Multi range current source based on Howland
20 ohms and 2 amps requires a voltage of 40 volts so that is the smallest voltage power supply you need at one end of the scale. 60 ohms and 2 amps requires a supply voltage of at least 120 volts and, given a few volts lost in the pass transistor and/or op-amp, the minimum supply voltage should now be regarded as 125 volts.

The problem I see is that with a 125 volt supply and a 20 ohm load that only needs 40 volts across it to conduct 2 amps, is the volt drop across the "silicon" i.e. 85 volts at 2 amps - this is a power of 170 watts. The maximum power dissipation of the APEX amplifier is 125 watts.

You need a substantially different approach and I would recommend using PWM to produce a current into your load - should you not be able to live with the ripple voltage then an inductor and capacitor will also be needed BUT, this solution will not self-heat with 170 watts.

Something like this may be of some interest topologically: -



It produces a constant current up to 1.5 amps into a string of LEDs and, of course, the LEDs could be replaced by your load. Or maybe something that works from a lower supply voltage and steps up to produce a bigger voltage (again this is limited to only 1 amp): -