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The input impedance, load impedance, or external impedance of a circuit or electronic device is the Thévenin equivalent impedance looking into its input.
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In audio systems
Generally in audio and hi-fi, the input impedance of components is several times higher than the output impedance connected to them. This is called voltage bridging or impedance bridging. In this case,
- Zload >> Zsource
In general, this configuration will be more resistant to noise (particularly power line hum). The best circuits are created by using a voltage follower in the source with the pull-up (load) resistor at the signal receiver.
In video and high frequency signal systems
In video and other systems the impedance of inputs, transmission lines, and outputs are designed to be the same. This is known as reflectionless impedance matching or a matched connection. In this case, the impedances must be matched in order to prevent reflected waves from the signal receiver. In 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).
- Zload = Zline = Zsource
In radio frequency power systems
In circuits carrying high power it is important that the impedances be matched for several reasons.
- 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 (which may be 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)
- 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
External links
Sources
- The Art of Electronics, Winfield. Hill, Paul Horowitz, Cambridge University Press, ISBN 0521370957
- "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
Wikipedia content modification information:
- This page was last modified on 12 August 2008, at 03:02.
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