Glossary term
Y-Parameters
The admittance-parameter representation of a linear two-port electrical network.
Definition
modelY-parameters describe a two-port network by relating port currents to port voltages through an admittance matrix.
For a linear small-signal two-port, admittance parameters are useful when currents are natural outputs and voltages are natural inputs. They support circuit modelling, amplifier analysis, filter design, network conversion, and small-signal representation of devices, but depend on port definitions, frequency, bias point, reference direction, and termination conditions.
Y-parameters are a two-port network model in which port currents are expressed as functions of port voltages:
Each parameter has units of admittance. The diagonal terms describe input and output admittance under the specified test conditions, while the off-diagonal terms describe reverse and forward transfer behaviour. In the classical definition, parameters are obtained with the opposite port short-circuited, so they are often called short-circuit admittance parameters.
Engineering use
Y-parameters are useful for small-signal transistor models, amplifiers, filters, feedback networks, microwave network conversion, and nodal circuit analysis. They align naturally with Kirchhoff current law because currents are summed at nodes and voltages are node variables.
The values are usually frequency-dependent and may also depend on temperature, bias point, operating region, and device geometry. For active devices, y21 may represent controlled current gain, while y12 captures reverse coupling that can affect stability and isolation.
Common mistakes
A common mistake is treating Y-parameters as universal constants of a component. They are valid only for the specified port reference directions, operating point, frequency range, and linearization. Another mistake is assuming that the short-circuit measurement condition is easy at high frequency. Practical RF work often uses scattering measurements and converts between parameter sets because ideal shorts at the port plane are difficult. A strong two-port model states port definitions, reference impedance where relevant, frequency, bias, temperature, measurement method, and conversion assumptions.