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Why is BJT called current controlled device and FET voltage controlled ones?

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nigosawa

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hi all
i want to know why why is BJT called current controlled device n FET voltage controlled ones..i just dont get it because the thing is that even in BJT that base current(which they say controls the larger controller current) is actually dependant on voltage applied to base-emitter terminals. so when this current itslef depends on voltage then why dont we say that controller current depends on base-emitter voltage

please help...this is something m trying to find answer of for so long but havent got a single satisfying and clarifing answer yet :cry::cry:
 

Re: basic querry

The most importat effect in a BJT, is that collector current is controled by base current. Ic=B*Ib

In a FET, channel's thickness is controlleb by gate-source voltage.
 

Re: basic querry

nigosawa,
It is true that the base current is a function of Vbe. This is a highly non-linear (exponential) relationship. At the device design level, this relationship is important. At the circuit level, it is more convenient to deal with the Ib/Ic relationship (Ic = Beta*Ib)for two reasons:
1) The relationship is relatively linear for small variations in current
2) Beta is a parameter that is specified
by transistor manufacturers. The Ic/Vbe relationship is not.
Regards,
Kral
 

basic querry

simply u hae the c/c's of the MOS and BJT , the MOS has a vairable Current Ids with respect to Vds and the curves are changed with respect to Vgs "voltage" ,while in BJT current Ic is varried with Vce and the curves are changed with respect to IB ,
i.e. to get a current output u are in saturation region in MOS where the current Ids is constant for a certain Vgs while in BJT u r in active region and current Ic is constant for a certain Ib
 

Re: basic querry

OK,first Observe when the voltage is increasing upto what value it increases? It remains constant at .8 Volts ,then it is not going to change.Means the voltage cannot controll the output.
 

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