Question:

For CE transistor amplifier, the audio signal voltage across the collector resistance of 2kΩ is 4V. If the current amplification factor of the transistor is 100 and the base resistance is 1kΩ, then the input signal voltage is?

10mV

20mV

30mV

5mV

Given that:
Ic/Ib = 100; RB = 1000Ω ⇒ β = 100 = Ic/Ib ⇒ Vo = 4V
⇒ Vo/Vi = βRL/RB ⇒ Vi = 4 × 1000 / (100 × 2000) = 0.02 = 20 mV