% Author: Dr. Matthias Jung, DL9MJ
% Year: 2020
\begin{circuitikz}
    \draw[-Triangle](0,0) -- (5.0,0.0) 
        node[anchor=north east] () {$f$};
    \draw[-Triangle](0.0,0.0) -- (0.0,3.0)
        node[anchor=north east] () {$U$};

    \draw [thick] plot [rounded corners=10mm] coordinates {
        (1.80,2.50)
        (2.50,0.00)
        (3.20,2.50)
    };

    \draw [dashed] (2.5,0) -- (2.5,3);
\end{circuitikz}