\documentclass[tikz]{standalone}
\begin{document}

    \begin{tikzpicture}
        % Input Layer
        \node (x1) at (-3, 2) {$x_1$};
        \node (x2) at (-3, 1) {$x_2$};
        \node at (-3, 0.5) {$\vdots$};
        \node (xi) at (-3, 0) {$x_i$};
        \node at (-3, -0.5) {$\vdots$};
        \node (xn-1) at (-3, -1) {$x_{n-1}$};
        \node (xn) at (-3, -2) {$x_n$};

        % Neuron
        \node[draw, circle,minimum size=50pt] (neuron) at (0, 0) {$\theta$};

        \draw[->] (x1) to node [sloped, above] {$w_1$} (neuron);
        \draw[->] (x2) to node [sloped, above] {$w_2$} (neuron);
        \draw[->] (xi) to node [sloped, above] {$w_i$} (neuron);
        \draw[->] (xn-1) to node [sloped, above] {$w_{n-1}$} (neuron);
        \draw[->] (xn) to node [sloped, above] {$w_n$} (neuron);

        % Output Layer
        \draw[->] (neuron) to node [above] {$f\left(\sum\limits_{i=1}^nw_i x_i-\theta\right)$} (4, 0);

    \end{tikzpicture}
\end{document}