Hello,

I have a problems with equations in bjt.Let me explain with an example :


Circuit

It is clear that the base current depends on RB by mesh analysis :


Mesh-analysis

However if we think about the collector current formula which is Ic = Is . e^(Vbe/Vt)

In this formula Is, Vbe and Vt are constant.(at least for ideal bjt).So collector current is constant.Then , base current is constant(Ic=β.Ib).Finally emitter current is also constant because it is sum of base and collector current.As a result, there is no importance of resistance in circuit for currents.Because currents are already determined by formula.I know this is not true but I couldn’t find an explanation.Can you help me?

Thank you,

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The transistor is considered a current source, according to the equation, therefor the resistor RC is not used to limit collector current, but rather used to develop a voltage across it with respect to the voltage across the transistor.
The transistor and RC form a variable voltage divider.

 

Vbe and Vt are constant.(at least for ideal bjt).

Vt is constant but Vbe is not constant.
It depends upon the base current which is determined by Rb.
The base emitter voltage is basically a forward diode drop as given by this equation where I is the base current and V is Vbe:

 

Vt is constant but Vbe is not constant.
It depends upon the base current which is determined by Rb.
The base emitter voltage is basically a forward diode drop as given by this equation where I is the base current and V is Vbe:
View attachment 140070

Thank you crutschow! Now bjt's are more friendly for me