BJT parameter identification: the (dc) current gain of a bipolar junction transistor is defined as the ratio of collector current to which transistor current?

Introduction / Context:
Designers often speak of a transistor’s “beta” (β) or h_FE, the dc current gain in common-emitter configuration. Knowing the correct definition is critical when sizing base resistors, estimating load capability, and biasing amplifier stages.

Given Data / Assumptions:

• Device: bipolar junction transistor (BJT), common-emitter usage.
• Currents: I_C (collector), I_B (base), I_E (emitter).
• We focus on dc current gain β.

Concept / Approach:
The dc current gain β is defined as β = I_C / I_B (for a given operating point). The small-signal gain h_fe is a related incremental parameter but conceptually aligned with the ratio of changes in collector and base currents. This definition underpins equations like I_C ≈ β * I_B, with I_E ≈ I_B + I_C.

Step-by-Step Solution:
Recall β (h_FE) definition: β = I_C / I_B.Identify the denominator: base current I_B.Confirm other currents are not in the definition’s denominator.Choose “base current.”

Verification / Alternative check:
Datasheets tabulate h_FE or provide graphs of I_C vs I_B at specified V_CE, validating β as the ratio I_C/I_B in common-emitter configuration.

Why Other Options Are Wrong:
B: I_E is the sum of base and collector currents and is not used in the standard β ratio. C: “Supply current” is ambiguous and not a BJT terminal current. D: Using collector current as both numerator and denominator yields 1, not β.

Common Pitfalls:
Assuming β is constant across all currents and temperatures; forgetting to check datasheet conditions for h_FE; mixing small-signal h_fe with dc β.

Final Answer:
base current