Which of the following is an ideal op-amp characteristic?

• A. Infinite voltage gain
• B. Finite input impedance
• C. Nonzero output impedance
• D. Gain depends on frequency

Answer: (A)

Infinite open-loop gain is the defining ideal op-amp characteristic because it makes the differential input voltage drive the output response in a way that, under negative feedback, the two input terminals are forced to be nearly equal. With infinite gain, even an infinitesimal difference between V+ and V− would produce a finite output only if the feedback network adjusts to cancel that difference, which is why the inputs appear to be at the same voltage in linear operation. This virtual short between the inputs lets the closed-loop gain be determined almost entirely by the feedback network rather than the amp itself, giving predictable, precise amplification. In real devices the gain is large but finite, input impedance is very high but not infinite, and the bandwidth is finite, which is why the ideal model uses infinite gain to simplify analysis.