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A signal source of internal resistance 1.2 kΩ is connected to the input of an amplifier with a voltage gain of 140. What is the value of input resistance the amplifier should have to get an overall gain of 100?

A signal source of internal resistance 1.2 kΩ is connected to the input of an amplifier with a voltage gain of 140. What is the value of input resistance the amplifier should have to get an overall gain of 100? Correct Answer 3 kΩ

Overall gain = Gain(AV) × Ri/(Ri + Rs)

Where,

Ri = input resistance

Rs = source resistance

Therefore, applying formula we get,

100 = 140 × Ri/(Ri + 1.2 k ohm)

Ri­ = 3 K ohm

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