Reading the source — the circuitikz behind four schematics

The previous post taught the build-time TikZ pipeline to draw electrical schematics with circuitikz¹ and showed off four classic circuits. It made the point that none of those figures are images — each is a dozen lines of source the build compiles into vector SVG — but it left that source mostly implicit. This post is the companion: each diagram, re-rendered, with the exact circuitikz block that produced it printed underneath and walked through.

The whole vocabulary is small. A component is a path operation: to[R=$R$] draws a resistor between the previous coordinate and the next one and labels it \(R\). Relative moves use ++(dx,dy); the |- and -| operators route an orthogonal path between two points; o-o and -* decorate the ends of a segment with open or filled terminals. That is very nearly all four circuits need. The TikZ manual’s native circuit library covers the same ground with different syntax².

1 · An RC low-pass filter

A resistor in series, a capacitor to ground, output taken across the capacitor³.

And the source:

\begin{circuitikz}[scale=1.1, transform shape]
\draw (0,0) node[ground]{} to[sV=$v_\text{in}$] ++(0,3) coordinate(t);
\draw (t) to[R=$R$, o-o] ++(3,0) coordinate(o);
\draw (o) to[C=$C$] (o |- 0,0) -- (0,0);
\draw (o) to[short,-o] ++(1.2,0) node[right]{$v_\text{out}$};
\end{circuitikz}

Read it line by line. The first \draw starts at the origin, drops a ground node there, then draws the source sV (a sinusoidal voltage source) straight up three units, naming the top coordinate(t). The second runs the resistor right across to coordinate(o), with o-o putting open terminals on both ends of that top rail. The third drops the capacitor from o down to the point (o |- 0,0) — “the x of o, the y of the origin” — then closes the loop back to the source. The last line taps the output node off o with a short wire ending in an open terminal. Its transfer function and corner frequency are

\[H(j\omega) = \frac{1}{1 + j\omega RC}, \qquad f_c = \frac{1}{2\pi RC}.\]

2 · A series RLC circuit

The whole loop is a single path, which is why this one is the shortest source of the four³.

The source:

\begin{circuitikz}[scale=1.2, transform shape]
\draw (0,0) to[sV=$v_\text{in}$] ++(0,3) to[R=$R$] ++(2.4,0) to[L=$L$] ++(2.4,0) to[C=$C$] ++(0,-3) -- (0,0);
\end{circuitikz}

One \draw chains four components around a rectangle: up through the source, right through R=$R$, right again through the inductor L=$L$, down through the capacitor, and a plain -- back to the origin to close it. Each ++(dx,dy) is relative to wherever the previous component ended, so the loop is described purely by the deltas — no point is named. It resonates at \(\omega_0\) with quality factor \(Q\):

\[\omega_0 = \frac{1}{\sqrt{LC}}, \qquad Q = \frac{1}{R}\sqrt{\frac{L}{C}}.\]

3 · An inverting op-amp amplifier

The active example, and the first to use a multi-terminal node — the op amp shape exposes named anchors (.+, .-, .out) you draw to and from⁴.

The source:

\begin{circuitikz}[scale=1.2, transform shape]
\draw (0,0) node[op amp] (A) {};
\draw (A.+) -- ++(-0.7,0) node[ground]{};
\draw (A.-) -- ++(-1,0) coordinate(s);
\draw (s) to[R=$R_\text{in}$, o-] ++(-2.4,0) node[left]{$V_\text{in}$};
\draw (s) -- ++(0,2) coordinate(t) to[R=$R_f$] (t -| A.out) -- (A.out);
\draw (A.out) to[short,-o] ++(1.4,0) node[right]{$V_\text{out}$};
\end{circuitikz}

The first line places the triangle and names it A. The non-inverting input A.+ is wired to ground; the inverting input A.- runs left to a summing node coordinate(s). From s, the input resistor R_\text{in} goes out to the source terminal (o- gives it one open end). The feedback path is the clever line: from s it goes up two units to coordinate(t), through R_f to the point (t -| A.out) — “the y of t, the x of A.out”, i.e. straight above the output — then down into A.out, bridging input and output exactly as the feedback resistor should. The gain is the resistor ratio set by that loop:

\[\frac{V_\text{out}}{V_\text{in}} = -\frac{R_f}{R_\text{in}}.\]

4 · A full-wave bridge rectifier

The showcase — four diodes in a diamond. Here named coordinates pay off, because every diode and wire is expressed relative to the four corners⁴.

The source:

\begin{circuitikz}[scale=1.25, transform shape]
\coordinate (L) at (0,0); \coordinate (R) at (3,0);
\coordinate (P) at (1.5,1.5); \coordinate (N) at (1.5,-1.5);
\draw (L) to[D] (P); \draw (N) to[D] (L);
\draw (R) to[D] (P); \draw (N) to[D] (R);
\draw (L) -- ++(-1.6,0) coordinate(sa);
\draw (sa) to[sV=$v_\text{ac}$] (sa|-N) -- (R|-N) -- (R);
\draw (P) to[short,-*] ++(1.8,0) coordinate(op) node[right]{$+$};
\draw (N) -| (op|-N) to[short,-*] (op|-N);
\draw (op) to[R=$R_L$] (op|-N);
\end{circuitikz}

It opens by naming the diamond’s four corners — left L, right R, positive apex P, negative apex N — and then everything is anchored to them. The four to[D] calls lay the diodes so all of them point toward the high side (into P, or out of N), which is what makes the bridge rectify. The AC source hangs off the left corner: down the left edge to (sa|-N) (below sa, level with N), across the bottom rail to (R|-N), and up into R. The output taps the top apex P to a node op (with -*, a filled junction dot), the bottom apex routes orthogonally with -| to meet it, and the load R_L spans the two — op to (op|-N), the output rail. After smoothing, the average output of an ideal full-wave rectifier is

\[V_\text{dc} = \frac{2\,V_\text{peak}}{\pi}.\]

The point of printing the source

Side by side, the four blocks show the same handful of moves doing all the work: to[...] for components, ++ for relative geometry, |- and -| for clean right-angle routing, and named coordinates when a circuit gets a shape worth holding onto. Because the figure on the page is this text run through lualatex and dvisvgm, the source above isn’t a transcription that could fall out of sync — it is, character for character, what the build compiled to draw the diagram a few lines above it. That is the whole argument for schematics that compile.

References