An amplifier without feedback has a voltage gain of 50, input resistance of 1 kΩ & Output resistance of 2.5 kΩ.The input resistance of the current shunt negative feedback amplifier using the above amplifier with a feedback factor of 0.2 is

Answer: Option A

The correct answer is Option A: $\frac{1}{11}$ kΩ

Explanation:

The input resistance of a current shunt negative feedback amplifier is given by the formula:

$$R_{in(f)}=\frac{R_{in}}{1+A\beta }$$

where,

$R_{in}$ = Input resistance without feedback = 1 kΩ

$A$ = Open-loop voltage gain = 50

$\beta$ = Feedback factor = 0.2

Substituting the values:

$$R_{in(f)}=\frac{1}{1+(50\times 0.2)}$$

$$R_{in(f)}=\frac{1}{1+10}=\frac{1}{11}\text{ kΩ}$$

Thus, the input resistance of the current shunt negative feedback amplifier is $\frac{1}{11}$ kΩ, which corresponds to Option A.