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Question:

An NPN transistor is used in common emitter configuration as an amplifier with 1kΩ load resistance. Signal voltage of 10mV is applied across the base-emitter. This produces a 3mA change in the collector current and 15µA change in the base current of the amplifier. The input resistance and voltage gain are:

0.33kΩ,300

0.67kΩ,300

0.33kΩ,1.5

0.67kΩ,200

Solution:

Correct option is D. 0.67kΩ,300
input current = 15 × 10⁻⁶; output current = 3 × 10⁻³
resistance output = 1000
Vinput = 10 × 10⁻³
Vinput = rinput × iinput
10 × 10⁻³ = rinput × 15 × 10⁻⁶; rinput = 2000/3 = 0.67kΩ
voltage gain = Voutput/Vinput = 1000 × 3 × 10⁻³/10 × 10⁻³ = 300