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  4. Consider an n-p-n transistor amplifier in common emitter configuration. The current gain of the transistor is 100. If the collector current changes by 1 mA, what will be the change in emitter current?

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Q. Consider an $n-p-n$ transistor amplifier in common emitter configuration. The current gain of the transistor is $100$. If the collector current changes by $1\, mA$, what will be the change in emitter current?

AIIMSAIIMS 2005Semiconductor Electronics: Materials Devices and Simple Circuits

Solution:

Current gain is defined as ratio of collector current to base current.
Current gain $=\frac{\text { change in collector current }}{\text { change in base current }}$
$\beta=\frac{\Delta i_{C}}{\Delta i_{B}}$
Also $i_{E}=i_{B}+i_{C}$
$\Rightarrow \Delta i_{E}=\Delta i_{B}+\Delta i_{C}$
$\beta=\frac{\Delta i_{C}}{\Delta i_{E}-\Delta i_{C}}$
Given, $\beta=100, \Delta i_{C}=1\, mA$
$\therefore 100=\frac{1}{\Delta i_{E}-1}$
$\Delta i_{E}-1 =\frac{1}{100}=0.01$
$\Delta i_{E} =1+0.01=1.01\, mA.$
In common-emitter amplifier, the output voltage signal is $180^{\circ}$ out of phase with the input voltage signal.