For a bipolar junction transistor, which control quantity primarily determines the collector current in normal operation?

Introduction:
Transistor operation involves interrelated currents and voltages. In a BJT, the collector current is largely controlled by the base–emitter junction and the resulting base current, with device gain β relating I_C and I_B. Recognizing the primary control input is crucial for amplifier design and switching analysis.

Given Data / Assumptions:

• Forward-active region of operation (not breakdown).
• Constant temperature and neglecting second-order effects for first-pass analysis.
• Reasonable transistor β (e.g., 50–200) so that I_C ≈ β * I_B.

Concept / Approach:
In forward-active region, collector current is approximately proportional to base current: I_C ≈ β * I_B, where β is the common-emitter current gain. Although collector–emitter voltage influences I_C via the Early effect, the primary control knob is base current. Collector resistance influences voltage drops but does not directly set I_C absent changes in bias/current drive.

Step-by-Step Solution:

Verification / Alternative check:
Load-line experiments show that varying base current shifts the Q-point vertically (I_C axis) along the load line, confirming I_B as the main determinant of collector current for a given device.

Why Other Options Are Wrong:

• collector voltage: Affects I_C modestly (Early effect) but is not the primary control variable.
• collector resistance: Sets V_CE with I_C but does not directly control I_C without changing I_B.
• all of the above: Overstates secondary effects; the dominant control is base current.

Common Pitfalls:
Confusing MOSFET gate-voltage control with BJT base-current control, or assuming I_C is independent of V_CE (it is not perfectly independent due to Early effect).

Final Answer:
base current