# input impedance

The input resistance is the resistance

formed by the input circuit of an electronic circuit. In most cases, the input resistance is a frequency-dependent complex resistor and thus represents an input impedance. Input impedance is a critical parameter for matching other components, sensors, transmission media, circuits, or other two- or four-terminal

devices.

Impedance For optimum matching, it must be high impedance or low impedance, depending on the source impedance of the upstream component or circuit. For voltage matching, the input impedance must be high impedance; for current matching, it must be low impedance; and for optimum power matching, it must match the output impedance of the upstream circuit.