Question

What is the CMRR (Common Mode Rejection Ratio) of an ideal Operational Amplifier?

• A. 0
• B. 1
• C. Infinite
• D. Very small

Hint:

Ideal Op-Amp → $A_d = \infty$, $A_c = 0$ → CMRR = $\infty$.

Answer 1

The Correct Option is C

Concept: CMRR (Common Mode Rejection Ratio) is a measure of an operational amplifier's ability to reject common-mode signals (signals that are present simultaneously and in-phase on both inputs).
Step 1: Definition of CMRR.
\[ \text{CMRR} = \frac{A_d}{A_c} \] where:

• $A_d$ = Differential gain
• $A_c$ = Common-mode gain

Step 2: Ideal Op-Amp condition.
For an ideal operational amplifier:

• Differential gain ($A_d$) is infinite
• Common-mode gain ($A_c$) is zero

Step 3: Resulting CMRR.
\[ \text{CMRR} = \frac{\infty}{0} = \infty \]
Step 4: Interpretation.
An infinite CMRR means the op-amp completely rejects common-mode signals and amplifies only the difference between inputs.
Step 5: Conclusion.
Thus, the CMRR of an ideal op-amp is infinite.