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  4. The input resistance of a silicon transistor is 665 Ω. Its base current is changed by 15 μ A which results in the change in collector current by 2 mA. This transistor is used as a common emitter amplifier with a load resistance of 5 k Ω. What is the voltage gain of the amplifier?

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Q. The input resistance of a silicon transistor is $665 \,\Omega$. Its base current is changed by $15 \, \mu \,A$ which results in the change in collector current by $2 \,mA$. This transistor is used as a common emitter amplifier with a load resistance of $5\, k\,\Omega$. What is the voltage gain of the amplifier?

JIPMERJIPMER 2013Semiconductor Electronics: Materials Devices and Simple Circuits

Solution:

$R_{i}=665 \Omega, \Delta I_{B}=15 \mu A =15 \times10^{-6} A $
$\Delta I_{C}=2 mA = 2 \times10^{-3} A ,R_{L}=5 k\Omega =5\times10^{3} \Omega$
$\beta=\frac{\Delta I_{C}}{\Delta I_{B}} =\frac{2\times10^{-3}}{15\times10^{-6}} =\frac{400}{3} $
Voltage gain, $A_{V }=\beta. \frac{R_{L}}{R_{i}}$
$ A_{V}=\frac{400}{3}\times\frac{5\times10^{3}}{665}=1002.5$