Bipolar Junction Transistor
A three-layer (NPN or PNP) device where a small base current controls a much larger collector current.
A BJT has an emitter, base and collector. In an NPN transistor, forward-biasing the base–emitter junction (~0.7 V) lets electrons flood from the emitter through the thin base into the collector.
(or ) is typically 50–500 and varies a lot between parts, so good circuits don't depend on its exact value. More precisely, the collector current depends exponentially on :
Common parts: BC547/2N3904 (NPN small signal), BC557/2N3906 (PNP), TIP120 (Darlington power).
Sub-topics (4)
BJT Regions of Operation
Cut-off, active, saturation and reverse-active — which junctions are biased which way.
BJT as a Switch
Drive a load from a microcontroller pin by saturating an NPN transistor.
BJT Biasing
Set a stable DC operating point so an amplifier works regardless of β and temperature.
Darlington Pair
Two transistors cascaded so their current gains multiply.
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