When two equal signals are applied to the inputs of a balanced closed-loop op-amp circuit, the signal is what?

When two equal signals are applied to the inputs of a balanced closed-loop op-amp circuit, the signal is what?

• A. Twice the input
• B. In phase
• C. Noise
• D. Zero

Answer: (D)

The essential idea is that an op-amp in a balanced closed-loop setup responds to the difference between its inputs, not the same signal appearing on both inputs. When the same signal is applied to both inputs, the differential input voltage is zero, and the feedback loop drives the output until the difference remains zero. In an ideal device this yields an output of zero, since there’s no differential signal to amplify. In real hardware, tiny offsets or imperfections can produce a very small output, but the intended result is zero. This also illustrates why equal signals on both inputs don’t produce a larger or phase-shifted output—the circuit cancels common signals, focusing only on the difference.

The essential idea is that an op-amp in a balanced closed-loop setup responds to the difference between its inputs, not the same signal appearing on both inputs. When the same signal is applied to both inputs, the differential input voltage is zero, and the feedback loop drives the output until the difference remains zero. In an ideal device this yields an output of zero, since there’s no differential signal to amplify. In real hardware, tiny offsets or imperfections can produce a very small output, but the intended result is zero. This also illustrates why equal signals on both inputs don’t produce a larger or phase-shifted output—the circuit cancels common signals, focusing only on the difference.