% Author: Dr. Matthias Jung, DL9MJ
% Year: 2020
\begin{circuitikz}
    \draw[-Triangle](0.0,0.0) -- (5.0,0.0) node[anchor=north] () {$f$};
    \draw[-Triangle](0.0,0.0) -- (0.0,3.0) node[anchor=east] () {$U$} ;
    \draw[rounded corners=3mm, thick, black] 
    (0,0) --
    ++(0.6,0) --
    ++(0.6,2.5) --
    ++(3.7,0);
    \draw [dashed] (1.4,0) -- ++(0,3);
\end{circuitikz}