017 PNP BJT Current Source set with Zener Diode

Basic electronics circuit fragments and simple circuits 15 PNP BJT current source set by zener diode diagram by electronzap electronzapdotcom
Basic electronics circuit fragments and simple circuits 15 PNP BJT current source set by zener diode diagram by electronzap electronzapdotcom
  • Emitter resistor (connected between supply voltage/Vcc and PNP BJT emitter) limits the current through it, based on the voltage across it.
  • The PNP BJT Emitter to Collector, and the load (LEDs in some examples above), are in series with the emitter resistor. Series components pass the same amount of current.
  • The voltage across the PNP emitter resistor is approx. 0.6V less than the voltage across the reverse biased zener diode. Adding a forward biased rectifier diode in series with the RB zener diode will drop the zener voltage an additional 0.6V. That offsets the approx. 0.6V PNP BJT base to emitter rise.
    1. 5.6V zener diode will set 5.6V less than Vcc (positive supply voltage) at the PNP BJT base.
    2. If you only have 5.1V zener diodes, but want to be closer to a 5.6V zener voltage, then you can take the reverse biased 5.1V zener diode and add an approx. 0.6V forward voltage rectifier diode in series with it. Totaling approx. 5.7V.
  • The supply voltage must be high enough to provide 5V across the emitter resistor, plus whatever voltage the load needs to pass that amount of current, plus some extra voltage that the transistor E-C (Emitter to Collector) needs to drop.
  • Notice how the load for a PNP BJT current source is on the low side of the circuit (closer to ground). This is a more natural position for a load to be than when it is on the high side (positive supply voltage). NPN BJTs have their loads on the high side when wired as a current source.

Voltage:

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