Although I quoted Meggido's 32-yr-old LP algorithm in the comments, a simpler-to-implement randomized algorithm has expected linear time and space complexity, and is the most common implementation, e.g., as cite below the figure.
![EnclosingCirc][1]
(Image from [code.project.com](http://www.codeproject.com/Articles/22568/Computational-Geometry-C-and-Wykobi).)
A clear description of this algorithm may be found in this lecture prepared by Marc van Kreveld and Maarten Loffler: [PDF slides download](http://www.cs.uu.nl/docs/vakken/ga/slides4b.pdf).
CGAL implements both a heuristic algorithm, and the LP algorithm, in $\mathbb{R}^d$: CGAL implementation.