This is a linear driver based around an NPN transistor as the feedback amplifier, and a MOSFET as the series-pass transistor. The current through the LED or Laser Diode is approximately 0.5/R3, where R3 is the series resistor between the Source of the MOSFET and ground. Note: The current provided by this driver does vary a small amount with increased voltage.
Op-Amp + MOSFET
This is a linear driver based around an Op-Amp open-loop difference amplifier and a MOSFET as a series-pass transistor. The setup is similar to above. The output of the Op-Amp is connected to the Gate of the MOSFET. The Drain of the MOSFET is the negative output terminal, and VCC is the positive output terminal. The Source of the MOSFET is connected to a resistor of arbitrary value, typically a low value like 1 Ohm. The Op-Amp's Negative input pin is connected to the Source of the MOSFET. The Op-Amp's Positive input pin is connected to a resistive voltage divider or potentiometer, which sets the current based on your choice of resistor:
I = Vfb/R
Where Vfb is the Op-Amp's Negative input pin voltage. The Op-Amp and resistive voltage divider or potentiometer must use a fixed voltage power supply (within the limits of the Op-Amp's operating voltage) in order for this circuit to work. This can be achieved with either a fixed voltage regulator, like an LM7805, or by making a fixed voltage reference using a Zener diode and resistor, though this method is less accurate and current can vary with increasing voltage.
Probably the simplest of the linear drivers, this driver uses a common LM317 adjustable voltage regulator to regulate current to the load. A resistor is placed between the Vout and Adj pins of the LM317, and the positive output terminal is the Adj pin. The value of the resistor is selected to set the current according to:
I = 1.25/R
Note: This driver requires the source voltage to be approximately 2V higher than the load voltage.
This driver is the most difficult, but is limited only by the selected inductor, capacitor, MOSFET or transistor switch, and diode. This is typically done for very high currents, or a need for greater control.