\documentclass{minimal}
\usepackage{tikz}
\usetikzlibrary{circuits.logic.US,circuits.logic.IEC}
\begin{document}
\begin{tikzpicture}[minimum height=0.75cm]
\node[or gate US, draw,logic gate inputs=nnn] (A) {};
\foreach \a in {1,...,3}
\draw (A.input \a -| -1,0) -- (A.input \a);
\draw (A.output) -- ([xshift=0.5cm]A.output);
\end{tikzpicture}
\begin{tikzpicture}[minimum height=0.75cm]
\node[and gate US, draw,logic gate inputs=nnn] (A) {};
\foreach \a in {1,...,3}
\draw (A.input \a -| -1,0) -- (A.input \a);
\draw (A.output) -- ([xshift=0.5cm]A.output);
\end{tikzpicture}
\end{document}
By slight modification of this code you can acheive gates with more inputs (modifying inputs=nnn and \foreach \a in {1,...,3}).
For more, check TikZ & PGF manual section 29.3 Page 300
You are using circuitikz inside a node. The circuitikz environment is nothing but a tikzpicture environment in disguise. Using tikzpicture inside a node is not a good idea and it will cause odd things. You can use a box instead. I have created \mycircuita and \mycircuitb boxes (with 0 and 20V) and used them inside the callout node.
\documentclass[tikz,border=3mm]{standalone}
\usetikzlibrary{positioning,%
shapes,shapes.callouts%
}
\usepackage{fouriernc}
\usepackage[scaled=0.83]{helvet}
\usepackage[scaled=0.82]{luximono}
\usepackage{marvosym,pifont}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
%---------------------------------------------------------------%
\usepackage[european,siunitx]{circuitikz}
\usepackage{circuitikz}
%---------------------------------------------------------------%
\newsavebox{\mycircuita}
\sbox{\mycircuita}{%
\begin{circuitikz}[sharp corners]
\draw[ultra thick, blue] plot[smooth,domain=-0.25*pi:-0.75*pi, samples=36] (0.25*pi+\x,{-1*sin(2*\x r)});
\draw[ultra thick, red] plot[smooth,domain=-0.25*pi:-0.75*pi, samples=36] (0.25*pi+\x,{+1*sin(2*\x r)});
\draw[ultra thick, red] plot[smooth,domain=-0.75*pi:-1.25*pi, samples=36] (0.25*pi+\x,{+1*sin(2*\x r)});
\draw[ultra thick, blue] plot[smooth,domain=-0.75*pi:-1.25*pi, samples=36] (0.25*pi+\x,{-1*sin(2*\x r)});
\draw (0,3) node[right] {\SI{+5}{V}}
to [R=$R_s$,o-] (0,+1) node[right] {A}
to [R=$R_k$,*-*] (0,-1) node[right] {B}
to [R=$R_s$, -o] (0,-3)
node[right] {\SI{0}{V}};
\end{circuitikz}
}
\newsavebox{\mycircuitb}
\sbox{\mycircuitb}{%
\begin{circuitikz}[sharp corners]
\draw[ultra thick, blue] plot[smooth,domain=-0.25*pi:-0.75*pi, samples=36] (0.25*pi+\x,{-1*sin(2*\x r)});
\draw[ultra thick, red] plot[smooth,domain=-0.25*pi:-0.75*pi, samples=36] (0.25*pi+\x,{+1*sin(2*\x r)});
\draw[ultra thick, red] plot[smooth,domain=-0.75*pi:-1.25*pi, samples=36] (0.25*pi+\x,{+1*sin(2*\x r)});
\draw[ultra thick, blue] plot[smooth,domain=-0.75*pi:-1.25*pi, samples=36] (0.25*pi+\x,{-1*sin(2*\x r)});
\draw (0,3) node[right] {\SI{+5}{V}}
to [R=$R_s$,o-] (0,+1) node[right] {A}
to [R=$R_k$,*-*] (0,-1) node[right] {B}
to [R=$R_s$, -o] (0,-3)
node[right] {\SI{20}{V}};
\end{circuitikz}
}
\begin{document}
\begin{tikzpicture}
\coordinate (a) at (0,0);
\coordinate (b) at (4,0);
\draw (1,-0.1) -- (1,0.1);
\draw (2,-0.1) -- (2,0.1);
\draw (3,-0.1) -- (3,0.1);
\draw[ultra thick,*-*] (a) -- (b);
\node[shape=rectangle callout,
draw, rounded corners,
callout pointer width=3.3 mm,
callout pointer shorten=-2mm,
font=\sffamily\footnotesize,
align=center,
callout absolute pointer={(b)},
scale=0.5] at ([xshift=19mm,yshift=3mm] b)
{termination circuit\\
\usebox{\mycircuita}
};
\node[shape=rectangle callout,
draw, rounded corners,
callout pointer width=3.3 mm,
callout pointer shorten=-2mm,
font=\sffamily\footnotesize,
align=center,
callout absolute pointer={(a)},
scale=0.5] at ([xshift=-19mm,yshift=3mm] a)
{termination circuit\\
\usebox{\mycircuitb}
};
\end{tikzpicture}
\end{document}
As an alternative, you could also use pic facility of tikz but using a box is simpler in this case.
Best Answer
Here is a first approach to a smaller logic circuit, with more compact positioning, and colored label text.