Question Statement:
For an Op-Amp based negative feedback, non-inverting amplifier, which of the
following statements are true?
Options:
-
(A) Closed loop gain < Open loop gain
-
(B) Closed loop bandwidth < Open loop bandwidth
-
(C) Closed loop input impedance > Open loop input impedance
-
(D) Closed loop output impedance < Open loop output impedance
Official Answer Key:
A, C, D
Step-by-Step Solution & Explanation
### **Official Solution & Step-by-Step Derivation (GATE PH 2022 Q.20)**
**1. Analog Electronics Principles (Negative Feedback Non-Inverting Op-Amp)**:
- Let $A_{\text{OL}}$ be open-loop gain, $R_{\text{in}}$ be open-loop input impedance, $R_{\text{out}}$ be open-loop output impedance, and $\beta$ be the feedback factor ($0 < \beta \le 1$).
**2. Step-by-Step Property Verification**:
- **Gain**: Closed-loop gain $A_{\text{CL}} = \frac{A_{\text{OL}}}{1 + A_{\text{OL}}\beta} < A_{\text{OL}}$. **Statement (A) is TRUE**.
- **Bandwidth**: The gain-bandwidth product is conserved, so closed-loop bandwidth is $BW_{\text{CL}} = BW_{\text{OL}}(1 + A_{\text{OL}}\beta) > BW_{\text{OL}}$. Statement (B) is FALSE.
- **Input Impedance**: For non-inverting amplifier (series input feedback), input impedance increases:
$R_{\text{in, CL}} = R_{\text{in}}(1 + A_{\text{OL}}\beta) > R_{\text{in}}$
**Statement (C) is TRUE**.
- **Output Impedance**: For voltage shunt feedback at the output, output impedance decreases:
$R_{\text{out, CL}} = \frac{R_{\text{out}}}{1 + A_{\text{OL}}\beta} < R_{\text{out}}$
**Statement (D) is TRUE**.
**3. Official Verification**:
The verified official IIT Master Answer is **Options (A, C, D)**.
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