In operational amplifier applications, a voltage follower (unity-gain buffer) is configured with 100% negative feedback. Which characteristic best describes its closed-loop behavior?

Introduction:
A voltage follower, also called a unity-gain buffer, is a core operational amplifier configuration used to isolate stages, present very high input impedance, and provide very low output impedance without amplifying the voltage. This question checks recognition of the defining closed-loop characteristic of a follower.

Given Data / Assumptions:

• Ideal op-amp assumptions: infinite open-loop gain, infinite input impedance, zero output impedance, and sufficient bandwidth for the frequencies of interest.
• Feedback is 100% from output to inverting input; the input signal is applied to the non-inverting input.
• We focus on closed-loop characteristics, not open-loop parameters.

Concept / Approach:
With unity feedback, the closed-loop transfer drives the op-amp to make the output equal to the input. Hence the closed-loop voltage gain Av_cl equals 1 (unity). This preserves the input signal level while providing buffering between source and load. The circuit is widely used to prevent loading effects and to build active filters and instrumentation chains.

Step-by-Step Solution:
1) In a follower, route Vout directly to the inverting input and apply Vin to the non-inverting input.2) The op-amp forces its differential input toward zero; thus Vout approaches Vin.3) The closed-loop gain Av_cl = Vout / Vin equals 1 for ideal conditions.4) The key benefit is buffering: high input impedance and low output impedance, minimizing source loading.

Verification / Alternative check:
Write the non-inverting gain formula Av_cl = 1 + (Rf / Rin). For a follower, Rin approaches infinity and Rf short equals 1. In the canonical derivation for unity feedback, the limit yields Av_cl = 1, confirming the identity.

Why Other Options Are Wrong:
Small open-loop voltage gain: Open-loop gain of an op-amp is typically very large, not small, and the question asks closed-loop behavior.

Closed-loop bandwidth of zero: Incorrect; followers often extend bandwidth compared to higher closed-loop gains.

Large closed-loop output impedance: The follower provides low output impedance, not large.

Closed-loop voltage gain greater than unity: By definition the follower has a gain of 1.

Common Pitfalls:
Confusing open-loop and closed-loop parameters, or believing that any feedback changes voltage gain above unity. In a follower, feedback sets the gain exactly to 1 while improving drive capability and linearity.

Final Answer:
closed-loop voltage gain of unity