2.2a.2 Supernodes

Supernode

If two extraordinary nodes are connected by a voltage source, the two nodes can be combined into a supernode. If one of the extraordinary nodes that make up a supernode is the ground node, it is called a quasi-supernode:
[DIAGRAM]
A supernode can be represented as a single node in terms of currents flowing out of it, plus an auxiliary equation that defines the voltage difference between the two nodes.

\usepackage{circuitikz}
 
\begin{document}
 
\begin{circuitikz}
    % Define nodes
    \node (V1) at (0,4) {$V_1$};
    \node (V2) at (8,4) {$V_2$};
    \node (N1) at (2,4) {};
    \node (N2) at (4,4) {};
    \node (N3) at (6,4) {};
    
    % Draw circuit
    \draw (V1) to [short, i=$i_1$] (N1);
    \draw (N1) to [R, l=$R_1$] (N2);
    \draw (N2) to [V, v^=$V_2$, *-*) (V2);
    \draw (N2) to [R, l=$R_2$] (N3);
    \draw (N3) to [short, i=$i_2$] (V2);
    
    \draw (N1) to [short] (-1,4) to [V, v^=$V_{in}$, *-*) (-1,0);
    \draw (N3) to [short] (9,4) to [R, l=$R_3$] (9,0) to [short] (V2);
    
    % Quasi-super node
    \draw [dashed, blue] (-1,3.5) rectangle (2,4.5);
    \node at (0.5,4.2) {Quasi-Super node};
    
    % Supernode
    \draw [dashed, red] (4,3.5) rectangle (9,4.5);
    \node at (6.5,4.2) {Supernode};
\end{circuitikz}
 
\end{document}