Question

The input resistance of a silicon transistor is 100. Base current is changed by 40mA which results in a change in collector current by 2 mA. This transistor is used as a common emitter amplifier with a load resistance of 4K. The voltage gain of the amplifier is:
(A) 3000
(B) 4000
(C) 1000
(D) 2000

Answer 1

Hint In this question, the base current is changed and it is reflected upon by the collector current. The emitter current in this case is constant. Hence, this configuration is that of a common emitter transistor. First we need to find the current gain in the circuit. Using this value of current gain, we will find the voltage gain in the circuit using the input and output resistance.

Complete step by step solution
We are given a common emitter circuit in this question. The current gain in common emitter transistor is given as:
 $$\beta ={\displaystyle \frac{\mathrm{\Delta }{I}_C}{\mathrm{\Delta }{I}_B}}={\displaystyle \frac{2\times {10}^{-3}}{40\times {10}^{-6}}}$$
 $$\beta =50$$
To find the voltage gain, we need to use input and output resistance of the transistor.
 $$Voltagegain=\beta {\displaystyle \frac{R_{out}}{R_{in}}}=50{\displaystyle \frac{4000}{100}}=2000$$

Therefore, the option with the correct answer is option D.

Note
In this question, the emitter was made common, i.e. its voltage and current were kept constant. We can make similar transistors which have the common base, or transistors with a common collector. However, the common emitter transistor is highly used in amplifying signals and many other applications.