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Transistor AMPLifier HelP

  1. Dec 3, 2009 #1
    Transistor AMPLifier HelP !!!!

    could some1 explain it entire circuit in terms of electron movement i.e how the voltage drops in each component in terms of electron flow

    1)i cant understand the voltage divider bias i find it extremely difficult
    plz explain it interms of electron movement in any one of the circuit diagrams mentioned below (explain the voltage drops for the electron current


    2)also to be frank i cant understand what basically Vbe and Vce is
    is it the barrier voltage between base emitter and base collector respectively
    if so then how can voltage drop occur at CB junction
    it experiences no impedance and only attraction then how can the voltage drop there
    what is meant by emitter base juction has less resistance and collector base junction has high resistance.But actually electrons from emitter get attracted to the collector easily how come that region has high resistance


    i have posted the circuit diagram
    i have many doubts there about how the voltage drop between R1 and R2 gives forward bias to the base emitter
    also which current's voltage is getting dropped there
    in my diagram its showing collector current
    but collector current itself arises from the emitter
    also u have not biased the emitter till now then how did u get Ic

    forward bias provided by the voltage drop of Ic
    see u have not biased it and u r about to bias the emitter only from voltage drop of Ic
    then where did u get that Ic from
    Ic comes only after biasing EB

    so could some1 explain the entire circuit qualitatively in terms of electron current starting from the ground plz dont explain in terms of conventional current. I want only electron flow
    i want to know how can the voltage drop cause forward bias to the base emitter
     

    Attached Files:

  2. jcsd
  3. Dec 4, 2009 #2
    Voltage divider bias

    i cant understand the voltage divider bias
    plz can any1 explain in terms of electron movement i.e how direction of electron flow
    and how a voltage drop can cause a bias i am totally confused and cant understand it
    because it is necessary for me to understand single stage ce amp
    i have provided the circuit
     

    Attached Files:

  4. Dec 4, 2009 #3

    berkeman

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    Staff: Mentor

    Re: Voltage divider bias

    Is it that you are having trouble understanding how a voltage divider works (not likely), or how BJT bias works for CE amps? What can you tell us so far about how a CE BJT is biased? What does your textbook say? Does the wikipedia introductory article help?

    http://en.wikipedia.org/wiki/Bipolar_transistor_biasing

    .
     
  5. Dec 4, 2009 #4
    Re: Transistor AMPLifier HelP !!!!

    What's a relationship between collector and emitter current?

    Consider how the base voltage forces the emitter voltage due to a diode voltage drop?
     
  6. Dec 4, 2009 #5
  7. Dec 4, 2009 #6
    Re: Voltage divider bias

    i have a serious doubt about biasing a pn diode and npn transistor
    first i'll mention about pn diode biasing
    the basic doubt i have in pn diode is that
    during forward bias + is connected to P and - is connected to N (+terminal -terminal)
    what i read in hyperphysics is that the electrons in N type cross the holes and go to the positive terminal and equal electrons from negative terminal flows to the N type again
    in this way the diode forward bias explanation is convincing
    but what i read in this site
    http://www.tpub.com/content/neets/14179/css/14179_32.htm
    is that the electrons first start from the negative terminal and go to N type and cross barrier potential and go to p type and go back to positive terminal
    in the latter explanation the pn junction is not anything special
    it is just like a ordinary small resistor dropping .7 volts.
    but the latter explanation is used everywhere even in rectifier they say diode does not allow current to pass from the battery in the other direction and allows the current to pass through it only in one direction
    then what is the use of inventing it calling it electronics devices
    nothing special is happening becuase no1 is explaining in terms of what is first happening in diode(my first explanation from hyperphysics).
    everywhere they are explaining in terms of electrons or current coming from the battery
    i want to know how rectifier bridge rectifier works i.e starting from the diode and not from supply. because if we start only from the diode we can say its electronics
    if we simply start from the supply its none other than simply electrical

    I have the same doubt in npn transistor which i will mention in the subsequent posts
    plz some1 clear my doubt
     

    Attached Files:

  8. Dec 4, 2009 #7
    Re: Voltage divider bias

    to be frank i cant understand ur circuit
    basically i cant understand voltage divider bias
    plz what ever circuit u show me can u plz mark me the electron movement
    in this circuit i have many questions
    1)plz can u mark direction of electron flow
    2)the circuit is open how can current of electron flow
    3)i cant understand voltage divider bias i.e how can a voltage drop cause bias
    4)why Re ( resistance ) is used. in my book its mentioned stabilisation what is that
    5)why capacitor C1 is used
    6)major doubt
    from where does the electron flow start
    this one is making me mad
    in the working its given the electrons flow form emitter to base and to collector i.e its given it starts from the emitter
    but in single stage ce amplifier its give its statrs form Vcc and signal
    which is right?
     

    Attached Files:

  9. Dec 5, 2009 #8
    Re: Transistor AMPLifier HelP !!!!

    Electrons are pushed out of the -Ve terminal of battery and attracted by +Ve terminal.

    A diode has P and N terminals, N with excess -Ve charges(electrons), P with excess +Ve charges(holes). N is connected to -Ve of battery, N's excess -Ve charges are pushed to wards P and pulled in by Battery

    If N is connected to +V of battery and P to -Ve of battery, then the excess -Ve charges are being pulled by +Ve bat but can't escape, similarly for the other end.

    A transistor is not equal two diodes. It is a special device. Its collector current depends upon base current. BE voltage remain about 0.7V in forward bias but CE voltage depend upon base current.

