In a standard, generic op-amp model, which statement correctly describes open-loop gain, input impedance, and how closed-loop gain is set?

• A. Avol is large in open loop; Input impedance is high; In closed-loop, gain is set by feedback; Important parameters include the gain-bandwidth product and slew rate.
• B. Avol is large in open loop; Input impedance is low; In closed-loop, gain is unchanged; Key parameters include the supply voltage.
• C. Avol is large in open loop; Input impedance is low; In closed-loop, input bias current dominates.
• D. Avol is moderate; Input impedance is moderate; Bandwidth is unlimited.

Answer: (A)

The main idea being tested is how an op-amp behaves with and without feedback and what sets its gain in each case. In a standard generic op-amp model, the open-loop gain is very large, meaning even a tiny difference between the two input terminals yields a large output swing. The input impedance is high, so little current is drawn from the input signal. When you apply negative feedback (close the loop), the gain is no longer set by that enormous open-loop gain. Instead, the closed-loop gain is determined by the feedback network (the resistors you connect around the op-amp). The amplifier adjusts its output to drive the input difference toward zero, giving a predictable, stable gain set by the feedback.

Real devices also have practical limits described by parameters like the gain-bandwidth product and the slew rate. The gain-bandwidth product means that as you push the closed-loop gain higher, the usable bandwidth gets smaller. The slew rate limits how fast the output can change in response to large input changes, affecting transient performance. That combination—very large open-loop gain, high input impedance, and a feedback-determined closed-loop gain with practical bandwidth and speed limits—is what makes this description the best fit.

The other possibilities don’t align with how real op-amps behave: some imply low input impedance in open loop, or that closed-loop gain stays the same regardless of feedback, or that the open-loop gain is only moderate and bandwidth is unlimited. Those clash with the standard model and its behavior under feedback.