If V2 exceeds V1, what is the sign of the output Vout in a standard difference amplifier?

If V2 exceeds V1, what is the sign of the output Vout in a standard difference amplifier?

• A. Positive
• B. Negative
• C. Zero
• D. Undefined

Answer: (A)

The key idea is that a standard difference amplifier outputs a voltage proportional to the difference between the two inputs, with a positive gain set by the resistor ratios. When the resistor values are chosen correctly, the output equals (R2/R1) times (V2 − V1). Because R2/R1 is positive, the sign of the output follows the sign of (V2 − V1). If V2 is larger than V1, the difference is positive, so the output is driven positive (assuming the op-amp remains within its supply rails). Conversely, if V2 were smaller than V1, the output would be negative, and if they’re equal, the output would be zero.

The key idea is that a standard difference amplifier outputs a voltage proportional to the difference between the two inputs, with a positive gain set by the resistor ratios. When the resistor values are chosen correctly, the output equals (R2/R1) times (V2 − V1). Because R2/R1 is positive, the sign of the output follows the sign of (V2 − V1). If V2 is larger than V1, the difference is positive, so the output is driven positive (assuming the op-amp remains within its supply rails). Conversely, if V2 were smaller than V1, the output would be negative, and if they’re equal, the output would be zero.