The
input impedance of an
electrical network is the
impedance from the source into the network being connected. In other words, the input impedance is the impedance, if placed across the input terminals, that would produce the same voltage across and current through the input terminals as the electrical network being connected. Therefore, the input impedance of the network being connected and the output impedance of the source determines the
transfer function from the source to the input terminals of the circuit.