Match circuit types to key small-signal characteristics: (A) Darlington pair, (B) Common-Base (CB) amplifier, (C) Cascade amplifier (multi-stage) — identify the best mapping among voltage gain, input impedance, and high-frequency isolation.

Introduction / Context:Small-signal amplifier topologies are characterized by input/output impedance, voltage gain, and frequency behavior. This question distinguishes between the Darlington emitter follower, CB stage, and cascaded multi-stage amplifiers often seen in RF front ends and audio preamps.

Given Data / Assumptions:

• Darlington pair behaves like a high-gain emitter follower.
• Common-Base amplifiers show low input impedance and good high-frequency isolation.
• Cascade (multi-stage) connection can deliver high overall voltage gain.

Concept / Approach:Match each topology to its hallmark: Darlington → very high input impedance and voltage gain near 1; CB → low input impedance, useful for wideband/high-frequency stages; cascade → stacking stages multiplies voltage gain.

Step-by-Step Solution:

Verification / Alternative check:Emitter follower’s Av ~ 0.9–0.99; CB stage’s Rin ~ tens of ohms and wide bandwidth; cascading two CE stages yields Av ≫ 1. These align with the selected descriptors.

Why Other Options Are Wrong:

• A-1, B-2, C-3: Misattributes large voltage gain to Darlington and high input impedance to CB.
• A-2, B-1, C-3: Gives high gain to CB and isolation to cascade—reversed.
• A-3, B-1, C-2: Swaps multiple properties simultaneously.

Common Pitfalls:

• Assuming Darlington provides large voltage gain; it primarily boosts current gain and input impedance.
• Confusing cascade (multi-stage) with cascode (CE–CB pair). Cascode has different properties.

Final Answer:A-2, B-3, C-1