The RF spectrum extends upward in frequency to well over 300 GHz. The exact lower limit is a matter of disagreement in the literature. Some texts put it at 3 kHz, some at 9 kHz, some at 10 kHz, and some at the upper end of the AF range, which is normally considered to be 20 kHz.
Weak-Signal Amplifiers versus Power Amplifiers
Some RF amplifiers are designed for weak-signal work. The front end, or first amplifying stage, of a radio receiver requires the most sensitive possible amplifier. Sensitivity is determined by two factors: the gain, which has already been discussed here, and the noise figure, a measure of how well a circuit can amplify desired signals while generating a minimum of electronic noise.
All bipolar transistors or FETs create some white noise because of the movement of the charge carriers among the atoms. In general, JFETs produce less noise than bipolar transistors. Gallium arsenide FETs, also called GaAsFETs (pronounced “gasfets”), are the least noisy of all. The higher the frequency at which a weak-signal amplifier is designed, the more important the noise figure gets. This is because there is less atmospheric noise at the higher radio frequencies, as compared with the lower frequencies.
A tuned RF amplifier for use at about 10 MHz. Resistances are in ohms. Capacitances are in microfarads (μF) if less than 1, and in picofarads (pF) if more than 1. Inductances are in microhenrys (μH).
At 1.8 MHz, for example, the airwaves contain much atmospheric noise, and it doesn’t make any difference if the receiver introduces a little noise itself. But at 1.8 GHz, atmospheric noise is almost nonexistent, and receiver performance depends critically on the amount of internally generated noise. Weak-signal amplifiers almost always use resonant circuits. This optimizes the amplification at the desired frequency, while helping to cut out noise on unwanted frequencies. A typical tuned GaAsFET weak-signal RF amplifier is diagrammed in above figure. It is designed for operation at about 10 MHz.
A broadband RF power amplifier, capable of producing a few watts output. Resistances are in ohms. Capacitances are in microfarads (μF). Inductances are in microhenrys (μH).
At RF, a PA can be either broadband or tuned. The main advantage of a broadband PA is ease of operation, because it does not need tuning. The operator need not worry about critical adjustments, nor bother to change them when changing the frequency. However, broadband PAs are slightly less efficient than tuned PAs. Another disadvantage of broadband PAs is the fact that they will amplify any signal in the design frequency range, whether or not this is desired. For example, if some earlier stage in a radio transmitter is oscillating at a frequency different from the intended signal frequency, and if this undesired energy falls within the design frequency range of the broadband PA, it will be amplified. The result will be unintended RF emission from the radio transmitter. Such unwanted signals are called spurious emissions.
Above figure is a schematic diagram of a typical broadband PA. The NPN bipolar transistor is a power transistor. It will reliably provide several watts of continuous RF power output over a range of frequencies from 1.5 MHz through 15 MHz. The transformers are a critical part of this circuit. They must be designed to work efficiently over a 10:1 range of frequencies.
A tuned RF power amplifier, capable of producing a few watts output. Resistances are in ohms. Capacitances are in microfarads (μF) if less than 1, and in picofarads (pF) if more than 1. Inductances are in microhenrys (μH).
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 ad dition 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 transistor. 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 high. Solid-state devices overheat quickly under these conditions. A tuned RF PA, providing a few watts’ output at 10 MHz or so, is shown in above figure. The transistor is the same type as for the broadband amplifier discussed previously. The tuning and loading controls (left-hand and right-hand variable capacitors, respectively) should be adjusted for maximum RF power output as indicated by an RF wattmeter.