NPN BJT current source – Bipolar Junction Transistor – 2N3904

NPN Bipolar Junction Transistors (BJT) is technically used to make a current sink, but commonly anything that sets a specific current through series components or load is called a current source.

NPN BJT diagrams

2N3904 NPN BJT pin layout schematic symbol and current paths diagram by electronzap electronzapdotcom
2N3904 NPN BJT pin layout schematic symbol and current paths diagram by electronzap electronzapdotcom

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NPN bipolar junction transistor BJT current source sink circuit schematic diagram by electronzap electronzapdotcom
NPN bipolar junction transistor BJT current source sink circuit schematic diagram by electronzap electronzapdotcom

Basic properties

  • Voltage is applied to the base of the transistor.
  • That voltage minus the base to emitter diode drop (approx. 0.6-0.7) is applied to the emitter resistor.
  • Emitter resistor sets the current (voltage across it, divided by it’s resistance) through it, plus through the collector to emitter, and any series components on the collector side of the transistor.
  • The supply voltage must be high enough to power all components for the amount of current.
  • NPN BJT current source is on the more negative side of the load. So, it actually sinks current as far as accurate analysis is concerned.

Video

NPN BJT current source trimpot controlled circuit 2N3904 bipolar junction transistor electronics

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