CMRR is often expressed in decibels. It is the ratio of which two quantities?

• A. Common-mode to differential gain
• B. Differential gain to common-mode gain
• C. Input resistance to output resistance
• D. Voltage gain to current gain

Answer: (B)

CMRR measures how strong the amplifier’s response to a true differential input is compared with its response to a signal that’s the same on both inputs. In other words, it’s the ratio of differential gain to common-mode gain. If Ad is the gain for the difference between the inputs and Acm is the gain for a signal common to both inputs, then CMRR = Ad / Acm. When expressed in decibels, this becomes 20 log10(|Ad / Acm|). So the best choice is the one that uses differential gain divided by common-mode gain. The other options don’t describe this ratio or involve unrelated quantities like input/output resistance or different types of gain.