f(x) f'(x) puzzle

Enter a trial function f(x) that can depend on up to two parameters a and b. The goal is to find a function such that the product of the function and its derivative is constant for all x greater than some x0. If both x and f(x) are quantities rather than numbers [see "Semantic Aspects of Quantity" on the mathMINDhabits website] explain how and why this constant can be adjusted.