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Impedance match

Electricity

It is usually very important to transfer power from one stage of an electronic device to the next. To transfer the maximum amount of power, the output impedance of one stage must be the same as the input impedance of the next stage.

Impedance is the resistance of a circuit to alternating current. If a small impedance is connected to a big impedance, then the power that can pass through the connection is limited by the larger impedance.

To solve this problem, engineers use combinations of transformers, resistors, inductors and capacitors.

Impedances matched by transformers are used for high power circuits. A transformer converts alternating current at one voltage to another voltage, however the power remains the same, except for conversion losses. The side with the lower voltage is attached to the low impedance, because more current can flow through the lower resistance. The side with the higher voltage goes to the higher impedance, because more voltage can get through the higher resistance. The most visible examples are the power transformers used to distribute power from high impedance transmission lines to low impedance retail use.

Resistive impedance matches are the easiest to design. They limit the power deliberately. They are used to transfer low-power signals such as unamplified audio or radio frequency signals in a radio receiver. Almost all digital circuits use resistive impedance matches, usually built into the structure of the switching element. See resistor.

Some special situations, such as radio tuners and transmitters, use tuned filters to match impedances for specific frequencies. These can distribute different frequencies to different places in the circuit.

Sound

A similar impedance match problem exists when transfering sound from one medium to another. If the acoustical impedance of the two media are very different, then most of the sound energy will be reflected, rather than transfered across the border. A similar effect occurs when light transfers between two mediums with different refractive indices. The the closer the impedances of the materials match, the more light is refracted, not reflected.

Sound transfer impedance from a loudspeaker to air is the ratio of the diameter of the speaker to the wavelength of the frequency it is playing, i.e. bigger speakers play louder and deeper (low frequency bass) than small speakers. Oval speakers act like large speakers lengthwise, and like small speakers crosswise.



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