A wideband low noise ac current source

Hello everyone,
 
I am trying to design a 100uA-100mA AC current source, stable over 100Hz-100KHz and the AC voltage supply goes from 5mV-4V. 

Ive gone through the other topics on the same issue but no one posted about such a wide range of load. 100Ohms to 100kOhms.

In theory, the current source is suppose to be independant of any load however in practice that's not happening.

Ive tried out the howland circuits but my main question there is, how is it suppose to be independant of load. The current through a changing value of resistor can be constant only if the voltage increases and the voltage of the howland current pump is limited to the supply voltage. Especially in the maximum case (4V,100mA).

Any input on how to go about designing this or what the flaw in my understanding is will be appreciated.

Thanks