Question

Determine the closed loop voltage gain from the given circuit. (Where gain of op-amp= 10^5).

(a) 1090.9

(b) 9821.43

(c) 9166.66

(d) 10000

I got this question in an interview for internship.

Question is from Voltage Series Feedback Amplifier in section Operational Amplifier Characteristics of Linear Integrated Circuits

Answer 1

To calculate the closed-loop voltage gain for an op-amp with feedback, we use the formula:

AF=A1+A⋅BA_F = \frac{A}{1 + A \cdot B}AF​=1+A⋅BA​

Where:

• AFA_FAF​ is the closed-loop voltage gain,
• $A$ is the open-loop gain of the op-amp (given as 10510^5105),
• $B$ is the feedback factor (which you would need to determine based on the given circuit, but let's assume you have this value).

In the absence of a specific value for $B$, we will assume that $B$ is given or can be derived from the feedback network in the circuit.

For example, if we assume $B=0.01$ (just as an example), then the closed-loop voltage gain is:

AF=1051+105⋅0.01=1051+1000=1051001≈99.9A_F = \frac{10^5}{1 + 10^5 \cdot 0.01} = \frac{10^5}{1 + 1000} = \frac{10^5}{1001} \approx 99.9AF​=1+105⋅0.01105​=1+1000105​=1001105​≈99.9

But based on the options given (1090.9, 9821.43, 9166.66, 10000), the answer should match with one of these, so either the feedback factor $B$ is different, or there is additional context in the circuit that defines $B$.

Given this information, if we do not know the exact value of $B$, it's difficult to calculate precisely. However, based on the options, the answer seems to be around 10000, which suggests a low feedback factor $B$.

Thus, the correct answer is likely:

(d) 10000

If you'd like, you can share the specific circuit details, and I can help you calculate the exact value for AFA_FAF​.