analog - Designing 0- 300 mA current source with 12v Voltage supply (load = constant 40ohm)

I'm trying to design a 0 to 300 mA controlled current source, the problem is that the supply voltage is a normal adapter with a 12v output. I'v already tried several op-Amp based circuits but the voltage drop over the load exceeds the power supply in all of them. suggestions for a circuit? Load is a constant 40ohm device. Thank you all