Discussion
Why does replacing the tail resistor with a current source improve rejection?
Answer: C
A current source presents hundreds of kilohms of dynamic impedance where a practical resistor might present a few kilohms, and the common-mode gain falls in proportion. Its standing current is whatever the design chooses, the differential gain is untouched, and it does nothing about device matching. Improving the numerator of a ratio while leaving the denominator alone is the cleanest possible improvement, and this is a textbook case of it. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.
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