Radio-frequency amplification

Chapter Radio-frequency amplification

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

  • Radio-frequency amplificationThe RF spectrum begins at about 9 kHz and extends upward in frequency to well over300 GHz, or 300,000,000,000 Hz. A complete discussion of RF amplifier design wouldoccupy a book. Therefore, again, only a sketch of the most important characteristicscan be given here.Weak-signal versus power amplifiersSome RF amplifiers are designed for weak-signal work. The general circuits, shown ear-lier in this chapter, are representative of such amplifiers, when the capacitors have val-ues of about 1 µF or less. The higher the frequency, the smaller the values of thecapacitors.The front end, or first amplifying stage, of a radio receiver requires the most sen-sitive possible amplifier. Sensitivity is determined by two factors: gain and noise fig-ure.The noise figure of an amplifier is a measure of how well it can amplify the desiredsignal, without injecting unwanted noise. All bipolar transistors or FETs; create somewhite noise because of the movement of charge carriers. In general, JFETs produceless noise than bipolar transistors. Gallium arsenide FETs, also called GaAsFETs (pro-nounced “gasfets”), are the least noisy of all.The higher the frequency at which a weak-signal amplifier is designed, the moreimportant the noise figure gets. This is because there is less atmospheric noise at thehigher radio frequencies, as compared with the lower frequencies. At 1.8 MHz, for ex-ample, the airwaves contain much atmospheric noise, and it doesn’t make a significantdifference if the receiver introduces a little noise itself. But at 1.8 GHz the atmosphericnoise is almost nonexistent, and receiver performance depends much more critically onthe amount of internally generated noise.Weak-signal amplifiers almost always use resonant circuits. This optimizes the am-plification at the desired frequency, while helping to cut out noise on unwanted fre-quencies. A typical tuned GaAsFET weak-signal RF amplifier is diagrammed in Fig.24-12. It is designed for about 10 MHz.Broadband PAsAt RF, a PA might be either broadband or tuned.The main advantage of a broadband PA is ease of operation, because it does notneed tuning. A broadbanded amplifier is not “particular” with respect to the fre-quency within its design range, such as 1.5 MHz through 15 MHz. The operator neednot worry about critical adjustments, nor bother to change them when changing thefrequency.One disadvantage of broadband PAs is that they are slightly less efficient thantuned PAs. This usually isn’t too hard to put up with, though, considering the conve-nience of not having to fiddle with the tuning.The more serious problem with broadband PAs is that they’ll amplify anything inthe design range, whether or not you want it to go over the air. If some earlier stage in atransmitter is oscillating at a frequency nowhere near the intended signal frequency, and450 Amplifiers