Definition check — The ratio of differential gain (Ad) to common-mode gain (Acm) in a differential amplifier is called what?

Introduction:
Differential amplifiers should amplify differences while rejecting signals common to both inputs. A precise metric quantifies this ability and appears on every op-amp datasheet, often in decibels for convenience.

Given Data / Assumptions:

• Differential gain Ad amplifies V1 − V2
• Common-mode gain Acm amplifies (V1 + V2) / 2
• Linear small-signal regime

Concept / Approach:

The figure of merit is CMRR, defined as CMRR = Ad / Acm. In practice, it is reported in dB: CMRR_dB = 20 * log10(Ad / Acm). A large CMRR means excellent rejection of common-mode interference such as power-line hum or ground shifts.

Step-by-Step Explanation:

Verification / Alternative check:

Datasheets list both Ad and CMRR; dividing Ad by Acm matches published CMRR, confirming the definition.

Why Other Options Are Wrong:

• amplitude / phase: Not ratios of the specific gains.
• differential-mode rejection: Nonstandard term; the amplifier should not reject differential signals.
• common-mode rejection: Describes the phenomenon, not explicitly the ratio unless stated as “ratio”.

Common Pitfalls:

• Confusing CMR (concept) with CMRR (quantified ratio).
• Mixing power (dB) with voltage gain without the 20× factor.

Final Answer:

common-mode rejection ratio