Common-mode signals at the inputs of a differential amplifier: A signal is said to be common-mode if the two input nodes see signals with which shared properties?

Introduction / Context:
Differential amplifiers and op-amps respond primarily to differences between the two inputs. Signals that appear identically on both inputs are called common-mode signals, and good amplifiers reject them to minimize interference and noise transmission.

Given Data / Assumptions:

• Both inputs are driven by the same time-varying waveform.
• Definitions are in the voltage domain.
• Amplifier is linear around its operating point.

Concept / Approach:
A common-mode signal is identical at the two inputs in amplitude, phase, and frequency (and ideally in waveform shape). Because the amplifier ideally amplifies only the difference v(+) − v(−), such a shared signal should produce no output in the ideal case. Real devices have finite CMRR, so a small residual output can occur.

Step-by-Step Solution:

Verification / Alternative check:
CMRR tests inject the same sinusoid (same amplitude, phase, frequency) on both inputs and measure the small output; this matches the definition used by manufacturers.

Why Other Options Are Wrong:
Picking only one property is incomplete; all listed properties must match to be truly common-mode.

Common Pitfalls:
Confusing common-mode with differential-mode; assuming frequency alone defines common-mode without checking amplitude and phase equality.

Final Answer:
All of the above