Some circuits need a constant-current source that does not necessarily connect to a power-supply rail or to ground. The circuit in the Figure 1 shows a simple method for achieving that configuration.
|Figure 1.||The simple method achieves a constant-current source
that does not necessarily connect to a power-supply
rail or to ground.
The LM317 voltage regulator develops 1.25 V between the OUT and the ADJ pins. Placing a resistor between those pins produces a constant current. Thus, the circuit's output current is 1.25 V/RADJ. The transistor lets the circuit source more current than the regulator alone can provide once the current through R1 creates enough voltage to turn the transistor on. Otherwise, the regulator supplies the load current.