It is generally challenging and difficult to deal with RF signals in their physical circuits. They are inherently high-frequency signals (GHz and above are commonplace now), usually are at low voltage levels (nominally 1 V maximum, but often far lower), quickly accumulate added noise from many sources, are easily overloaded, and lack drive capability. Therefore, it is necessary to use an RF amplifier (often shortened to the word “amp,” but not to be confused with current and amperes) to modify and boost them as needed.
What is the Function of a RF Amplifier?
There are three major functions that RF amplifiers provide (Figure 1).
Figure 1: This simple, standard schematic symbol of an amplifier does not show the wide variety of uses, applications, functions, or roles that the amplifier must support even though it provides no signal processing or change in signal shape (power and ground connections not shown.
RF Amplifier Gain
Gain: this is needed when the RF signal amplitude is minuscule and too low to be useful elsewhere in the circuit, needs to be boosted so the overall SNR (signal-to-noise ratio) will not deteriorate as the signal passes through the rest of the circuit, or its amplitude must match the input range of a component such as an A/D converter. Many RF signals, such as from an antenna, are in the microvolt (μV) or millivolt (mV) range, while signal-processing circuits work far more effectively with signals that have a typical maximum value ranging between 1 and 10 V (depending on the design).
A gain amplifier focuses on one thing: boosting the signal level while adding minimum noise or distortion. Gain amplifiers designed to be used with extremely low-level signals, such as from an antenna, are usually called low-noise amplifiers (LNAs). Some RF amplifiers offer a single fixed-gain value, while others allow the user to select among fixed-gain values (such as ×10 and ×100, or ×2, ×4, ×8, ×16) via an external jumper or resistor. Another type of RF amplifier, called a variable gain amplifier (VGA), lets the user set and change the gain as needed within a wide range, using an external physical resistor, digitally-programmable resistor, or an analog control voltage (usually between 0 and 1 V).
RF Buffer Amplifier
Buffer: this is needed when a circuit function or signal must maintain its shape and amplitude even as its load changes, or it must connect to a load that is greater than it can normally accept (lower impedance, or reactive). For example, a ±1-V amplified signal from an antenna LNA output may need to connect another stage that has some inductance. The buffer amplifier ensures that the presence of this inductance does not affect the fidelity of the ±1-V signal or induce distortion. A buffer RF amplifier can also be used to match (via complex conjugate) the impedance of output of the circuit that is sourcing a signal to the impedance of the input of its load circuit, to maximize the transfer of power. In addition to frequency range, a critical specification for buffers is the range of resistive and reactive loads it can drive while not distorting the signal.
RF Driver Amplifier
Driver: a driver RF amplifier’s primary role is to source and sink sufficient current at the operating frequency to drive a low-impedance load such as a 50- or 75-Ω coaxial cable. RF drivers can also be considered as RF power amplifiers, when their role is to provide a power boost (current and/or voltage) to drive a load such as an antenna. The slew rate (dI/dt) and sink/source capability they must provide with sufficient current is fairly high at the frequencies at which these RF amplifiers operate, beyond the capabilities of a general-purpose RF buffer amplifier.
Some drivers provide gain, while others have fixed-unity gain. In addition, these drivers often connect “outside the box” to cables and user-handled interfaces, so they are usually deigned to withstand short circuits to ground and DC power rails (whether a result of user error or connection fault). Key parameters for these drivers are their source/sink rating as well as short-circuit and other misconnection “withstand” ratings for time and applied voltage.
Types of RF Amplifiers
Note that some RF amplifiers combine more than one basic function in a single device. For example, it is possible to find a buffer that also has gain. In some cases, these are attractive bill of material (BOM) choices; in other cases, the circuit needs performance specifications that can only be met by focused, single-function RF amplifiers that do not have any of the inherent tradeoffs, which are sometimes unavoidable with multi-role devices.
A good example of a low-noise RF amplifier is the BGM781N11 from Infineon Technologies AG, which is optimized for performance at 1575.42 MHz, for GPS and Galileo satellite navigation products (Figure 2). In this application, the RF signal strength is inherently extremely low, so a basic LNA must provide gain with very low noise. For the BGM781N11, gain is 18.6 dB while the noise figure is 1.7 dB; out-of-band rejection of the adjacent cellular bands is 80 dBc. The device, in a tiny TSNP-11-2 leadless package, requires just 3.3 mA from a 1.5 V to 3.6 V supply, includes narrowband filters to minimize out-of-band noise, and is internally matched at both input and output to 50 Ω drive and load, respectively.
Figure 2: The Infineon BGM781N11 low-noise RF amplifier is specially designed to boost the weak signals from the antenna of a GPS or Galileo satellite navigation receiver; it not only provides gain with low noise, but it also includes passband filters to suppress signals on either side of the 1575.42 MHz carrier. (Source: Infineon Technologies)
The LTC6431-20 from Linear Technology Corp. is a basic buffer with high linearity extending to frequencies beyond 1 GHz, along with low noise and low power dissipation (Figure 3). Like the Infineon part, it is internally matched for 50-Ω interfaces on both input and output from 20 MHz to 1.5 GHz, easing the interconnection challenge in wideband designs.
See related product
It can provide up to 20 dB of gain, and is primarily used as a buffer in the IF stages of the RF chain. Noise figure for this IC is 2.6 dB at 240 MHz, with 0.6nV/√Hz total input noise. The single-ended buffer draws only 93 mA from a single 5-V supply.
Figure 3: The LTC6431-20 from Linear Technology Corp. operates to above 1 GHz while providing gain of +20 dB; as an interstage buffer, it will usually see a 50-Ω source and a 50-Ω load and so is designed to match those impedances. (Source: Linear Technology)
A representative RF driver amplifier is Avago Technologies’ MGA-30489, a highly linear 0.25-W device in a SOT-89 standard plastic package, designed for operation between 250 MHz and 3 GHz (Figure 4). It can be easily impedance-matched with the standard 50-Ω wireless infrastructure used in applications such as Cellular/PCS/W-CDMA/WLL and next-generation wireless technologies, to achieve optimum power and linearity across the band. It operates from a 5-V supply, requires 97 mA (typical), and features a noise figure of 3 dB and fixed gain of 13.3 dB.
Figure 4: Avago Technologies’ MGA-30489 RF driver amplifier can deliver RF power at up to 0.25 W from 250 MHz and 3 GHz, and is well-suited for driving coaxial cables as well as antennas. (Source: Avago Technologies)
Conclusion
The simple term “RF amplifier” actually encompasses a very wide array of amplifier functions across the RF spectrum, from a few MHz into the multi-GHz range. Though the RF amplifier does not change the shape of the signal nor perform any analog signal processing, it still has vital and different roles at each stage of the signal chain. Among these are basic amplification typified by the LNA or matching signal to converter span; acting as a buffer for RF signals so each stage in the signal chain operates somewhat independently of the prior and succeeding stages, and providing the signal power needed to drive low-impedance or non-resistive loads at high speed with minimum distortion.
Some RF amplifier parameters are common to all these (and other) applications, but there are also parameters that are more important and prominent in some RF amplifier functions than they are in others. Furthermore, there are RF amplifiers that combine two or more basic amplifier functions, yet single-function devices are still quite popular because they can provide the ultimate in performance if needed.