GATE 2015 EE – Question 24

Analog and Digital Electronics · Amplifiers: biasing, equivalent circuit and frequency response · 1 mark · Numerical answer

In the given circuit, the silicon transistor has $\beta = 75$ and a collector voltage $V_C = 9$ V. Then the ratio of $R_B$ and $R_C$ is ________.

A 15 V supply feeds a collector resistor $R_C$. The base resistor $R_B$ is connected from the collector to the base of an NPN transistor with its emitter at ground.

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Correct answer: 100 to 110

Explanation

The collector resistor drops $15 - 9 = 6$ V while carrying $I_C + I_B = 76\,I_B$. The base resistor drops $9 - 0.7 = 8.3$ V while carrying $I_B$. So $\frac{R_B}{R_C} = \frac{8.3/I_B}{6/(76 I_B)} = \frac{8.3 \times 76}{6} = 105.1$.