    Potential divider place a certain voltage at base. (V*R1/(R1+R2). Emitter voltage are less by 0.7V. Emitter current is Ve/Re. Base current is Ie/(beta+1), collector current is beta*Ib.
     
  10. Dec 5, 2009 #9
    Re: Transistor AMPLifier HelP !!!!


    How does collector current depend upon the base current?
    I want a clear explanation on this on the basis of electron motion.


    What is BE voltage and CE voltage?IS it the barrier voltage? If it is so since BE is forward biased the barrier potential disappears then where does the .7 volts come from?

    Potential divider place a certain voltage at base. (V*R1/(R1+R2). Emitter voltage are less by 0.7V. Emitter current is Ve/Re. Base current is Ie/(beta+1), collector current is beta*Ib. [/QUOTE]

    How did u get that formula? I saw the book but actually the resistances R1 and R2 are in parallel, then how did u simply add it up?
    How is emitter current is less than .7V?

    Hey my question is very big.You didnt give answers for everything!!!
     
  11. Dec 5, 2009 #10

    sophiecentaur

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    Science Advisor
    Gold Member

    Re: Transistor AMPLifier HelP !!!!

    Sudar
    The first sentence of your OP worries me. I think you want too much too fast. It seems that you have been overwhelmed with too much info. Would I be right?
    It can't be done in the way you ask, because the whole system is too complicated. You have to approach it in stages and in layers.
    First, you could get to understand the electron flow in a simple, metallic conductor and find how it gives you Ohm's law behaviour. Then get familiar with the use of Ohm's law and all the normal equations used for simple circuits, which just discuss Current, charge and Volts. (No Electronics Engineer sits down to design a circuit with electrons in his mind - except if something is wrong or he's breaking new ground).
    When you have done that, you will understand the operation of a potential divider.
    Some time later (!!) you may want to look at how a Transistor operates but that is several orders of magnitude harder. There really isn't an easy way to understand the details of solid state electronics - Solid State Physics is Degree level stuff. You can easily, however, get to appreciate what they do and how the volts and currents associated with the three terminals are related. The way that one forward biased and one reverse biased diode work as a transistor can just about be described satisfactorily in an arm waving kind of a way. I recommend that you get into the subject that way.
    This is not a put-down. I'm being realistic and pragmatic about it, having been there myself.
     
  12. Dec 5, 2009 #11
    Re: Transistor AMPLifier HelP !!!!

    Sudar-
    Please refer to the two attached thumbnails, and refer to them when asking questions. The first one shows the quiescent voltages and electron(e) currents for an npn emitter follower circuit with a collector resistor. The second thumbnail shows the base, emitter, and collector voltages when a 1-volt peak ac voltage is applied to the base.

    Bob S
     

    Attached Files:

  13. Dec 5, 2009 #12
    Re: Transistor AMPLifier HelP !!!!

    Ahhhh!!!
    BoBS you always came to my rescue right from the beginning i joined PF.
    1)What is V2 and V1 in the figure?

    2)Also you have marked 4 volts to the base,but actually the 4 volts gets dropped and the electrons have 8 volts (12-4) remaining at the base.

    3)You have calculated the values for conventional i.e u have marked 12v at the top and 0 at the bottom, but actually if we consisder electron flow 12v must be at the bottom and 0 must be at the top.

    4)V emitter must be 8.6 (12-3.4) 3.4 is the voltage dropped.

    5)Also electron current through R2(500 ohm) must be .016 i.e 16mA ( 8/500=.016=16mA)
    8 volts came from 8.6-.6 (.6 is the barrier voltage).

    6)BOBS i have calculated all the values taking the electron voltage to be 12V
    but i cant type it in 'microsoft paint'.

    7)Why that capacitor is used? What do u mean by coupling, i want a detailed explanation on it

    8) What is Vce , there is no barrier voltage extending from emitter to collector then what do u mean by Vce .Also what is Vcc ( in CE configuration)

    9)Also my most important doubt .
    From where does the electron flow start???
    From the supply Vcc or from the emitter ??
    If the latter then the 12 V electrons starting from the emitter drops .6 in the depletion region then 8.6 V in R2,but it has to drop all the 12v
    plz clarify this

    dont mind about the points 2 ,3,4,5 mentioned by me .I'll take care of it.
    But plz clarify point 1 ,7,8,9
    Thanks BOBS for ur efforts and understanding my difficulties.:approve:

    p.s Also if u consider the electron flow movement then in the graph Vemitter must be above than Vbase
     
    Last edited: Dec 5, 2009
  14. Dec 5, 2009 #13
    Re: Transistor AMPLifier HelP !!!!

    1) V1 is a 12 volt dc power supply. V2 is a 1-volt peak ac sine wave power supply. It is ac-coupled to the npn base.

    2) I am measuring the volts above ground. This is very much like a cathode-bias vacuum tube triode: cathode= emitter; grid = base, and plate = collector; same voltage scheme.

    3) Electrons flow from the bottom to the top just like vacuum tubes.

    5) Current through R2 is 3.4volts/500 ohms = 6.8 milliamps.

    7) Capacitor is used so as not to disturb the dc operating point of transistor.

    8) Vce just means the dc collector-to-emitter voltage. In this circuit, Vce = 8.6 V – 3.4 V = 5.2 volts.

    9) Electrons start at ground, flow through R1 to npn emitter, out the npn collector, through R2 to + 12 volts.

    ps) The base current (electrons) flow in the emitter and out the base.

    I hope this helps.

    Bob S
     
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