Glossary term
Admittance
The reciprocal of impedance, expressing how easily alternating current flows through a circuit, component, or network.
Definition
quantityThe reciprocal of impedance, expressing how easily alternating current flows through a circuit, component, or network.
Admittance is a complex AC quantity that combines conductance and susceptance. It is especially useful when analysing parallel branches, nodal equations, power-system networks, filters, and frequency-dependent behaviour because currents can be written directly as products of admittance and voltage.
Admittance is the inverse of impedance:
For sinusoidal steady-state analysis it is normally treated as a complex quantity:
where G is conductance, B is susceptance, and j is the imaginary unit. Conductance represents the in-phase current component associated with real power. Susceptance represents the quadrature current component associated with energy storage in electric and magnetic fields.
Engineering role
Admittance is useful whenever a circuit is easier to describe by the current that flows for a given applied voltage. Parallel branches combine directly by addition of admittances, which makes the quantity central to nodal analysis, equivalent circuits, small-signal models, filter design, and power-system load-flow calculations. In power engineering, bus-admittance matrices are used to describe how network nodes are electrically connected.
Capacitive and inductive behaviour
In AC analysis, admittance depends on frequency. An ideal capacitor has admittance:
so its magnitude increases with angular frequency \omega. An ideal inductor has admittance:
so its magnitude decreases as frequency rises. Real components also include parasitic resistance, leakage, dielectric loss, winding resistance, and stray capacitance, so measured admittance can depart significantly from the ideal expression.
Measurement and modelling
Admittance is measured in siemens (S). Precision measurements require a defined frequency, voltage level, bias condition, temperature, fixture compensation, and reference plane. In electronics, admittance may be extracted from network parameters or impedance analyser data. In power systems, it may represent lines, transformers, shunts, loads, or equivalent network models.
Common mistakes
A common error is treating admittance as a real scalar when the reactive part is important. Another is inverting a magnitude only and losing phase information: |Y| = 1/|Z| does not preserve the full complex relationship unless phase is carried through. Engineers should also avoid using an admittance measured at one frequency, bias, or temperature as if it were valid across a broad operating range. For nonlinear devices, admittance may be a local small-signal quantity rather than a fixed property.