FAA Aviation Mechanic General (AMG)Fundamentals of Electricity and ElectronicsMedium
A technician is analyzing an NPN transistor operating in its normal amplifying (active) mode. What is the correct bias condition for the base-emitter and base-collector junctions?
- ABoth junctions are reverse biased
- BBoth junctions are forward biased
- CBase-emitter junction forward biased, base-collector junction reverse biased
- DBase-emitter junction reverse biased, base-collector junction forward biased
Show answer & explanationAnswer & explanation
Correct answer: C. Base-emitter junction forward biased, base-collector junction reverse biased
For proper amplifying operation, an NPN transistor requires the base-emitter junction to be forward biased (allowing current injection into the base) while the base-collector junction is reverse biased (allowing collector current collection without direct conduction).
Why the other options are wrong
- A. Both junctions reverse biased would put the transistor in cutoff, blocking all current flow.
- B. Both junctions forward biased would drive the transistor into saturation, not linear amplification.
- D. This bias arrangement is reversed and does not occur in normal transistor operation.
NPN Transistor Biasing
For active-mode amplification, an NPN transistor's base-emitter junction must be forward biased while the base-collector junction is reverse biased.
- NPN = layers Negative-Positive-Negative
- Forward bias on base-emitter allows base current
- Reverse bias on base-collector allows amplified collector current
Memory trick: 'Forward the Base-Emitter, Reverse the Collector' for active amplification.