Emitter follower (common-collector) gain property: for a properly biased emitter follower, what is the approximate small-signal voltage gain from base to emitter?

Introduction / Context:
The emitter follower is widely used as a buffer. Although it does not provide voltage gain above unity, it offers high input resistance and low output resistance, making it ideal for impedance matching and isolation. This question checks your understanding of the emitter follower’s hallmark: a voltage gain very near, but slightly less than, unity with no phase inversion.

Given Data / Assumptions:

• Proper biasing so the transistor remains in its active region.
• Small-signal linear operation about the bias point.
• Load not so heavy that it forces significant emitter voltage droop.

Concept / Approach:
In a common-collector stage, the output is taken from the emitter, which follows the base voltage minus approximately VBE. Under small-signal AC analysis, the incremental gain is Av ≈ RE / (RE + re), typically close to 1 when RE ≫ re (where re is the intrinsic emitter resistance). The output is in phase with the input (no inversion), and because of the low output resistance, the stage can drive loads better than a common-emitter stage at the same bias current.

Step-by-Step Solution:

Verification / Alternative check:
Simulate or calculate with a practical example: re ≈ 25 mV / IE. For IE = 2 mA, re ≈ 12.5 Ω; with RE = 1 kΩ, Av ≈ 1000 / (1012.5) ≈ 0.988, which is very close to one.

Why Other Options Are Wrong:
Much less than one: contradicts the high follower gain typical of a buffered stage.
Greater than one: emitter followers do not provide voltage gain beyond unity.
Zero: would imply no output response; incorrect.
Negative and large: describes common-emitter behavior (inversion), not emitter follower.

Common Pitfalls:
Confusing DC level shift (input minus VBE) with AC gain; assuming the gain is exactly 1.000 regardless of load; overlooking the effect of re and load on the exact value.

Final Answer:
Approximately equal to one