Coupling methods

Chapter Coupling methods

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

  • for voice communications must work from 300 Hz to 3 kHz, a 10: 1 span of frequencies.In digital communications, audio amplifiers are designed to work over a narrow range offrequencies, sometimes less than 100 Hz wide.Hi-fi amplifiers are usually equipped with resistor-capacitor (RC) networks that tai-lor the frequency response. These are tone controls, also called bass and treble con-trols. The simplest hi-fi amplifiers use a single knob to control the tone. Moresophisticated “amps” have separate controls, one for bass and the other for treble. Themost advanced hi-fi systems make use of graphic equalizers, having controls that af-fect the amplifier gain over several different frequency spans.Gain-versus-frequency curves for three hypothetical audio amplifiers are shownin Fig. 24-9. At A, a wideband, flat curve is illustrated. This is typical of hi-fi systemamplifiers. At B, a voice communications response is shown. At C, a narrowband re-sponse curve, typical of audio amplifiers in Morse code or low-speed digital-signal re-ceivers, is illustrated.Volume controlAudio amplifier systems usually consist of two or more stages. A stage is one bipolartransistor or FET (or a push-pull combination), plus peripheral resistors and capaci-tors. Stages are cascaded one after the other to get high gain.In one of the stages in an audio system, a volume control is used. This controlis usually a potentiometer that allows the gain of a stage to be adjusted without af-fecting its linearity. An example of a simple volume control is shown in Fig. 24- 10.In this amplifier, the gain through the transistor itself is constant. The ac output sig-nal passes through C1 and appears across R1, a potentiometer. The wiper (indi-cated by the arrow) of the potentiometer “picks off ” more or less of the ac outputsignal, depending on the position of the control shaft. When the shaft is fully coun-terclockwise, the arrow is at the bottom of the zig-zaggy line, and none of the signalpasses to the output. When the shaft is fully clockwise, the arrow is at the top of thezig-zaggy line, and all of the signal passes to the output. At intermediate positionsof the control shaft, various proportions of the full output signal will appear at theoutput. Capacitor C2 isolates the potentiometer from the dc bias of the followingstage.Volume control is usually done in a stage where the audio power level is quitelow. This allows the use of a small potentiometer, rated for perhaps 1 W. If volumecontrol were done at high audio power levels, the potentiometer would need to beable to dissipate large amounts of power, and would be needlessly expensive.Coupling methodsIn all of the amplifiers you’ve seen so far, with the exception of the push-pull circuit(Fig. 24-6), capacitors have been used to allow ac to pass while blocking dc. But thereis another way to do this, and in some amplifier systems, it is preferred. This is the useof a transformer to couple signals from one stage to the next.An example of transformer coupling is shown in Fig. 24-11. Capacitors C1 and C2keep one end of the transformer primary and secondary at signal ground. Resistor R1Coupling methods447