Input resistance

The input impedance of an electrical network is the measure of the opposition to current flow (impedance), both static (resistance) and dynamic (reactance), into the load network being connected that is external to the electrical source. The input admittance (1/impedance) is a measure of the load'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.

If the load network were replaced by a device with an impedance equal to the input impedance of the load network, the characteristics of the source-load network would be the same from the perspective of the connection point. And so the voltage across and current through the input terminals would be identical to the original load network.

Therefore, the input impedance of the load network being connected and the output impedance of the source determines how the source current and voltage change i.e. the transfer function from the source to the input terminals of the circuit.

The Thévenin's equivalent circuit of the electrical network uses the concept of input impedance to determine the impedance of the equivalent circuit.

If one were to create a circuit with equivalent properties across the input terminals by placing the input impedance across the load of the circuit and the output impedance in series with the signal source, Ohm's Law could be used to calculate the transfer function. To calculate the input impedance, short the input terminals together and reduce the circuit by determining the equivalent circuit with only one component.

The values of the input and output impedance are often used to evaluate the electrical efficiency of networks by breaking them up into multiple stages and evaluating the efficiency of the interaction between each stage interdependently. To minimize electrical losses, the output impedance of the signal should be insignificant in comparison to the input impedance of the network being connected as the gain is equivalent to the ratio of the input impedance to the total impedance (input impedance + output impedance). In this case,

In AC circuits carrying power, the losses due to the reactive component of the impedance can be significant. These losses manifest themselves in a phenomenon called phase imbalance, where the current is out of phase (lagging behind or ahead) with the voltage. Therefore, the product of the current and the voltage is less than what it would be if the current and voltage were in phase. With DC sources, reactive circuits have no impact, therefore power factor correction is not necessary.

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Wikipedia