% ---------------------------------------------------------------
% Álgebra de blocos: redução de uma malha de realimentação negativa.
% ---------------------------------------------------------------
\begin{tikzpicture}[>=Latex, line width=0.9pt, node distance=1.3cm,
  bloco/.style={draw, minimum width=1.8cm, minimum height=1.0cm, fill=blue!8},
  soma/.style={draw, circle, minimum size=0.6cm, inner sep=0pt},
  seta/.style={->}]

  \node (x) at (0,0) {$X(s)$};
  \node[soma, right=1.0cm of x] (sum) {};
  \node[bloco, right=1.1cm of sum] (g) {$G(s)$};
  \node[right=1.0cm of g] (y) {$Y(s)$};
  \node[bloco, below=0.9cm of g] (h) {$H(s)$};

  \draw[seta] (x) -- (sum);
  \draw[seta] (sum) -- (g);
  \draw[seta] (g) -- (y);
  \draw[seta] (g.south) -- (h.north);
  \draw[seta] (h.west) -| (sum.south);
  \node[font=\small] at ($(sum.north west)+(-0.12,0.12)$) {$+$};
  \node[font=\small] at ($(sum.south west)+(-0.12,-0.12)$) {$-$};

  \node[font=\Large, right=1.1cm of y] (eq) {$\equiv$};

  \node[right=1.1cm of eq] (x2) {$X(s)$};
  \node[bloco, right=of x2, minimum width=3.2cm] (geq) {$\dfrac{G(s)}{1+G(s)H(s)}$};
  \node[right=of geq] (y2) {$Y(s)$};
  \draw[seta] (x2) -- (geq);
  \draw[seta] (geq) -- (y2);

\end{tikzpicture}