Functional identification — A circuit whose output is proportional to the algebraic difference between two input signals is best described as which type of amplifier?

Introduction:
Many analog front ends and op-amp configurations are based on differential principles. Recognizing the building block that outputs a scaled version of V1 − V2 is crucial to understanding instrumentation amplifiers, comparators in linear mode, and active filters.

Given Data / Assumptions:

• Two input signals applied to a device or network
• Interest in the arithmetic difference of those inputs
• Linear small-signal behavior

Concept / Approach:

A differential amplifier produces Vout = Ad * (V1 − V2). Common-mode components ideally cancel, and only the difference is amplified. An operational amplifier is a general-purpose active device; when configured appropriately, it implements a differential amplifier among many other functions.

Step-by-Step Explanation:

Verification / Alternative check:

Examine instrumentation amplifier datasheets: the first stage is a precision differential amplifier where output tracks the difference between inputs.

Why Other Options Are Wrong:

• common-mode: Refers to signals common to both inputs, not difference.
• darlington: A transistor pair for high current gain, not differential operation per se.
• operational: Describes the device class, not the configured function.
• transconductance: Output current proportional to input voltage, different transfer.

Common Pitfalls:

• Assuming any op-amp circuit is inherently differential—configuration matters.
• Ignoring input resistor matching, which degrades CMRR.

Final Answer:

differential