In an NPN transistor, the device turns on when the base is:

• A. Positive with respect to the emitter
• B. Negative with respect to the emitter
• C. At the same potential as the emitter
• D. Floating with respect to the emitter

Answer: (A)

In an NPN transistor, turning on relies on forward biasing the base–emitter junction. When the base is more positive than the emitter by about 0.6–0.7 volts (for silicon), the base–emitter diode conducts, allowing base current to flow. This small base current enables a much larger current to flow from collector to emitter, effectively turning the transistor on.

If the base is negative relative to the emitter, the junction is reverse-biased and the transistor stays off. If the base and emitter are at the same potential, there is no forward bias, so it remains off. If the base is floating, there’s no defined bias to turn it on, so it won’t conduct either.