In an ideal op-amp, what is the input impedance?

In an ideal op-amp, what is the input impedance?

• A. Zero input impedance
• B. Very low input impedance
• C. Finite input impedance
• D. Infinite input impedance

Answer: (D)

The main idea is that an ideal op-amp presents infinite input impedance. In the ideal model, no current flows into either input terminal, so the inputs do not draw any current from the source. That means the source sees essentially no load from the op-amp itself—the impedance looking into each input is infinite. In real devices, the input impedance is very high but finite, determined by the input transistor design and biasing. Remember, the exact impedance the source experiences can depend on the surrounding circuit (for example, input resistors in some configurations), but the op-amp’s own input impedance in the ideal sense is infinite.

The main idea is that an ideal op-amp presents infinite input impedance. In the ideal model, no current flows into either input terminal, so the inputs do not draw any current from the source. That means the source sees essentially no load from the op-amp itself—the impedance looking into each input is infinite. In real devices, the input impedance is very high but finite, determined by the input transistor design and biasing. Remember, the exact impedance the source experiences can depend on the surrounding circuit (for example, input resistors in some configurations), but the op-amp’s own input impedance in the ideal sense is infinite.