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At RF, a PA might be either broadband or tuned. The main advantage of a broadband PA is ease of operation, because it does not need tuning. A broadbanded amplifier is not particular with respect to the frequency within its design range, such as 1.5 MHz through 15 MHz. The operator need not worry about critical adjustments, nor bother to change them when changing the frequency. One disadvantage of broadband PAs is that they are slightly less efficient than tuned PAs. This usually isn t too hard to put up with, though, considering the convenience of not having to fiddle with the tuning. The more serious problem with broadband PAs is that they ll amplify anything in the design range, whether or not you want it to go over the air. If some earlier stage in a transmitter is oscillating at a frequency nowhere near the intended signal frequency, and
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Radio-frequency amplification 451
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24-12 A tuned RF amplifier for use at about 10 MHz. Resistances are in ohms. Capacitances are in F if less than 1, and in pF if more than 1. Inductances are in H.
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if this undesired signal falls within the design frequency range of the broadband PA, it will be amplified. The result will be unintended (and illegal!) RF emission from the radio transmitter. Such unwanted signals are called spurious emissions, and they occur more often than you might think. A typical broadband PA circuit is diagrammed schematically in Fig. 24-13. The NPN bipolar transistor is a power transistor. It will reliably provide about 3 W of continuous RF output from 1.5 MHz through 15 MHz. The transformers are a critical part of this circuit; they must be designed to work well over a 10:1 range of frequencies. This circuit is suitable for use on the ham radio bands at 160, 80, 75, 40, 30, and 20 meters.
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A tuned RF power amplifier offers improved efficiency compared with broadband designs. Also, the tuning helps to reduce the chances of spurious signals being amplified and transmitted over the air. Another advantage of tuned PAs is that they can work into a wide range of load impedances. In addition to a tuning control, or resonant circuit that adjusts the output of the amplifier to the operating frequency, there is a loading control that optimizes the signal transfer between the amplifier and the load (usually an antenna). The main drawback of a tuned PA is that the adjustment takes time, and improper adjustment can result in damage to the amplifying device (bipolar transistor or FET). If the tuning and/or loading controls are out of kilter, the efficiency of the amplifier will be extremely low sometimes practically zero while the dc collector or drain power input is unnaturally high. Solid-state devices overheat quickly under these conditions.
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24-13 A broadband RF power amplifier, capable of producing a few watts output.
24-14
A tuned RF power amplifier, capable of producing a few watts output.
A tuned RF PA, providing 3 W output at 10 MHz or so, is shown in Fig. 24-14. The transistor is the same as for the broadband amplifier discussed above. The tuning and loading controls should be adjusted for maximum RF power output as indicated on a wattmeter in the feed line going to the load.
Quiz 453
Quiz
Refer to the text in this chapter if necessary. A good score is at least 18 correct. Answers are in the back of the book. 1. The decibel is a unit of: A. Relative signal strength. B. Voltage. C. Power. D. Current. 2. If a circuit has a voltage-amplification factor of 20, then the voltage gain is: A. 13 dB. B. 20 dB. C. 26 dB. D. 40 dB. 3. A gain of 15 dB in a circuit means that: A. The output signal is stronger than the input. B. The input signal is stronger than the output. C. The input signal is 15 times as strong as the output. D. The output signal is 15 times as strong as the input. 4. A device has a voltage gain of 23 dB. The input voltage is 3.3 V. The output voltage is: A. 76 V. B. 47 V. C. 660 V. D. Not determinable from the data given. 5. A power gain of 44 dB is equivalent to an output/input power ratio of: A. 44. B. 160. C. 440. D. 25,000. 6. A resistor between the base of an NPN bipolar transistor and the positive supply voltage is used to: A. Provide proper bias. B. Provide a path for the input signal. C. Provide a path for the output signal. D. Limit the collector current.
454 Amplifiers 7. The capacitance values in an amplifier circuit depend on: A. The supply voltage. B. The polarity. C. The signal strength. D. The signal frequency. 8. A class-A circuit would not work well as: A. A stereo hi-fi amplifier. B. A television transmitter PA. C. A low-level microphone preamplifier. D. The first stage in a radio receiver. 9. In which of the following FET amplifier types does drain current flow for 50 percent of the signal cycle A. Class A. B. Class AB1. C. Class AB2. D. Class B. 10. Which of the following amplifier types produces the least distortion of the signal waveform A. Class A. B. Class AB1. C. Class AB2. D. Class B. 11. Which bipolar amplifier type has some distortion in the signal wave, with collector current during most, but not all, of the cycle A. Class A. B. Class AB1. C. Class AB2. D. Class B. 12. How can a class-B amplifier be made suitable for hi-fi audio applications A. By increasing the bias. B. By using two transistors in push-pull. C. By using tuned circuits in the output. D. A class-B amplifier cannot work well for hi-fi audio. 13. How can a class-C amplifier be made linear A. By reducing the bias.
Quiz 455 B. By increasing the drive. C. By using two transistors in push-pull. D. A class-C amplifier cannot be made linear. 14. Which of the following amplifier classes generally needs the most driving power A. Class A. B. Class AB1. C. Class AB2. D. Class B. 15. A graphic equalizer is a form of: A. Bias control. B. Gain control. C. Tone control. D. Frequency control. 16. A disadvantage of transfer coupling, as opposed to capacitive coupling, is that: A. Transformers can t match impedances. B. Transformers can t work above audio frequencies. C. Transformers cost more. D. Transformers reduce the gain. 17. A certain bipolar-transistor PA is 66 percent efficient. The output power is 33 W. The dc collector power input is: A. 22 W. B. 50 W. C. 2.2 W. D. None of the above. 18. A broadband PA is: A. Generally easy to use. B. More efficient than a tuned PA. C. Less likely than a tuned PA to amplify unwanted signals. D. Usable only at audio frequencies. 19. A tuned PA must always be: A. Set to work over a wide range of frequencies. B. Adjusted for maximum power output. C. Made as efficient as possible. D. Operated in class C.
456 Amplifiers 20. A loading control in a tuned PA: A. Provides an impedance match between the bipolar transistor or FET and the load. B. Allows broadband operation. C. Adjusts the resonant frequency. D. Controls the input impedance.
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