Common-base (CB) amplifier usage: Among the classic single-transistor configurations, which type of gain is primarily provided by the common-base stage, making it useful for wide-band, low-input-impedance applications?

Introduction:
The common-base configuration is less common than the ubiquitous common-emitter, but it serves distinct purposes. Recognizing what kind of gain it excels at clarifies when to choose it for RF front ends or impedance transformation tasks.

Given Data / Assumptions:

• BJT operated in linear region, common-base configuration.
• Signal applied at emitter; output taken at collector; base is AC-grounded.
• We focus on the type of gain most associated with CB.

Concept / Approach:
Common-base amplifiers have very low input impedance at the emitter, relatively high output impedance at the collector, and current gain approximately equal to alpha (close to 1). Thus, they offer little current gain but can provide substantial voltage gain (Av often greater than 1), leading to useful power gain even though current gain is near unity.

Step-by-Step Solution:
Assess current gain: Ai ≈ α ≈ 1 → minimal current amplification.Evaluate voltage gain: with a high collector load and low input impedance, Av can be large.Infer power gain: since voltage gain > 1 and current gain ≈ 1, power gain is achieved.Conclude CB is primarily a voltage-gain stage with wide bandwidth.

Verification / Alternative check:
Small-signal models demonstrate low input resistance r_e and large collector load resistance reflected at the emitter, yielding Av ≈ gm * RC (in magnitude), which is often several times unity.

Why Other Options Are Wrong:
Current: CB provides ~unity current gain; CE is chosen when current gain is needed.Resistance: not a gain type; CB has low input resistance but that is not a 'gain'.Power: CB can have power gain, but the primary distinguishing gain type is voltage.

Common Pitfalls:
Assuming every amplifier provides both voltage and current gain. CB is mainly used where voltage gain and wide bandwidth with low input impedance are required.

Final Answer:
voltage