Why is a common-collector (emitter follower) stage often used as the final stage before a low-resistance load, and what is its principal function?

Introduction:
The common-collector (CC) configuration, also known as an emitter follower, is widely used as an output buffer in amplifier chains. Its hallmark is high input impedance, low output impedance, and near-unity voltage gain, making it ideal for driving low-resistance loads without loading previous gain stages.

Given Data / Assumptions:

• Load has relatively low resistance compared to the preceding amplifier's optimal load.
• Signal integrity (amplitude and bandwidth) must be preserved.
• Standard BJT operation with appropriate biasing.

Concept / Approach:
The CC stage provides impedance transformation: it accepts a signal with minimal loading (high input impedance) and delivers current into the load (low output impedance). Voltage gain is approximately 1, but current gain is high, enabling power transfer to the load. This protects earlier voltage-amplifying stages and improves overall efficiency and linearity when driving speakers, cables, or other low-resistance loads.

Step-by-Step Solution:

Verification / Alternative check:
Small-signal models show output resistance roughly Re || (re' + (β+1) * emitter network), which is low; input resistance is approximately (β+1) times the emitter network, which is high—quantitatively confirming buffering behavior.

Why Other Options Are Wrong:

• provide voltage gain: CC has near-unity voltage gain by design.
• provide phase inversion: CC is non-inverting; CE inverts.
• provide a high-frequency path: Frequency response may be good, but this is not the defining role.

Common Pitfalls:
Expecting large voltage gain from a CC stage or overlooking the importance of proper bias and emitter bypass choices for linear operation.

Final Answer:
buffer the voltage amplifiers from the low-resistance load and provide impedance matching for maximum power transfer