Input impedance

Simple source and load circuit

The input impedance, load impedance, or external impedance of a circuit or electronic device is the Thévenin equivalent impedance looking into its input.

Termination requirements

Audio systems

Generally in audio and hi-fi, components have input impedance several times higher than the output impedance connected to them. This is called voltage bridging or impedance bridging. In this case,

Z_load >> Z_source

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.

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, one must match the impedances 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).

Z_load = Z_line = Z_source

Radio frequency power systems

In circuits carrying high power, matching the impedances is important for at least two reasons:

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, Z_load = Z_source* (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 Z_load = Z_source (no complex conjugate).

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

See also

• Impedance
• Damping factor
• Voltage divider
• Dummy load

External links

• Calculation of the damping factor and the damping of impedance bridging
• Interconnection of two audio units - Input impedance and output impedance
• Impedance and Reactance

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