Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.
Please review the calculation in the appended pdf file. In the general case, the output current isn't independent of output voltage, in so far your schematic is incomplete. You usually try to implement a current source with (ideally) infinite output resistance, the conditions are derived in the document.
It assumes however infinite OP gain, introducing finite OP gain further increases the complexity.
- - - Updated - - -
There are also various previous threads about the Howland current source, e.g. https://www.edaboard.com/threads/317636/