feedback2

In this example the summation circles have a different character in them. It is of course simple to change to e.g. the one used previously, or to change the previous examples to use this summation circle style.

Download feedback2.tex

\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}