Patent Derivatives...

Can you tell me or point me in the right direction... regarding patent derivatives, if there is such a thing... I know about copyright derivatives... and that they differ... but if someone (incl. a subsidiary company) enhances your patented work, who owns the derivative or enhanced work?

I am a law school graduate. What I offer is mere information, not to be construed as forming an attorney client relationshp.

Basically, patents are only limited in time. So, if someone makes a derivative product outside the time the patent is enforceable, not much original creator can do.

http://www.xilinx.com/prs_rls/00113itc.htm