Inverting input drive — output polarity: When an input signal is applied to the inverting input of an op-amp and the noninverting input is grounded, what is the polarity of the output relative to the input (within the linear range)?

Introduction / Context:
The inverting op-amp configuration is one of the first linear circuits taught. Its hallmark is a 180-degree phase inversion between input and output. Recognizing this helps when chaining stages, summing signals, or designing active filters with correct phase response.

Given Data / Assumptions:

• Ideal op-amp with negative feedback.
• Noninverting input tied to ground reference.
• Signal applied through an input resistor to the inverting input.

Concept / Approach:
With the noninverting input at 0 V, feedback keeps the inverting input near 0 V (virtual ground). The closed-loop gain is −Rf/Rin, a negative value indicating inversion. Thus, a positive input produces a negative output, and vice versa, until limited by the supply rails.

Step-by-Step Solution:

Verification / Alternative check:
Oscilloscope shows input and output sinusoidal waveforms inverted by half a cycle with amplitude ratio equal to |Rf/Rin|. Step response also inverts polarity instantly, within slew limits.

Why Other Options Are Wrong:

Common Pitfalls:
Ignoring output swing limits; forgetting to include input bias cancellation when using large resistances; miswiring Rin and Rf leading to positive feedback.

Final Answer:
Opposite polarity (inverted) with magnitude set by the closed-loop gain