Bipolar Junction Transistor

  • #1
Does the collector current in a transistor depend on the base-collector voltage in a common base configuration? If so,how does it depend mathematically?
 

Answers and Replies

  • #2
LvW
819
220
The principle behaviour of a BJT does not depend on the specific configuration (common emitter, base or collector).
In any case, the famous Shockley equation applies: Ic=Is[exp(Vbe/VT) -1].
Hence, the collector current always is determined and controlled by the base-emitter voltage only.
I am aware that in some textbooks and other publications (in particular: Internet) it is stated that Ic would be controlled by Ib because of the relation Ic=B*Ib.
This relation holds, of course, and it is often used for designing simple transistor stages - however, it does not tell anything about cause and effect.
There are many properties and effects to be observed in BJT applications that can be explained ONLY with the above mentioned equation Ic=f(Vbe).
(I would not be surprised if this reply would cause some disagreements from other forum members; I have corresponding experiences.).
 
  • #3
The principle behaviour of a BJT does not depend on the specific configuration (common emitter, base or collector).
In any case, the famous Shockley equation applies: Ic=Is[exp(Vbe/VT) -1].
Hence, the collector current always is determined and controlled by the base-emitter voltage only.
I am aware that in some textbooks and other publications (in particular: Internet) it is stated that Ic would be controlled by Ib because of the relation Ic=B*Ib.
This relation holds, of course, and it is often used for designing simple transistor stages - however, it does not tell anything about cause and effect.
There are many properties and effects to be observed in BJT applications that can be explained ONLY with the above mentioned equation Ic=f(Vbe).
(I would not be surprised if this reply would cause some disagreements from other forum members; I have corresponding experiences.).
Thanks. Can you also explain physically why it does not depend on the voltage of the other junction?
 
  • #5
LvW
819
220
Thanks. Can you also explain physically why it does not depend on the voltage of the other junction?
It depends! (I assume the question is if Ic depends, correct?).
The collector current is mainly determined by the base-emitter voltage - however, there is also a small dependency on the collector-base voltage (backward biased).
This dependency is visualized by the "Early voltage" which is a measure of the BJT`s output conductance (finite slope of the Ic=f(Vce) characteristics).
 

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