Input impedance

The input admittance (the reciprocal of impedance) is a measure of the load network's propensity to draw current.

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.

In this case, In AC circuits carrying power, the losses of energy in conductors due to the reactive component of the impedance can be significant.

The resulting equivalent circuit is purely resistive in nature, and there are no losses due to phase imbalance in the source or the load.

Since the characteristic impedance for a homogeneous transmission line is based on geometry alone and is therefore constant, and the load impedance can be measured independently, the matching condition holds regardless of the placement of the load (before or after the transmission line).

Pre-amplifiers designed for high input impedance may have a slightly higher effective noise voltage at the input (while providing a low effective noise current), and so slightly more noisy than an amplifier designed for a specific low-impedance source, but in general a relatively low-impedance source configuration will be more resistant to noise (particularly mains hum).

Signal reflections caused by an impedance mismatch at the end of a transmission line can result in distortion and potential damage to the driving circuitry.

In high-speed digital systems, such as HD video, reflections result in interference and potentially corrupt signal.

The standing waves created by the mismatch are periodic regions of higher than normal voltage.

If this voltage exceeds the dielectric breakdown strength of the insulating material of the line then an arc will occur.

This in turn can cause a reactive pulse of high voltage that can destroy the transmitter's final output stage.