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  1. #1
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    a simple basic question

    hey guys
    i want to know why do we use biasing resistors in BJT. i mean i common emitter mode for example, we use a resistor with in series with base

    whats the reason for this. someone in class said that its because we cant directly connect 5 volts to a transistor. i couldnt digest it because if we cant do it then why is emitter connected without a resistor to same voltage source. so this reason was definitely not right

    can anyone tell me the real reason as i am kinda stuck at it. the teacher keeps on saying its necessary for biasing but doesnt tell how and why

    •   Alt21st April 2007, 08:14

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    Re: a simple basic question

    BJT is current device. The main equations of work is Ie = Ib + Ic and Ic = B × Ib.
    Resistor converts voltage to control current of BJT.



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    Re: a simple basic question

    TO set the base current.



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    Re: a simple basic question

    but our teacher said that for MOS device its not needed. we can bias it without a resistor too but for BJT its needed
    so there must be some good reason for it other than just controlling the base current



    •   Alt21st April 2007, 11:17

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    Re: a simple basic question

    BJT is current controlled device. for setting up the current one converter should required you can say now I =v/R by changing value of r you can change the current at 5 v level this is the reason we use the biasing resisstor.



    •   Alt21st April 2007, 12:50

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    Re: a simple basic question

    Biasing the BJT means making its base emitter junction forward biased and base collector junction reverse biased (seeing them as diode junctions)to work for example as an amplifier so say we use a 5V supply , In order to make that supply do all the work (forward biasing -emitter/base- and reverse biasing-collector base )we shall put much of the volt on the collector base junction and a small portion of it on the emitter base junction and this is achieved by the resistor.While for MOS we only need a voltage on the gate greater than the threshold voltage (with respect to the source )to work (i.e. only one target is needed).



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    Re: a simple basic question

    Can you tell me what happens if you connect a diode (Forward biased) directly across a 5V supply?

    Well, the eqn of diode ( I = Is * e(nV/Vt)) tells us that a HUGE current will tend to flow; causing either the supply to reduce its terminal voltage from 5V volts (the rest of the voltage drops a/c the source resistance) ... or .. damaging the diode.

    Similarly, the E-B junction is simply a diode (pn junction)...thus it can not be connected directly a/c a 5V supply without a resistor.

    However, if you configuration you are using is not CE-configuration....then the emitter is not directly grounded ... and thus it is a floating (variable voltage) node....and can fix its voltage to make the voltage Vbe = 0.7V; no matter what DC source is applied..


    HOPE THIS HELPS!


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    Re: a simple basic question

    thanks quaternion and zeeshan...both of your replies were really helpful

    hey zeeshan your reply was very detailed one...could you suggest any good book on electronics that may clarify my conceptual problems like this one as you know most of the books dont give such minute conceptual details and neither do teachers in the class

    and yeah one thing.. what would u say about this darlington pair
    there is no biasing resistor...wouldnt it matter here?



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    Re: a simple basic question

    mujee,
    The BJTs, including the Darlington, are current controlled devices. In other words, the collector current is proportional to the base current. The MOSFET is a voltage controlled device. The drain current is proportional to the gate voltage. In a "self biased circuit" where you have a resistor between emitter and ground, you can connect a voltage source to the base. The emitter voltage will be one diode drop below the base voltage. The emitter current wil be equal to the emitter to ground voltage divided by the emitter resistance. The base current will automatically adjust itself to provide the required emitter current.
    Regards,
    Kral

    Added after 1 minutes:

    I recommend "The Art of Electronics" by Horowitz and Hill as an introductory book on electronics.



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    Re: a simple basic question

    Quote Originally Posted by Kral
    mujee,


    Added after 1 minutes:

    I recommend "The Art of Electronics" by Horowitz and Hill as an introductory book on electronics.
    check here for student manual

    h**p://www.edaboard.com/ftopic245191.html



  11. #11
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    a simple basic question

    The resistors control the base current as well as put the transistor in the active region. If you directly connect 5V it will go to saturation. Biasing is all about poitioning the transistor int he correct region of operation.



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