Differential vs. common-mode inputs — What is the key difference between common-mode and differential-mode input signals for a differential amplifier (such as an op-amp front end)?

Introduction:
Differential amplifiers respond to differences between two input nodes while rejecting signals that are common to both. Distinguishing between common-mode and differential-mode signals is crucial for instrumentation, noise rejection, and accurate sensor interfacing.

Given Data / Assumptions:

• Two input terminals: noninverting (+) and inverting (−)
• Linear operation of the differential front end
• Focus on signal composition rather than absolute amplitudes

Concept / Approach:

Common-mode (CM) signals are those that are substantially the same on both inputs (same magnitude and phase). Differential-mode (DM) signals are equal in magnitude but opposite in polarity (180° phase difference) or simply unequal, creating a nonzero difference. Thus, the defining distinction is their phase/polarity relationship across the two inputs.

Step-by-Step Explanation:

Verification / Alternative check:

Examine waveforms: identical waveforms in phase on both inputs are CM; two equal-amplitude, opposite-phase waveforms are DM. This shows phase relationship is the salient differentiator.

Why Other Options Are Wrong:

• voltage/current/apparent power/impedance: These may vary, but they do not fundamentally define CM vs DM; the phase/polarity relationship does.

Common Pitfalls:

• Assuming any equal amplitudes imply CM even when phases differ.
• Ignoring that practical amplifiers have finite CMRR, so some CM leaks into the output.

Final Answer:

phase relationship