Inductors at audio frequency

Chapter Inductors at audio frequency

Teach Yourself Electricity and Electronics Third Edition Book
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Summary of Contents

Teach Yourself Electricity and Electronics Third Edition Book

  • Inductors at audio frequencyInductors at audio frequencies range in value from a few millihenrys up to about 1 H.They are almost always toroidally wound, or are wound in a pot core, or comprise partof an audio transformer.Inductors can be used in conjunction with moderately large values of capacitancein order to obtain audio tuned circuits. However, in recent years, audio tuning hasbeen taken over by active components, particularly integrated circuits.Inductors at radio frequencyThe radio frequencies range from 9 kHz to well above 100 GHz. At the low end of thisrange, inductors are similar to those at audio frequencies. As the frequency increases,cores having lower permeability are used. Toroids are quite common up through about30 MHz. Above that frequency, air-core coils are more often used.In radio-frequency (rf) circuits, coils are routinely connected in series or in parallelwith capacitors to obtain tuned circuits. Other arrangements yield various characteris-tics of attenuation versus frequency, serving to let signals at some frequencies pass,while rejecting signals at other frequencies. You’ll learn more about this in the chapteron resonance.Transmission-line inductorsAt radio frequencies of more than about 100 MHz, another type of inductor becomespractical. This is the type formed by a length of transmission line.A transmission line is generally used to get energy from one place to another. In ra-dio communications, transmission lines get energy from a transmitter to an antenna,and from an antenna to a receiver.Types of transmission lineTransmission lines usually take either of two forms, the parallel-wire type or the coax-ial type.A parallel-wire transmission line consists of two wires running alongside each otherwith a constant spacing (Fig. 10-10). The spacing is maintained by polyethylene rodsmolded at regular intervals to the wires, or by a solid web of polyethylene. You haveseen this type of line used with television receiving antennas. The substance separatingthe wires is called the dielectric of the transmission line.A coaxial transmission line has a wire conductor surrounded by a tubular braid orpipe (Fig. 10-11). The wire is kept at the center of this tubular shield by means of poly-thylene beads, or more often, by solid or foamed polyethylene dielectric, all along thelength of the line.Line inductanceShort lengths of any type of transmission line behave as inductors, as long as the line isless than 90 electrical degrees in length. At 100 MHz, 90 electrical degrees, or 1⁄4Transmission-line inductors193