\documentclass{standalone}

\usepackage{tikz,pgfplots}
\usepackage{siunitx}
\usepackage[siunitx,european]{circuitikz}
\usepackage{mathrsfs}
\usetikzlibrary{arrows}
\usepackage{amsbsy}
\usetikzlibrary{fadings}
\usetikzlibrary{patterns}
\usetikzlibrary{automata,positioning}
\usepackage{mwe,subcaption,tikz}
\tikzset{boximg/.style={remember picture,red,thick,draw,inner sep=0pt,outer sep=0pt}}
\pgfplotsset{compat=1.11}
\begin{document}
\definecolor{mygreen}{rgb}{0,0.78,0}
\begin{tikzpicture}

	\fill[opacity=0.15] (0.75,-0.75) rectangle(3.25,2.5);
	\draw (2,2.4) node[color=black!60, above]{internal};
	\draw (-1,2) to [R, l=$R_g$, o-] (0.75,2)
	to [short, i=$i_g$] (2,2)
	to [C, l=$C_{gd}$, v=$u_g$] (2,0) node[ground]{};

\end{tikzpicture}
\end{document}