PNP bipolar junction transistor BJT current source circuit
PNP BJT current sources are nice because they provide current from the positive side of a load. Therefore, I put together the following demonstration video.
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Basic properties of simple PNP current source circuit
- The emitter resistor sets the current based on the voltage across it. Since this circuit uses a PNP BJT, the emitter resistor is between the Vcc (positive side of the power supply). I = V/RE (current equals voltage across emitter resistor divided by the resistor’s resistance.
- The voltage at the PNP base, which can be set by a trimpot in relationship to Vcc, will be about 0.7V higher than the voltage across the current setting resistor. Remember that the trimpot’s voltage is in relationship to the positive side of the power supply (Vcc), not the negative side of the power supply (ground).
Trimpot controlled PNP BJT constant current source video:
Zener diode controlled 2N3906 PNP BJT constant current source video:
Showing how the power supply voltage splits up across the transistor collector-emitter and load, in order to hold a steady voltage across the current setting resistor.
Voltage division measurements of PNP BJT constant current source 2N3906
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