In this circuit, the output voltage Vo must be

In this circuit, the output voltage Vo must be

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

Answer: (B)

In this kind of circuit, what the output does is determined by the sign of the input difference V+ − V− when negative feedback is present. The output moves in the direction that reduces that difference, trying to make V− track V+. Here, the non-inverting input is driven higher than the inverting input (as set by the input source and the feedback network from Vo to V−). To bring V− up to match V+, the output must rise, so Vo goes positive. A negative output would push V− further away from V+, causing the amplifier to swing back toward positive until balance is reached. Thus the stable, required output is a positive voltage.

In this kind of circuit, what the output does is determined by the sign of the input difference V+ − V− when negative feedback is present. The output moves in the direction that reduces that difference, trying to make V− track V+. Here, the non-inverting input is driven higher than the inverting input (as set by the input source and the feedback network from Vo to V−). To bring V− up to match V+, the output must rise, so Vo goes positive. A negative output would push V− further away from V+, causing the amplifier to swing back toward positive until balance is reached. Thus the stable, required output is a positive voltage.