I have just read this: OPEN-LOOP GAIN Unlike the ideal op amp, a practical op amp has a finite gain. The open-loop dc gain (usually referred to as AVOL) is the gain of the amplifier without the feedback loop being closed, hence the name open-loop. For a precision op amp this gain can be vary high, on the order of 160 dB (100 million) or more. From the section, I can infer that 160 dB here is power gain. But I just wonder why we often use power gain like this not directly amplitude gain. Is power easy to measure than amplitude?