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A circuit node is a set of electrically connected points treated as having a common electric potential in a lumped-element model. Ideal connecting wires belong to the same circuit node even if the drawing places them far apart.

Conservation of electric charge leads to Kirchhoff's current law: the algebraic sum of currents entering an ideal node is zero when unmodelled charge accumulation is negligible. For example, a current entering a junction equals the sum of two outgoing branch currents. A capacitor connected at that node does not invalidate the law: its charging current must be included as another branch current.

A lumped-element model represents an electrical circuit by discrete ideal elements such as resistors, capacitors and inductors, linked by equipotential circuit nodes. Each element is assigned terminal variables, rather than a fully resolved spatial electromagnetic field.

This approximation is effective when signal-propagation times across the circuit are short compared with the timescales of interest, and the relevant dimensions are small compared with the wavelength. A long cable carrying a rapid pulse may instead require a transmission line model: different points on the cable can have different voltages at the same instant.

A non-inverting operational amplifier applies the signal to its non-inverting input and returns part of the output to its inverting input through negative feedback. Connect resistor $R_f$ from the output to the inverting input, and resistor $R_g$ from that input to a reference circuit node at electric potential $V_{\mathrm{ref}}$.

In stable, unsaturated ideal operation, the inputs draw no electric current and their electric potentials are equal. Writing the signal as $V_{\mathrm{in}}$, Kirchhoff's current law at the inverting input givessoFor a grounded reference circuit node, the voltage gain is $1+R_f/R_g$. For example, $R_f=20\mathrm{k\Omega }$ and $R_g=10\mathrm{k\Omega }$ give gain three. The equal-input-potential approximation requires sufficiently large loop gain; amplifier saturation or unstable feedback invalidates it.^[ref]