Common-collector (emitter follower) as the final stage: What is the primary function of this stage before driving a low-resistance load?

Introduction / Context:The common-collector (emitter follower) stage is ubiquitous as the last stage of small-signal amplifiers. It offers high input resistance and low output resistance, making it ideal to interface sensitive voltage-gain stages with a relatively heavy load such as a speaker, cable, or subsequent module input.

Given Data / Assumptions:

• BJT operated in its linear region.
• Emitter follower biased for approximately unity voltage gain.
• Load resistance is comparatively low.

Concept / Approach:Because the emitter follower has voltage gain near +1 but can supply higher current due to its low output resistance, it ”buffers” a preceding high-gain, high-impedance stage from the loading effects of R_L. This preserves the voltage gain of earlier stages and improves overall drive capability without additional voltage amplification.

Step-by-Step Solution:

Verification / Alternative check:Small-signal analysis shows R_out ≈ (re + emitter resistance)/(1 + β) in simplified models, which is low compared with typical load resistances, confirming buffering action.

Why Other Options Are Wrong:

• Provide voltage gain: Emitter followers have near-unity gain.
• Phase inversion: CC output is in phase with the input.
• Provide a high-frequency path: Not the defining purpose; compensation networks address HF response.

Common Pitfalls:Expecting significant voltage amplification from a CC stage; its strength is current drive and isolation.

Final Answer:buffer the voltage amplifiers from the low-resistance load