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A BJT shows a current gain α = 0.99 in common-base (CB) configuration. What is the corresponding current gain in common-collector (CC, emitter follower) configuration?

Difficulty: Easy

Correct Answer: 100

Explanation:


Introduction / Context:
Current gains in different BJT configurations are related. Converting between α (CB gain) and β (CE gain) enables quick estimates of follower (CC) current gain, which is approximately (β + 1). Understanding these relations helps in rapid hand analysis and design sanity checks.


Given Data / Assumptions:

  • Common-base current gain α = IC/IE = 0.99.
  • Relation between α and β: β = α / (1 − α).
  • Common-collector current gain (approx.) = β + 1 (ratio IE/IB).


Concept / Approach:
First compute β from α, then obtain the CC gain. A high α implies a high β, and consequently a CC current gain near β + 1, often close to 1 for voltage gain but high for current gain, which is the essence of the emitter follower behavior.


Step-by-Step Solution:

Compute β: β = α/(1 − α) = 0.99 / 0.01 = 99.Compute CC current gain: ≈ β + 1 = 100.Therefore the current gain in CC configuration is 100.


Verification / Alternative check:

Check that IE = IC + IB and IC ≈ α IE; with α = 0.99, IB is small, giving a large IE/IB ratio ~ 100.


Why Other Options Are Wrong:

99 overlooks the +1 term in CC gain.1.01 or 0.99 confuse current gain with voltage gain (which is near unity in CC).“~50” is arbitrary and inconsistent with α = 0.99.


Common Pitfalls:

Mixing up current gain (high for CC) with voltage gain (≈ 1) of an emitter follower.


Final Answer:

100
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