Input impedance

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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.

Simple source and load circuit (ZS is the output impedance seen by the load, and ZL is the input impedance seen by the source).

Termination requirements[edit]

Audio systems[edit]

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,

Zload >> Zsource
The input impedance of the driven stage (load) is much larger than the output impedance of the driver stage (source).

In general, this configuration will be more resistant to noise (particularly mains 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.

Video and high frequency (RF) systems[edit]

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. Signal reflections can 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 50 Ω and 75 Ω. 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.

Zload = Zline = Zsource

Radio frequency power systems[edit]

In circuits carrying high power, there are two different ways "matching" impedance, serving two different purposes:

  1. The maximum power at maximum efficiency will be transferred when the impedances are complex conjugate matched throughout the power chain, from the transmitter output, through the transmission line (a balanced pair, a coaxial cable, or a waveguide), to the antenna system, which consists of an impedance matching device and the radiating element(s). For maximum power, Zload = Zsource* (where * indicates the complex conjugate)
  2. Failure to match impedances will create standing waves on the transmission line due to reflections. These will be 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. For reflectionless matching Zload = Zsource (no complex conjugate).

In the case of purely resistive impedances (no reactive components), the two types of impedance matching are identical.

See also[edit]

External links[edit]

Sources[edit]

  • The Art of Electronics, Winfield Hill, Paul Horowitz, Cambridge University Press, ISBN 0-521-37095-7
  • "Aortic input impedance in normal man: relationship to pressure wave forms", JP Murgo, N Westerhof, JP Giolma, SA Altobelli pdf
  • An excellent introduction to the importance of impedance and impedance matching can be found in A practical introduction to electronic circuits, M H Jones, Cambridge University Press, ISBN 0-521-31312-0