%%%------------------------------------------------------------------------------------------------------------
\begin{tikzpicture}
\begin{axis}[  
  name=sigmoid,
  width=6cm, height=4.5cm, 
  xtick={-5,-2.5,...,6},
  ytick={0,0.5,1.0},
  xlabel={$x$},
  ylabel={$y$},
  xlabel style={xshift=2.4cm,yshift=0.7cm},
  axis y line=middle,%y轴居中
  ylabel style={xshift=0.1cm,yshift=0cm},
  x axis line style={->},%x轴箭头
  axis line style={very thick},
  axis x line*=bottom,%将上面的x轴去掉，仅保留下面的x轴
  xmin=-6,
  xmax=6,
  ymin=0,
  ymax=1.4]
\addplot[draw=red,very thick]{1/(1 + exp( -x))};
\end{axis}
\node [anchor=north] (labelc) at (2.3,-0.5) {\footnotesize{(a)}};
\begin{axis}[
  at={(sigmoid.east)},
  anchor=east, 
  xshift=6cm,
  yshift=0cm,  
  width=6cm, height=4.5cm, 
  xtick={-4,-2,...,4},
  ytick={0,0.1,0.2},
  xlabel={$x$},
  ylabel={$y$},
  xlabel style={xshift=2.4cm,yshift=0.7cm},
  axis y line=middle,%y轴居中
  ylabel style={xshift=0.1cm,yshift=0cm},
  x axis line style={->},%x轴箭头
  axis line style={very thick},
  axis x line*=bottom,%将上面的x轴去掉，仅保留下面的x轴
  xmin=-6,
  xmax=6,
  ymin=0,
  ymax=0.3]
\addplot[draw=ublue,very thick]{(1/(1 + exp( -x)))*(1-(1/(1 + exp( -x))))};
\end{axis}
\node [anchor=north] (labelc) at (8.2,-0.5) {\footnotesize{(b)}};
\end{tikzpicture}
%%%------------------------------------------------------------------------------------------------------------