Step 4: More
1) Adjust R3 - You can easily produce a different current by changing the resistance of R3. The voltage drop over R3 will remain constant (due to the op amp), so the current can be calculated as before from ohms law: I = 1.36V/ R3. The upper limit of the current is determined by the power tolerance of R3. When R3 gets smaller, the current is larger, but you may burn out your resistor if you exceed the resistor’s power rating. If you want to easily adjust the current out, R3 could be replaced by a potentiometer.
2) Adjust the voltage divider - Another way to change the current is to change the voltage from the voltage divider. Adjusting R1 and R2 augments the voltage in the Ohm’s law calculation. However, the larger the voltage drop over R3, the smaller voltage drop is available over the load. Remember that your power supply a limited voltage (15 V in our circuit).
*Note that the transistor must have at least about 0.2 V drop from the collector to the emitter terminal (corresponding to the point that connects to the R3 and the point connected to the load). Without this drop, the transistor no longer functions properly and the current sags to almost 0 mA. One can see this sag begin around 1.4 k.
The current source can be useful in lighting up LEDs as they need only a small current and about a 0.6 V drop across.