The input impedance of an electrical network is the equivalent impedance "seen" by a power source connected to that network. If the source provides known voltage and current, such impedance can be calculated using Ohm's Law. The input impedance is the Thévenin's equivalent circuit of the electrical network, modeled by an RL (resistor-inductor) or an RC (resistor-capacitor) combination, with equivalent values that would result in the same response as that of the network. It is also called Z11 in terms of Z-Parameters. Generally speaking, the exact definition depends on the particular field of study.
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Generally in audio and hi-fi systems, amplifiers have an input impedance several orders of magnitude higher than the output impedance of the source device connected to that input. This concept is also called voltage bridging or impedance bridging. In this case,
In general, this configuration will be more resistant to noise (particularly power line hum). Also the loading effects on the driving amplifier stage are reduced. In certain circuits a voltage follower stage is used to match the source and load impedance, which results in maximum power transfer.
In RF systems, the input impedance of inputs, the characteristic impedance of the transmission line, and the load impedance all have to be equal (or "matched") to reduce signal reflections, which result in distortion and potential damage to the driving circuitry. This is known as a matched connection, and the process of correcting an impedance mismatch is called impedance matching. Typical values are 50ohm and 75ohm. In analog video circuits these reflections can cause "ghosting", where the time-delayed echo of the principle image appears as a weak and displaced image (typically to the right of the principal image). In high-speed digital systems, such as HD video, reflections result in interferences and potentially corrupt signal.
In circuits carrying high power, matching the impedances is important for at least two reasons:
In the case of purely resistive impedances (no reactive components), the two types of impedance matching are identical.