In a silicon bipolar junction transistor (BJT) operated in a common-emitter (C–E) configuration, what is the typical forward-bias base–emitter junction voltage V_BE at room temperature when the device is properly turned on?

Introduction:
The base–emitter junction of a silicon BJT behaves like a diode. In a common-emitter (C–E) amplifier or switch, the transistor conducts when this junction is forward biased to a characteristic voltage. Knowing the typical V_BE is essential for bias design, operating point prediction, and troubleshooting.

Given Data / Assumptions:

• Device: Silicon BJT (not germanium).
• Room-temperature operation (around 25 °C).
• Forward-biased base–emitter junction during normal conduction.
• Moderate collector current typically in the milliampere range.

Concept / Approach:
For silicon junctions, the forward conduction knee is near 0.6–0.7 V at room temperature. Practical V_BE rises slightly with current (approximately 60 mV per decade change) and falls about 2 mV/°C with increasing temperature. Designers therefore use 0.7 V as a convenient nominal value in hand calculations for silicon BJTs in conduction.

Step-by-Step Solution:

Verification / Alternative check:
Datasheets often show V_BE(on) around 0.65–0.75 V for currents of a few milliamperes. Lab measurements with a DMM across base–emitter while the device conducts corroborate this range. At very low currents, V_BE may be nearer 0.55–0.6 V; at higher currents, nearer 0.8 V—0.7 V remains the standard design heuristic.

Why Other Options Are Wrong:

• voltage-divider bias: A biasing method, not a V_BE value.
• 0.4 V: Too low for silicon; more typical of germanium devices.
• emitter voltage: A node quantity, not the junction drop.

Common Pitfalls:
Confusing silicon with germanium values, ignoring temperature dependence, or assuming V_BE is constant regardless of current. Always treat 0.7 V as a nominal starting point and refine with datasheet curves when precision matters.

Final Answer:
0.7 V