In a non-inverting amplifier with feedback resistor R1 and ground resistor R2, what is the closed-loop gain?

• A. 1 + (R2/R1)
• B. R1/R2
• C. 1 + (R1/R2)
• D. R2/R1

Answer: (C)

In a non-inverting amplifier, the feedback network between the output and the inverting input determines how much of the output is fed back to compare with the input. The inverting input sits at a node of a divider formed by the two resistors: one from the output back to the node, and the other from the node to ground. The voltage at that node is V- = Vout · (R2 / (R1 + R2)).

With an ideal op-amp and negative feedback, the voltages at the two inputs are equal, so Vin ≈ V-. Substituting gives Vin ≈ Vout · (R2 / (R1 + R2)). Rearranging to solve for the gain Vout/Vin yields Vout/Vin = (R1 + R2) / R2 = 1 + (R1 / R2).

So the closed-loop gain is 1 + (R1 / R2). This positive gain reflects the non-inverting setup, where part of the output is fed back without inverting the phase. The form 1 + (R2/R1) or R1/R2 would correspond to different configurations or interpretations and do not apply to this arrangement.