In an NPN transistor, the device turns on when the emitter is positive with respect to the base.

In an NPN transistor, the device turns on when the emitter is positive with respect to the base.

• A. Emitter is positive with respect to the base
• B. Base is positive with respect to the emitter
• C. Collector is positive with respect to the base
• D. Emitter is negative with respect to the base

Answer: (A)

Turning on an NPN transistor relies on forward biasing the base–emitter junction. That junction behaves like a diode, so it conducts when the base is about 0.6–0.7 V higher in potential than the emitter. With that forward bias, electrons are injected from the emitter into the base, and the small base current controls a much larger current from the collector to the emitter, thanks to the transistor’s current gain. The collector–base junction remains reverse-biased in normal operation, allowing this amplified current to flow. If the emitter were more positive than the base, the base–emitter junction would be reverse biased, preventing significant base current and keeping the transistor essentially off.

Turning on an NPN transistor relies on forward biasing the base–emitter junction. That junction behaves like a diode, so it conducts when the base is about 0.6–0.7 V higher in potential than the emitter. With that forward bias, electrons are injected from the emitter into the base, and the small base current controls a much larger current from the collector to the emitter, thanks to the transistor’s current gain. The collector–base junction remains reverse-biased in normal operation, allowing this amplified current to flow.

If the emitter were more positive than the base, the base–emitter junction would be reverse biased, preventing significant base current and keeping the transistor essentially off.