Bipolar transistor consists of two PN-junctions — two diodes back to back.
How it works: when you apply a small current to the base (from emitter to base, for NPN type), a much larger current can flow through the transistor (from emitter to collector, for NPN type)
- One of them is usually grounded.
The choice between NPN and PNP is really arbitrary: all that matters is that the proper current directions are maintained.
- To get transistors to work,
- For NPN, collector must be more positive in voltage than emitter.
- For PNP, collector must be more negative in voltage than emitter.
- IB + IC = IE,
- IC is almost equal to IE, as the base current is very small.
- IC = bIB, b is called the current gain, typically 20 ~ 200
- Base(small) current is the only current that goes through the base wire of the transistor.
- Collector(large)current is the only current that goes through the collector wire of the transistor.
- emitter current is the sum of the base and collector current.