simulate this circuit – Schematic created using CircuitLab
I was wondering whether I could use this circuit as a constant current source provided my load circuit impedance was less than Rb/hfe? All points of view are welcome….:)
Electronic – Constant Current Source circuit improvisation
current-sourcetransistors
Related Solutions
There is no return path from ground back to the other side of the source, therefore no current will flow to ground. All of the current will flow through the resistor.
Try this:
simulate this circuit – Schematic created using CircuitLab
This circuit is capable of ~1% accuracy with good layout. If the battery is removed, the op-amp will current limit into the base, which should not hurt it. If you don't like that, or if you use another op-amp that doesn't current limit as well, add a small base resistor such as 100 ohms.
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Best Answer
It's not going to be an accurate constant current source because the voltage-current slope of the BJT is not that great but if you can live with this then that's OK: -
Ideally, once outside the saturation region, the lines would be horizontal to the base line of Vce - horizontal means that no-matter how you change the resistance of the load (which obviously changes the voltage across it and Vce, the current would remain the same. As you can see, the lines are not horizontal but have a gentle upward slope and get progressively less-horizontal for higher base currents. Added to this your circuit won't control the base current as neatly as you think - current through the emitter will slightly reduce the base current. If you want a better constant current source try one of these: -