\begin{tikzpicture}[auto, node distance=2.5cm, >=latex]
    % Estilo dos blocos
    \tikzstyle{block} = [draw, rectangle, minimum height=1.3cm, minimum width=1.4cm, line width=0.8pt]
    \tikzstyle{input} = [coordinate]
    \tikzstyle{output} = [coordinate]

    % Nós
    \node [input, name=input] {};
    \node [block, right of=input, text centered, text width=2.5cm] (system) {$G(s)$};
    \node [output, right of=system] (output) {};

    % Conexões
    \draw [draw, ->, line width=1pt] (input) -- node {$U(s)$} (system);
    \draw [draw, ->, line width=1pt] (system) -- node {$Y(s)$} (output);
\end{tikzpicture}