In an inverting amplifier, the closed-loop gain magnitude equals

In an inverting amplifier, the closed-loop gain magnitude equals

• A. Rin over Rf
• B. Rf over Rin
• C. Rf plus Rin
• D. Vin divided by Vout

Answer: (B)

In an inverting amplifier, the current through the input resistor Rin must flow through the feedback resistor Rf because the op-amp input draws no current and the inverting node sits at a virtual ground. This sets up the relation Vin/Rin = - Vout/Rf. Solving gives Vout = - (Rf/Rin) Vin, so the closed-loop gain magnitude is Rf/Rin. The other expressions don’t match the circuit behavior: Rin/Rf would invert the ratio, Rf plus Rin implies a series sum rather than a feedback ratio, and Vin divided by Vout is the reciprocal of the gain.

In an inverting amplifier, the current through the input resistor Rin must flow through the feedback resistor Rf because the op-amp input draws no current and the inverting node sits at a virtual ground. This sets up the relation Vin/Rin = - Vout/Rf. Solving gives Vout = - (Rf/Rin) Vin, so the closed-loop gain magnitude is Rf/Rin.

The other expressions don’t match the circuit behavior: Rin/Rf would invert the ratio, Rf plus Rin implies a series sum rather than a feedback ratio, and Vin divided by Vout is the reciprocal of the gain.