Hi Kingearlkwan,
I have attached a simple schematic of a voltage controlled current source. The analysis is straight forward. We have an op-amp configured as a follower and the negative op amp terminal attached to the source of the transistor. Since the op amp will have a very high open loop gain at DC the negative input termonal will see the same voltage as the positive terminal, Vin. Therefore the voltage presented across the resistor is the input voltage which you can vary. By selecting a sensible value for the resistor knowing the input voltage range that you wish to apply you can then set the output current range. So for an input range of 1V to 5V and a 1K Ohm resistor you can vary the output current from 1mA to 5mA.
Does this help answer your question?
Regards, Ken.