Input impedance

In electrical engineering, the input impedance of an electrical network is the measure of the opposition to current (impedance), both static (resistance) and dynamic (reactance), into a load network that is external to the electrical source network. The input admittance (the reciprocal of impedance) is a measure of the load network's propensity to draw current. The source network is the portion of the network that transmits power, and the load network is the portion of the network that consumes power.

The circuit to the left of the central set of open circles models the source circuit, while the circuit to the right models the connected circuit. ZS is the output impedance seen by the load, and ZL is the input impedance seen by the source.