The main purpose of RF amplifiers in our system is to enhance the signal level. Initially, two distinct cases existed, where we might have been interested in the capabilities of the module to dig up the weak signal from noise or to give our TX output rhe full power needed. Recently, however, monolithic microwave ICs (MMICs) have appeared with sufficient performance for both tasks—of course, with reasonable limits. The most important parameters | IS) describing an RF amplifier arc listed as follows:
- Frequency range;
- Amplitude transfer function (gain as a function of frequency);
- Phase transfer function {also for stability analysis);
- Input and output matching;
- NF (mainly of preamplifiers);
- Maximum output power (often at 1-dB compression, mainly of power stages);
- IP3 or third-order intercept point (distortion behavior);
- The dc power consumption and voltage (often also of preamplifiers due to the
cellular devices);
- Gain control range (when applicable);
- Cooling (when applicable).
The majority (the quantity in use) of today's amplifiers are based on semiconductors bur the highest powers still come from tubes. Particularly TWTs are indispensable in radar systems and certain satellite system TXs. These tubes are based first on an electron beam that is accelerated with a high dc voltage between the electrodes and additionally on a multiple resonator structure in which the interaction between the original input signal and the beam takes place. Exotic lower frequency applications can also make use of classic tetrodes and sometimes also klystron amplifiers turn out to be feasible. Most higher microwave and lower millimeter-wave amplifiers rely on GaAs MESFETs [16J, and above about 60 GHz also IMPATT and Gunn-diode re flection-type designs become practical.
Often the first question in system or equipment design related to an amplifier is "how much gain." Commercial devices start from about 10 dB, and the more expensive TWTs, for example, give above 50 dB. Many MMIC building blocks make a nice compromise around 20 to 30 dB with a variation of about 2 to 3 dB across their entire useful frequency range [17]. Figure 4.24 illustrates one less typical measured result. The higher microwave range is naturally more complicated, and we easily end with a cascade of four to five modules just for the same 30-dB net gain due to additional losses in connectors and transitions to and from the microstrip MMIC board. NFs have practically achieved the man-made noise limit so that a further reduction seldom makes sense. Of course, the millimeter-wave devices still have some progress to show. Typical commercially available figures range from 0.3 to 3 dB, depending on frequency. AGC blocks tend to have inherently poor noise performance.
The impedance matching of amplifier blocks has evolved considerably and thus quite easy-to-use modules have appeared requiring just one or two external components. Wideband units have SWR values generally below 2 at their inputs, but 4 or even 5 occasionally appears as a respective output parameter. The best NF is normally not obtained simultaneously with optimum matching. Figure 4.25 shows the actual measured input return loss of our sample amplifier. The maximum output power of normal laboratory-grade blocks at the 1-dB compression point lays somewhere between +10 dBm and +40 dBm, again depending on frequency. Up to 40 GHz we can rather easily get about 200 mW, but from there on semiconductors tend to exhaust. Commercial VHF/UHF transistor amplifiers arc available up to 20 to 40 kW of CW power, but these devices are actually rather complicated parallel amplifier matrixes used mainly for broadcasting and radar work.
Good spectral characteristics, which are mainly indicated as low spurious levels, are obtained at the expense of dc power consumption. Most transistor amplifier blocks work below 30% efficiency, but unfortunately very many cannot even achieve the 5% limit. The good thing in semiconductor blocks is that wc normally rely on dc voltages less than or equal to 24V. Tubes are known to require huge anode voltages, up to and above 50 kV, which makes system prototyping interesting and sometimes also colorful. Actually TWT power supplies tend to be as
complicated as the tubes themselves and, according to recent experience, have more faults.
A typical example of a set of amplifiers in a system is illustrated in Figure 4.26. There is one millimeter-wave LNA, and after the mixer we have a number of IF amplifiers, which have been arranged according to the best overall noi.se performance. This means that both gain and NF are taken into account. The total amplification in the chain is about 80 dB when we add the conversion loss in the mixer.
Cooling may be necessary both in LNAs and in final power stages. Cryogenic front ends often use liquid nitrogen. Extreme needs are fulfilled by helium, whieh
provides an operating temperature of about 20K. If no other means exist to satisfy the over ail noise floor requirement, this is the way to go, but operating complexity and costs tend to be considerable. Remember, that not all active modules can withstand such low temperatures either—the whole design must often be reconsidered. Even some conventional materials may suffer and become brittle. PAs from the 100-W class upward typically cannot rely solely on convection cooling through fins. Forced-air cooling is the most common choice, but its efficiency is limited. Systems involving liquids, mainly water, arc again complicated and expensive and cause reliability problems. Two main variants are employed. Systems in which normal tap water runs isolated from the blocks to be cooled is easier to maintain but the cooling capability is limited. If we use electrically purified water, which is produced through
ion-exchanging, for example, we can push the water directly in to the electrodes of the TX tube, but already the frequent change of the liquid may be too much of a burden. The highest cooling performance is obtained if we let the warer vaporize in the tube and later circulare rhis water through heat exchangers.
Many amplifier problems are related either to neglected cooling of seemingly low-power units or to poor connection arrangements, which cause oscillations and spurious emissions. First of all, even amplifiers operating at, say +10-dBm power levels, need proper cooling due to their extremely low efficiency. We often have to dissipate 1 to 2W of heat from a small module. Take care to prevent any unintentional RF coupling into an amplifier block through its power supply lines. Of course, any direct coupling between the input and the output may be disastrous. Sometimes the shielding as supplied from the manufacturer is inadequate and allows a coupling through the electromagnetic field. Oscillation problems tend to be more severe, if we have to cascade modules for higher gain. Often manufacturers indicate whether a certain device is not recommended to be use in a series connection (cascaded). A RF amplifier can oscillate totally outside its nominal frequency range, and this characteristic may not come out until complaints from other users of the spectrum start to arrive.
Libro: Circuits and Components for System Evaluations and Design
Autor: Pekka Eskelinen
Nombre: Josmar Eduardo Depablos Rodriguez
Asignatura: Circuitos de Alta Frecuencia
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