BJT current source

Current sources provide a set amount of current as long as there is enough voltage to do so.

NPN BJT current source 2N3904 set by zener diode for 5mA schematic and pictorial diagram by electronzap
NPN BJT current source 2N3904 set by zener diode for 5mA schematic and pictorial diagram by electronzap

NPN Bipolar Junction Transistors (BJTs) with a resistor from emitter to ground, holds a voltage across the emitter resistor. That voltage, and the resistance sets the current. That current flows through the resistor, and collector side of the transistor.

  • Base voltage, minus about 0.6V due to the base to emitter diode drop, is what is held across the emitter resistor as long as current can flow from collector to emitter. Base voltage – approx. 0.6V = voltage across emitter resistor.
  • The base is usually powered by a signal that can’t provide enough current to hold the desired voltage across the emitter resistor by itself. A small amount of base to emitter current however, results in a lot of collector to emitter current. So almost all of the current holding the voltage of the emitter resistor is from the collector. The signal voltage should hold steady.
  • Adding a load between the positive power supply (Vcc) and collector, does not change the current as long as it doesn’t happen to limit current even more than what is set by the emitter resistor.
  • Emitter resistor, NPN BJT collector to emitter, and load, all divide up the total power supply voltage.


As long as the load doesn’t consume too much power…

  • 5.6V zener diode sets base voltage to 5.6V, which sets the voltage across the emitter resistor to 5V due to the 0.6V base to emitter voltage drop.
  • 1,000Ω with 5V across it, passes 5V/1000Ω = 0.005A. 0.005A is usually referred to as 5mA.
  • All components connected in series with the emitter resistor will pass that same amount of current. That is the electrical property of components in series.

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