\documentclass{article}
\usepackage[pdftex,active,tightpage]{preview}
\setlength\PreviewBorder{2mm}
\usepackage{tikz}
\usetikzlibrary{shapes, calc}
\tikzstyle{vertex}=[draw,fill=black!15,circle,minimum size=20pt,inner sep=0pt]

\begin{document}
\begin{preview}
\begin{tikzpicture}[font=\sffamily,thick,level/.style={sibling distance=50mm/#1}]
    \node[vertex] {S}
        child {
            node[vertex] {A}
            child {
                node[vertex] {a}
            }
            child {
                node[vertex] {A}
                child { node[vertex] {a} }
                child {
                    node[vertex] {A}
                    child { node[vertex] {$\varepsilon$} }
                }
            }
        } child {
            node[vertex] {B}
            child { node[vertex] {b} }
        }
    ;
\end{tikzpicture}
\end{preview}
\end{document}
