\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:{{\Large$+$}}, minimum width=2em}}

\tikzset{every picture/.style={auto, line width=1pt, >=narrow,
    x=1mm, y=1mm, font=\small}}

\begin{document}
\begin{tikzpicture}
\node[block](G1){$G_1$};
\node[sumcircle,right=13mm of G1](sum2){};
\node[block,right=10mm of sum2](G2){$G_2$};
\node[block,below=9mm of sum2](G3){$G_3$};
\node[sumcircle,left=12mm of G1](sum){};
\node[coordinate, left=10mm of sum](ref){};
\node[coordinate, right=12mm of G2](output){};
\node[coordinate, above=10mm of sum2](dist){};
\draw[->](ref)--node[xshift=-1mm](){$R$}(sum);
\draw[->](sum)--node[xshift=-1mm](){$E$}(G1);
\draw[->](G1)--node(){$U$}(sum2);
\draw[->](sum2)--(G2);
\draw[->](dist)--node[yshift=2mm](){$V$}(sum2);
\draw[->](G2)--node[xshift=0mm](Y){$Y$}(output);
\draw[->](Y)|-(G3);
\draw[->](G3)-|(sum);
\end{tikzpicture}
\end{document}