What happens to the output of a differential amplifier when the inputs are the same and in phase?

In a differential amplifier, the main purpose is to amplify the difference between two input signals while rejecting any common-mode signals. When the inputs are the same and in phase, there is no difference between the two signals.

In this condition, the output of the differential amplifier is determined by the common-mode gain, which ideally is zero for a perfect differential amplifier. Thus, the output does not respond to the common input. Instead, it effectively cancels out the identical signals, leading to no differential output being produced.

Therefore, when the inputs are the same and in phase, the output of the differential amplifier will be zero, as there's no difference to amplify. This is why producing a maximum output or a fluctuating output is not applicable in this scenario; thus, the choice indicating no output aligns with the fundamental operation of a differential amplifier.