In the ideal op-amp model, what is the input impedance and how does negative feedback affect the relation between V+ and V−?

• A. Zero input impedance
• B. Infinite input impedance
• C. Finite input impedance equal to source impedance
• D. Infinite output impedance

Answer: (B)

The key idea is that an ideal op-amp presents infinite input impedance, so the input terminals draw essentially no current. With negative feedback in a linear configuration, the amplifier output adjusts so the voltages at the two inputs are driven very close together. Because the open-loop gain is taken as infinite in the ideal model, any tiny difference between V+ and V− would produce a large output, so the feedback forces that difference toward zero. This creates a virtual short between the inputs: V+ and V− become equal in practice, even though they’re not physically connected. The infinite input impedance means the source isn’t loaded by the op-amp’s inputs, which aligns with choosing the infinite input impedance as the correct description.