Which statement about the difference amplifier is true when v2 is greater than v1?

Which statement about the difference amplifier is true when v2 is greater than v1?

• A. Vo and v2 are in phase
• B. Vo is inverted with respect to v2
• C. Vo is independent of v2
• D. Vo equals v1

Answer: (A)

The difference amplifier produces an output proportional to the difference between its two inputs: Vo ≈ Ad (v2 − v1) with Ad > 0 when the resistor values give a positive differential gain. If v2 is greater than v1, the difference (v2 − v1) is positive, so Vo is positive and increases as v2 increases. Because changes in v2 produce changes in Vo in the same direction, Vo is in phase with v2. The other ideas don’t fit: Vo would not be inverted with respect to v2 under a positive, noninverting differential gain; Vo depends on the difference between inputs, not solely on v2; and Vo does not simply equal v1 in general.

The difference amplifier produces an output proportional to the difference between its two inputs: Vo ≈ Ad (v2 − v1) with Ad > 0 when the resistor values give a positive differential gain. If v2 is greater than v1, the difference (v2 − v1) is positive, so Vo is positive and increases as v2 increases. Because changes in v2 produce changes in Vo in the same direction, Vo is in phase with v2.

The other ideas don’t fit: Vo would not be inverted with respect to v2 under a positive, noninverting differential gain; Vo depends on the difference between inputs, not solely on v2; and Vo does not simply equal v1 in general.