In BJT operation, which bias condition is typically applied to the emitter-base junction (the “emitter diode”) during normal amplifier operation?

Introduction / Context:
A bipolar junction transistor has two pn junctions: emitter-base (EB) and collector-base (CB). Understanding their biasing in normal operation clarifies current flow, gain mechanisms, and why small base currents control large collector currents. In amplifier mode (forward-active region), the EB junction must promote carrier injection from the emitter into the base.

Given Data / Assumptions:

• Transistor used as a linear amplifier (forward-active region).
• Proper supply voltages provided.
• No saturation or breakdown intended.

Concept / Approach:
In forward-active operation, the emitter-base junction is forward-biased to inject majority carriers from the emitter into the base. The collector-base junction is reverse-biased to sweep these carriers into the collector region. This arrangement yields current amplification characterized by β = I_C / I_B. Reversing these biases moves the device into cutoff, saturation, or reverse active modes, none of which correspond to standard small-signal amplification.

Step-by-Step Solution:
Identify normal amplifier mode: forward-active.Recall EB junction condition: forward-biased to allow injection.Recall CB junction condition: reverse-biased for collection.Choose “Forward-biased” for the EB junction.

Verification / Alternative check:
Examine transistor characteristic curves: with EB forward and CB reverse, output characteristics show controlled collector current; datasheets and textbooks depict this mode explicitly.

Why Other Options Are Wrong:
Reverse-biased EB junction would cut off emitter injection. “Nonconducting” conflicts with amplifier operation. “Breakdown region” is undesirable and destructive for normal operation. “None” is incorrect because forward bias is required.

Common Pitfalls:
Confusing saturation (both junctions forward-biased) with forward-active; overlooking that small V_BE changes produce large I_C variations, demanding bias stabilization.

Final Answer:
Forward-biased