Increasing the feedback resistor RF in a noninverting amplifier while Rin is fixed causes the gain to

Increasing the feedback resistor RF in a noninverting amplifier while Rin is fixed causes the gain to

• A. Increase
• B. Decrease
• C. Stay the same
• D. Become negative

Answer: (A)

In a noninverting amplifier, the closed-loop gain is set by Av = 1 + RF/Rin. Since Rin is fixed, increasing RF raises the ratio RF/Rin, so the gain increases. Intuitively, the inverting input sits at a fraction of the output given by Rin/(RF + Rin); as RF grows, that fraction gets smaller, so to keep the inverting input equal to the noninverting input, the output must rise more, yielding a larger gain. The gain stays positive as long as the supply rails and resistor values are positive, so it does not become negative.

In a noninverting amplifier, the closed-loop gain is set by Av = 1 + RF/Rin. Since Rin is fixed, increasing RF raises the ratio RF/Rin, so the gain increases. Intuitively, the inverting input sits at a fraction of the output given by Rin/(RF + Rin); as RF grows, that fraction gets smaller, so to keep the inverting input equal to the noninverting input, the output must rise more, yielding a larger gain. The gain stays positive as long as the supply rails and resistor values are positive, so it does not become negative.