Question

Compute RIF for an inverting amplifier with feedback, where the value of input resistance of op-amp is 4.7kΩ.

(a) 4.7kΩ

(b) 9.4kΩ

(c) 2.35kΩ

(d) Data given is insufficient

I have been asked this question at a job interview.

I need to ask this question from Voltage Shunt Feedback Amplifier in division Operational Amplifier Characteristics of Linear Integrated Circuits

Answer 1

In an inverting amplifier with feedback, the input resistance with feedback RIFR_{IF}RIF​ is given by the formula:

RIF=Rin∥(Rf+1A)R_{IF} = R_{in} \parallel \left( R_f + \frac{1}{A} \right)RIF​=Rin​∥(Rf​+A1​)

Where:

• RinR_{in}Rin​ is the input resistance of the op-amp (given as 4.7kΩ in this case),
• RfR_fRf​ is the feedback resistor (which is not given in this question),
• $A$ is the open-loop gain of the op-amp (also not provided).

Since the problem only gives the value of RinR_{in}Rin​ (4.7kΩ) but does not provide information about the feedback resistor RfR_fRf​ or the open-loop gain $A$, the data is insufficient to compute RIFR_{IF}RIF​.

Thus, the correct answer is:

(d) Data given is insufficient.