# transistor biasing circuit : need help to calculte it

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#### Sebastie

##### Junior Member level 1
Hello all,
I would want to know he someone could help me to simplify the first schematic to the second one. i would want to know what is the method to calculate the Rb resistor ... ? (I suppose I have to use thevnin, but I don't success to do it).
Could you help me ?  thanks

Hello all,
I would want to know he someone could help me to simplify the first schematic to the second one. i would want to know what is the method to calculate the Rb resistor ... ? (I suppose I have to use thevnin, but I don't success to do it).
Could you help me ?
Hi sebastie
Thats so simple . if you neglect DC base current ( if that is so small ) then you can simplify it as : R6llR7 then this value will be in series with R1 . isn't it ? and you can also use a simple voltage divide calculations to find that voltage supply . easily .
Best Wishes
Goldsmith

Hello Goldsmith,
Many thanks for your help. Well, I have the answer but I don't understand how they found the expression :
for RB they have : RB=R1+(R6R7)/(R4 +R5+R6+R7)

do you know how they found this ?
regards

Does someone have any idea ?

Hi sebastie,

the problem is that you have two different circuit diagrams with different properties - for example: different feedback schemes.
(by the way: the first circuit looks very strange since R7 is not grounded, are you sure the drawing is correct?).
Now you ask how "to simplify the first schematic to the second one".

When comparing two circuit diagrams you must define some requirements that have to be fulfilled by both circuits.
Example: constant gain, constant collector current, constant power dissipation, constant input resistance....?

I hope you realize that no answer can be given without knowing your requirements.

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