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why bjt transistor is called current operated device

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Junior Member level 3
Jun 2, 2005
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a bjt is a

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

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.

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.

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)

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

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

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..

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.

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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