\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{shapes,positioning}
\usetikzlibrary{narrow}

\tikzset{block/.style={draw, rectangle, line width=2pt,
     minimum height=3em, minimum width=3em, outer sep=0pt}}
\tikzset{sumcircle/.style={draw, circle, outer sep=0pt, 
     label=center:{{$\sum$}}, minimum width=2em}}
\tikzset{every picture/.style={auto, line width=1pt,
          >=narrow,font=\small}}

\begin{document}
\begin{tikzpicture}

\node[block](G1){$G_1$};
\node[block,right=15mm of G1](G2){$G_2$};
\draw[->](G1)--node[above](U){$U$}(G2);
\node[block,below=12mm of U](G3){$G_3$};
\node[sumcircle,left=12mm of G1](sum){};
\node[coordinate, left=10mm of sum](ref){};
\draw[->](ref)--node[xshift=-2mm](){$R$}(sum);
\draw[->](sum)--node[xshift=-1mm](){$E$}(G1);
\node[coordinate, right=12mm of G2](output){};
\draw[->](G2)--node[xshift=0mm](Y){$Y$}(output);
\draw[->](Y)|-(G3);
\draw[->](G3)-|(sum);
\end{tikzpicture}
\end{document}