Discussion
Which of the following is not a way to improve common-mode rejection?
Increasing the collector load resistance
Using a current source in the tail
Taking the output differentially
Matching the two collector loads more closely
Answer: D
Increasing the collector load raises both gains in nearly the same proportion, so the ratio barely moves - what it buys is differential gain. The other three all act on the ratio itself, by raising the tail impedance, restoring the cancellation, or reducing the imbalance that converts common-mode into differential. A change affecting both terms of a ratio equally cannot improve it, which is worth checking before any such proposal is adopted. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.
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