Circuit Basics (2)– transistor

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)
3 leads/legs:
  • emitter,
  • base,
  • collector.
    • One of them is usually grounded.

transistor_1

transistor_2

The choice between NPN and PNP is really arbitrary: all that matters is that the proper current directions are maintained.
Some rules:
  • 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.
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