measures the voltage of the source and that for the transistor it justs provides electric field to destroy the depletion region
    since transistor is connected parallel to voltmeter and supply the voltage measured by voltmeter is the same applied for the transistor as it is connected parallel
    plz tell me why that rheostat


    The voltmeter draws very little current compared to the current drawn by the transistor. Ideally it should draw none, but all voltmeters draw some current.

    A rheostat used as in this diagram is called a potentiometer. (A rheostat only uses two connections but a potentiometer uses both ends of the device and the sliding contact.)

    It is a long piece of resistance wire wrapped on an insulating cylinder and a connection is made to each end and to a sliding contact which is available to tap onto the wire at regular intervals (ususally every turn) along its length.

    If a potential is placed across this potentiometer, it divides evenly along the length of the wire and you can use the sliding contact to vary the voltage out. This is just like a variable voltage supply.
    Some current is taken from the potentiometer and flows through the transistor. A lot more current flows in the potentiometer and returns to the battery. Note that this is only for testing the transistor and would not be used for normal operation of a transistor.

    .
     
  13. Nov 15, 2009 #12
    but in transistor the current (non conventional current) flows from negative terminal to the emitter of the transistor. in its path there is no rheostat.
    Moreover initially current does not flow through the transistor but only establishes electric field. then y do u need a rheostat or potentiometer there???
     
  14. Nov 15, 2009 #13

    vk6kro

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    OK.

    It looks like you have a problem with electrical circuits, not just transistors. This can be fixed.

    Transistor currents 3.PNG

    In the above diagram can you see that if you move the slider on the potentiometer from the bottom to the top, the voltage between the slider and the bottom connection will increase?

    Forget the transistor for a moment, but I have redrawn the currents as electron flow.
     
  15. Nov 15, 2009 #14
    i already know how potentiometer works
    i know that it can vary the voltage drops and all
    but i cant understand y is it used in the transistor characteristics
    because here the supply voltage just simply establishes electric field within the transistor
    and that no current is drawn from the battery
    in that case what is the use of this potentiometer which is primarily used to vary the voltage drops
    thats y in most of this circuit diagram they have marked only conventional current

    what i know is that the supply voltage is mainly used to provide the electric field to destroy the depletion region in the EB junction forward biased due to which electrons in emitter move to base.Only minute of them are attraced towards the positive terminal of Vbe while the rest flows to the Collector and from collector they flow to the positive terminal of Vcb
    and from -terminal of Vcb flows to the +terminal of Vbe and from - terminal of Vbe flow back to emitter to maintain the concentration of electrons in emitter.thus all the concentrations have been balanced and the result is that current has flown in the circuit
    i want u explain in terms of the current that i have mentioned.
    i can understand only if u mention the actual electron movement
    not conventional as it does not exist in reality
     
  16. Nov 15, 2009 #15

    vk6kro

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    As you increase the voltage across the transistor from zero, initially the transistor does not conduct at all. You could use a variable power supply for this or you could use a potentiometer, it doesn't matter. What matters is the voltage across the transistor C- E pins.

    When you reach about 1 volt, it will start to conduct if there is base current flowing.

    As you increase the collector to emitter voltage, (assuming constant base current) the current reaches a maximum at about 1.5 volts on the collector and stays like that as you increase the voltage from 1.5 volts to say 12 volts. This current is set by the base current times the gain of the transistor.

    You do not have to worry about conventional current vs electron current as the magnitudes of these are the same.
     
  17. Nov 16, 2009 #16

    thats what i want to know that how potentiometer can be used as a variable power supply
    since for transistor only the battery's electric field is action .
    If u use potentiometer u can vary the voltage drops.This is valid only if u draw current from the battery
    but for transistor only the battery's electric field is used there
    i cant understand how a potentiometer can vary the electric field of the battery acting on the transistor.
    but i know how potentiometer works i learnt from this site
    http://www.allaboutcircuits.com/vol_1/chpt_6/1.html
    i just cant digest how it is used in this transistor characteristics and what is its role and how can it vary the electric field of the battery acting the Emitter BAse
     
  18. Nov 16, 2009 #17

    vk6kro

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    i cant understand how a potentiometer can vary the electric field of the battery acting on the transistor.
    but i know how potentiometer works i learnt from this site
    http://www.allaboutcircuits.com/vol_1/chpt_6/1.html
    i just cant digest how it is used in this transistor characteristics and what is its role and how can it vary the electric field of the battery acting the Emitter BAse


    You understand how a potentiometer works but you don't understand how it produces a variable voltage?
    Its only function is to produce a variable voltage, so you do not understand how it works.
    That web site explains it very well.

    As you increase the voltage across the base emitter junction of a transistor (or any other diode) in the forward direction, the current through the diode starts off at zero and stays there at zero until the diode starts to conduct at about 0.6 volts for a silicon diode and at different voltages for other types of diodes. Above 0.6 volts, the diode current will increase rapidly and be limited only by the available current.

    You need a variable voltage to test the transistor and a potentiometer gives you a variable voltage. So, why is that a problem?
     
  19. Nov 16, 2009 #18


    u didnt understand my question
    i never told i didnt understand how potentiometer works
    see what i have asked

    "i cant understand how a potentiometer can vary the electric field of the battery acting on the transistor.
    but i know how potentiometer works i learnt from this site
    http://www.allaboutcircuits.com/vol_1/chpt_6/1.html
    i just cant digest how it is used in this transistor characteristics and what is its role and how can it vary the electric field of the battery acting the Emitter BAse"


    potentiometer is just an instrument used to vary the voltage drops
    if u send 10V by adjusting the slider u can drop 4V and send only 6V to the appliance
    this is possible if current flows through it
    in the case of transistor battery only establishes electric field within the transistor
    how can potentiometer vary the electric field which is acting on the transistor?????????
     
  20. Nov 16, 2009 #19

    vk6kro

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    There is a current flowing in the potentiometer, so the voltage that comes out of the slider is just as real as the voltage that comes out of the battery.

    Transistor currents 4.PNG

    In the above circuit, the voltmeter doesn't draw any current. What does it read when the slider is in the centre position on the potentiometer?

    If you put a transistor across the voltmeter, why would it not get the same voltage on it as the voltmeter?
     
  21. Nov 16, 2009 #20
    the voltmeter will draw little current and measure 6V as the remaining 6 would have already dropped.What u r saying is applicable only if we draw current from the battery.
    But in transistor the battery is just applying electric field and not drawing current.
    How can the electric field be varied but using potentiometer
    Mr vk6kro plz bear with me.If u feel that i am pestering u plz mention it
    here after i wont trouble u
     
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