Question

A transistor is used as a common emitter amplifier with a load resistance 2 kΩ. The input resistance is 150 Ω. Base current is changed by 20 μA which results in a change in collector current by 1.5 mA. The voltage gain of the amplifier is

• A. 1100
• B. 1200
• C. 900
• D. 1000

Answer 1

Answer: (D)

1000

Answer 2

The voltage gain of a common emitter amplifier can be calculated using the formula:
Voltage Gain = - β * $\frac {Load \ Resistance}{Input \ Resistance}$
where:
β is the current gain of the transistor, Load Resistance = 2 kΩ and Input Resistance = 150Ω.
To find the voltage gain, we need to determine the value of β, which is the current gain of the transistor. The current gain (β) is defined as the ratio of the change in collector current (ΔIc) to the change in base current (ΔIb).
β = $\frac {ΔIc}{ΔIb}$
Given that the change in collector current (ΔIc) is 1.5 mA and the change in base current (ΔIb) is 20 μA
β = $\frac {1.5 \ mA}{20\ μA}$ = 75
Now
Voltage Gain = - β * $\frac {Load \ Resistance}{Input \ Resistance}$
Voltage Gain = -75 * $\frac {2000\ Ω}{150\ Ω}$ = -75 * 13.333 = -1000
Since the voltage gain can be negative for a common emitter amplifier, the correct answer is (D) 1000.