Which statement best defines CMRR and its importance in differential amplifiers?

• A. The ratio of common-mode gain to differential gain; this ratio is CMRR and a higher value improves common-mode rejection.
• B. The ratio of differential gain to common-mode gain; higher CMRR reduces sensitivity to common-mode noise and interference.
• C. The ratio of input impedance to output impedance; CMRR measures bandwidth.
• D. A measure of input impedance mismatch in a differential pair.

Answer: (B)

CMRR measures how well a differential amplifier rejects signals that appear on both inputs. It’s defined as the ratio of the amplifier’s differential gain (how much it amplifies the difference between inputs) to its common-mode gain (how much it amplifies signals that are the same on both inputs). In formula form, CMRR = A_diff / A_cm, and in decibels it’s 20 log10(A_diff/A_cm). A larger ratio means the output follows only the difference between the inputs, with very little of the common noise getting through.

This matters because real-world noise and interference—like power-line hum or EMI—that appear equally on both inputs should ideally be canceled out. A high CMRR makes the differential amplifier less sensitive to that common-mode noise, preserving the actual signal of interest.

The other statements mix up what’s being compared or refer to concepts like impedance or bandwidth that aren’t what CMRR measures.