Reading Bipolar Junction Transistor BJT Schematics

There are many bipolar junction transistors part numbers to choose from. First off, the BJTs come in 2 types, either NPN or PNP. A schematic diagram with one or more Bipolar Junction Transistors (BJTs) will show the NPN or PNP schematic symbol, and what each transistor connects to. Hopefully the diagram will also include the part number for each BJT, and that you already have that part in your inventory. That exact part number does not have to be used. But, you should try to substitute it with one with similar properties. Ultimately, be aware that some circuits are more picky about the exact transistor used than others.

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Some primary bipolar junction transistor topics:

Bipolar Junction Transistor schematics are often accompanied by written text about the circuit.

  • NPN
  • PNP
  • Input – Signal
  • Output – Power
  • Collector (C) – Base (B) – Emitter (E) terminals
  • Gain
  • Base to Emitter Diode
  • Single Subscripts – VC, VB, VE, IC, IB, IE
  • Double Subscripts VBE, ICE, etc.

Some Bipolar Junction Transistor circuits

  • Switch – High side or low side
  • Emitter Follower
  • Current Source
  • Schmitt trigger
  • Current mirror
  • Pulse Generator

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