# 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.

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