What's the role of each component in voltage controlled current source circuit for VLC?

[Getoof]

Hi, I'm trying to design a driver circuit for the purposes of modulation of a white LED for visible light communication. The circuit I need to build has to be a voltage controlled current source.
The speed of modulation is between 30-50MHz. So really fast. This is the basic schematic of the circuit but i cant seem to get it working on Multi-Sim.



I think what I'm really missing is an understanding of what each component in the circuit is doing. The role of the op-amp and the FET. Because of the high speeds the components I'm using are:

AD8021 - Low noise, high speed amplifier ( http://www.kynix.com/uploadfiles/pdf/AD8021ARM.pdf )

PD55003-E (1GH, 2.5A)

If someone could explain the role of each of these components in the circuit i would be very grateful

Thanks

[Jean-Baptiste Kempf]

I think you are on the wrong forum.