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    a bjt is a

    why BJT is called current operated device allthough its output depends on the base voltage

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    Re: why bjt transistor is called current operated device

    i think it is because you change the Ic current changing the Ib current. In Fet´s you change the Vg to change Is. So fets are voltage operating devices.



    •   Alt13th December 2005, 18:06

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    Re: why bjt transistor is called current operated device

    A BJT transistor has its allowed current between Collector and Emitter terminals determined by the base current. The Vbe voltage is necessary to "turn on" the transistor and after a certain level it remains slightly constant, so the variation of the current flowing through the collector and emitter is determined by the variation of the base current. If no base current flows, there is no Vbe to turn the transistor on.



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    Re: why bjt transistor is called current operated device

    infact a bjt is called as current controlled device.
    output current (Ic) is controlled by input cuurent(Ib) by a factor β

    consider a base resistor baising circuit (Re = 0)

    i/p side:

    Ib = (Vcc - Vbe) / Rb

    Ib is a function of Rb in math terms
    in this equation, both Vcc n Vbe are constants. only Rb can be varied
    i.e., Ib depends on value of Rb (Ib descreases with increase in value of Rb)

    o/p side:

    Vce = Vcc - Ic.Rc

    Vcc is a function of Rc
    Vcc is constant as we said earlier, Ic = βIb. indicates current in output circuit is not at all depends on the component values (circuit configuration) of the output circuit.
    ** Thus output circuit current is independent of output circuit variables and depends only on input current.

    back to o/p eq.
    only variable in output eq. is Rc
    implies Vce(Collecter to Emitter Voltage) increases with decrese in Rc


    final notes:
    *** more the value of Rb --> less Ib --> less Ic --> more Vce
    these are voltage amplifiers (Resistances in KΩ, Ic in mA)
    *** less Rb --> more Ib --> more Ic --> less Vce
    these are power amplifiers (Resistances in Ω, Ic in A)



    •   Alt14th December 2005, 07:09

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  5. #5
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    why bjt transistor is called current operated device

    In BJT, gain depend on beta, and beta = IC/IB.
    it is not depend on VBE.



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    Re: why bjt transistor is called current operated device

    the current Ic depens on Ib. you can see the reason in some books for electronic devices and electronic physics.



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    Re: why bjt transistor is called current operated device

    b/c it is controlled by the base current Ib(not by base voltage)

    like FET are controlled by gate voltage(not gate current), so are called voltage controlled devices..
    i think its simple for u to understand..



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    Re: why bjt transistor is called current operated device

    output, Vce=Vcc-Ic. Rc, where Ic is the collector current.

    But this Ic depend on Ib(base current), as Ib=Ic/β

    now this Ib can be found out as Ib= (Vin-Vbe)/Rb , where Vin is the applied base voltage and Vbe=0.7( for silicon)
    =0.3(for germanium)

    So ffrm above u can understand how output of bjt is current controlled though it is inputed by voltage

    hope u understand!!!!!!!!!!!!!!



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    Re: why bjt transistor is called current operated device

    I agree to what davood says.BJT's output does not depend on the input voltage.The input base-emitter voltage is needed to turn ON the device.The out put in ON state depends on the input current.



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    Re: why bjt transistor is called current operated device

    BJT described as a current operated device because the collector current is controlled by the current flowing between base and emitter terminals.



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