In op-amp configurations, compare inverting and non-inverting in terms of input impedance and gain expressions; which statement is correct about noise sources?

• A. Inverting configuration has high input impedance and gain = -Rf/Rin; non-inverting has low input impedance and gain = 1 + Rf/Rin; resistor noise dominates in inverting, bias current noise matters in both.
• B. Inverting configuration has low input impedance and gain = -Rf/Rin; non-inverting has high input impedance and gain = 1 + Rf/Rin; resistor noise dominates in inverting, bias current noise matters in both.
• C. Inverting configuration has input impedance equal to source; non-inverting has high input impedance; gain expressions are swapped.
• D. Inverting configuration has gain independent of Rf; non-inverting gain depends on Rf and Rin.

Answer: (B)

The essential idea is that topology fixes both input impedance and how the gain scales with the surrounding resistors. In the inverting configuration the input is applied to the inverting input through Rin while the noninverting input is at a reference potential; the feedback forces the inverting node to a virtual ground, so the impedance seen by the source is roughly Rin, which is typically much lower than the op-amp’s own input impedance. In the non-inverting configuration the input is applied to the high-impedance noninverting input, so the overall input impedance is very large, close to the op-amp’s own input impedance.

The gain expressions follow directly from how the feedback network is arranged. For the inverting setup, the output scales the input by the ratio Rf/Rin with a sign flip, so Vout = - (Rf/Rin) Vin. For the non-inverting setup, the gain is positive and given by Vout = (1 + Rf/Rin) Vin, with the same Rin being the resistor from the inverting input to ground in the standard form. This contrast—low input impedance and negative gain for the inverting path versus high input impedance and positive, larger gain for the non-inverting path—is what the statement captures.

Regarding noise sources, the inverting path’s low input impedance means the thermal noise from the input and feedback resistors (Rin and Rf) tends to dominate the input-referred noise, while the op-amp’s bias currents also introduce noise voltages by flowing through these resistors. In the non-inverting path, the input impedance is very high, so resistor noise seen at the input is less dominant, but the op-amp’s input current noise and bias currents still contribute to the overall noise, especially through any source impedance. This combination aligns with the described expectation that resistor noise dominates in the inverting case and bias current noise matters in both configurations.