Impedance-transfer ratio

Chapter Impedance-transfer ratio

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

  • balanced and unbalanced parts of the systems are the same, then a 1:1 turns ratio isideal. But if the impedances differ, the turns ratio should be such that the impedancesare matched. This is discussed in the section on impedance transfer ratio that follows.Transformer couplingTransformers are sometimes used between amplifier stages in electronic equipmentwhere a large amplification factor is needed. There are other methods of couplingfrom one amplifier stage to another, but transformers offer some advantages, especiallyin radio-frequency receivers and transmitters.Part of the problem in getting a radio to work is that the amplifiers must operate ina stable manner. If there is too much feedback, a series of amplifiers will oscillate, andthis will severely degrade the performance of the radio. Transformers that minimize thecapacitance between the amplifier stages, while still transferring the desired signals,can help to prevent this oscillation.Impedance-transfer ratioOne of the most important applications of transformers is in audio-frequency (AF) andradio-frequency (RF) electronic circuits. In these applications, transformers are gener-ally employed to match impedances. Thus, you might hear of an impedance step-uptransformer, or an impedance step-down device.The impedance-transfer ratio of a transformer varies according to the square ofthe turns ratio, and also according to the square of the voltage-transfer ratio. Recall theformula for voltage-transfer ratio:Epri/EsecTpri/TsecIf the input and output, or source and load, impedances are purely resistive, andare denoted by Zpri (at the primary winding) and Zsec (at the secondary), thenZpri/Zsec(Tpri/Tsec)2andZpri/Zsec(Epri/Esec)2The inverses of these formulas, in which the turns ratio or voltage-transfer ratio are ex-pressed in terms of the impedance-transfer ratio, areTpri/Tsec(Zpri/Zsec)1/2andEpri/Esec(Zpri/Zsec)1/2The1/2 power is the same thing as the square root.Problem 18-3A transformer is needed to match an input impedance of 50.0 Ω, purely resistive, to anoutput impedance of 300 Ω, also purely resistive. What should the ratio Tpri/Tsec be?338 Transformers and impedance matching