Radio-frequency transformers

Chapter Radio-frequency transformers

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • The required transformer will have a step-up impedance ratio of Zpri/Zsec50.0/3001:6.00. From the above formulas,Tpri/Tsec(Zpri/Zsec)1/2(1/6.00)1/20.166671/20.40829A couple of extra digits are included (as they show up on the calculator) to preventthe sort of error introduction you recall from earlier chapters. The decimal value0.40829 can be changed into ratio notation by taking its reciprocal, and then writing “1:”followed by that reciprocal value0.408291:(1/0.40829)1:2.4492This can be rounded to three significant figures, or 1:2.45. This is the primary-to-secondary turns ratio for the transformer. The secondary winding has 2.45 times asmany turns as the primary winding.Problem 18-4A transformer has a primary-to-secondary turns ratio of 4.00:1. The load, connected to thetransformer output, is a pure resistance of 37.5 Ω. What is the impedance at the primary?The impedance-transfer ratio is equal to the square of the turns ratio. Therefore,Zpri/Zsec(Tpri/Tsec)2(4.00/1)24.00216.0This can be written 16.0:1. The input (primary) impedance is 16.0 times the sec-ondary impedance. We know that the secondary impedance, Zsec is 37.5 Ω. Therefore,Zpri16.0Zsec16.037.5600Anything connected to the transformer primary will “see” a purely resistive imped-ance of 600 Ω.Radio-frequency transformersIn radio receivers and transmitters, transformers can be categorized generally by themethod of construction used. Some have primary and secondary windings, just like util-ity and audio units. Others employ transmission-line sections. These are the two mostcommon types of transformer found at radio frequencies.Coil typesIn the wound radio-frequency (RF) transformer, powdered-iron cores can be used upto quite high frequencies. Toroidal cores are most common, because they areself-shielding (all of the magnetic flux is confined within the core material). The num-ber of turns depends on the frequency, and also on the permeability of the core.In high-power applications, air-core coils are sometimes used, because air, althoughit has a low permeability, also has extremely low hysteresis loss. The disadvantage ofRadio-frequency transformers339