What does "common mode rejection ratio" (CMRR) measure?

Common mode rejection ratio (CMRR) is a critical metric in the field of electronics, particularly when analyzing the performance of differential amplifiers. It quantifies the ability of an amplifier to reject signals that are common to both of its input terminals. In simpler terms, CMRR measures how effectively the amplifier can ignore noise or interference that affects both inputs equally, while still amplifying the difference between the two inputs.

Having a high CMRR means that the amplifier is effective at filtering out any unwanted signals that are common to both channels, allowing it to maintain fidelity when amplifying the desired differential signal. This characteristic is particularly important in applications like audio processing, communication systems, and instrumentation, where external noise can introduce errors if not appropriately rejected.

The other options either address unrelated aspects of electrical circuits or focus on different performance parameters that do not specifically pertain to the rejection of common signals. For instance, efficiency of a filter circuit pertains to how well it attenuates unwanted frequencies, the gain of an amplifier relates to its output power compared to input, and the frequency response describes how the gain of an amplifier varies with frequency. None of these directly define the CMRR.