In an inverting amplifier, the input impedance is approximately equal to which resistance?

• A. Rin
• B. Rf
• C. Rin || Rf
• D. Infinite

Answer: (A)

The main idea is that the input impedance of an inverting amplifier is set by the input resistor Rin because the inverting input is held at a virtual ground by negative feedback. The source voltage sees Rin directly to a node that is effectively at zero volts, so the input current flows through Rin into the summing junction. Since the source’s load is just Rin to this near-ground node, the impedance it experiences is approximately Rin.

Rf isn’t part of the path from the input to ground; it connects from the output back to the inverting input, so it doesn’t set the input impedance. The Rin || Rf combination would only apply if both resistors were connected from the input node to ground, which isn’t the case. The input impedance isn’t infinite because there is a real resistor Rin providing a path to ground through the virtual node.