GATE 2015 EE - SET 1 Question 14

Solution :

\frac{R_{B}}{R_{C}} = ?

 V_{C}= 9V , \beta = 75
V_{BE} = 0.7V

I_{C}= \beta I_{B}

I_{C} + I_{B}  =\frac{15-V_{C}}{R_{C}} = \frac{15-9}{R_{C}} = \frac{6}{R_{C}}

I_{C} + I_{B} = \frac{6}{R_{C}}

\beta I_{B} + \ I_{B} \frac{6}{R_{C}}

 \Rightarrow 76 I_{B} \frac{6}{R_{C}}  ---------(1)

 

I_{B}= \frac{9-0.7}{R_{B}} = \frac{8.3}{R_{B}}
\therefore I_{B}=  \frac{8.3}{R_{B}}

\therefore 76I_{B} = \frac{6}{R_{C}}

76 \left ( \frac{8.3}{R_{B}} \right ) = \frac{6}{R_{C}}

\frac{R_{B}}{R_{C}}= \frac{76*8.3}{6} =105.1