Audio-frequency transformers

Chapter Audio-frequency transformers

Teach Yourself Electricity and Electronics Third Edition Book
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Summary of Contents

Teach Yourself Electricity and Electronics Third Edition Book

  • Most solid-state devices use low voltages, ranging from about 5 V up to perhaps 50V. This equipment needs step-down power transformers in its power supplies.Solid-state equipment usually (but not always) consumes relatively little power, so thetransformers are usually not very bulky. The exception is high-powered audio-fre-quency or radio-frequency amplifiers, whose transistors can demand more than 1000watts (1 kW) in some cases. At 12 V, this translates to 90 A or more.Television sets have cathode-ray tubes that need several hundred volts. This is de-rived by using a step-up transformer in the power supply. Such transformers don’t haveto supply a lot of current, though, so they are not very big or heavy. Another type of de-vice that needs rather high voltage is a ham-radio amplifier with vacuum tubes. Such anamplifier requires from 2 kV to 5 kV.Any voltage higher than about 12 V should be treated with respect. The voltagesin televisions and ham radios are extremely dangerous, even after the equipmenthas been switched off. Do not try to service such equipment unless you are trainedto do so!Audio-frequency transformersTransformers for use at audio frequency (AF) are similar to those employed for 60-Hzelectricity. The differences are that the frequency is somewhat higher (up to 20 kHz),and that audio signals exist in a band of frequencies (20 Hz to 20 kHz) rather than atjust one frequency.Most audio transformers look like, and are constructed like miniature utility trans-formers. They have laminated E cores with primary and secondary windings woundaround the cross bars, as shown in Fig. 18-4.Audio transformers can be either the step-up or step-down type. However, ratherthan being made to produce a specific voltage, audio transformers are designed tomatch impedances.Audio circuits, and in fact all electronic circuits that handle sine-wave or com-plex-wave signals, exhibit impedance at the input and output. The load has a certainimpedance; a source has another impedance. Good audio design strives to minimizethe reactances in the circuitry, so that the absolute-value impedance, Z, is close tothe resistance R in the complex vector R+ jX. This means that X must be zero ornearly zero.In the following discussion of impedance-matching transformers, both at audio andat radio frequencies, assume that the reactance is zero, so that the impedance is purelyresistive, that is, ZR.Isolation transformersOne useful function of a transformer is that it can provide isolation between electroniccircuits. While there is inductive coupling in a transformer, there is comparatively lit-tle capacitive coupling. The amount of capacitive coupling can be reduced by usingcores that minimize the number of wire turns needed in the windings, and by keepingthe windings separate from each other (rather than overlapping).336 Transformers and impedance matching