Q. What are the Current relations in common emitter configuration?
In a CB configuration Ie is the input current and Ic is the output current. These currents are relayed through the equations given below
I_{e}=I_{c}+I_{b}
I_{c}=II_{e}+I_{cb}0
In CE configuration, Ib becomes the input current and the Ic is the output current. We are interested in knowing how the output Ic is related with the input current Ib. That is, we should find a relation such as
I_{c} = f(I_{b})
To obtain the relation, we simply substitute the expression of Ie from the equations given earlier,so that
I_{c} = ID_{c}(I_{c}+I_{b}) + I_{cbo}
or
(1-I_{dc})I_{c}=I_{dc}I_{b}+I_{cbo}
I_{c} = (I_{dc}/(1-I_{dc}))I_{b}+(1/(1-Id_{c}))I_{cbo}
In thjis equation,Ic is given in terms of Ib. The equations can be simplified somewhat by defining
? I_{dc}/(1-I_{dc})
and
I_{ceo}=I_{cbo}/(1-I_{dc})
thus equation becomes
Ic=?_{dc}Ib+I_{ceo}