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Understanding the Basics of Low-Noise and Power Amplifiers in Wireless Designs



In a wireless design, two components are the critical interfaces between the antenna and the electronic circuits, the low-noise amplifier (LNA) and the power amplifier (PA). However, that is where their commonality ends. Although both have very simple functional block diagrams and roles in principle, they have very different challenges, priorities, and performance parameters.

How so? The LNA functions in a world of unknowns. As the "front end" of the receiver channel, it must capture and amplify a very-low-power, low-voltage signal plus associated random noise which the antenna presents to it, within the bandwidth of interest. In signal theory, this is called the unknown signal/unknown noise challenge, the most difficult of all signal-processing challenges.

In contrast, the PA takes a relatively strong signal from the circuitry, with very-high SNR, and must "merely" boost its power. All the general factors about the signal are known, such as amplitude, modulation, shape, duty cycle, and more. This is the known-signal/known-noise quadrant of the signal-processing map, and the easiest one to manage. Despite this apparent simple functional situation, the PA has performance challenges as well.

In duplex (bidirectional) systems, the LNA and PA usually do not connect to the antenna directly, but instead go to a duplexer, a passive component. The duplexer uses phasing and phase-shifting to steer the PA's output power to the antenna while blocking it from the LNA input, to avoid overload and saturation of the sensitive LNA input (for more on antenna fundamentals, see the TechZone article “Understanding Antenna Specifications and Operation”); it also directs the antenna signal to the LNA and not to the PA (Figure 1a). The Avago Technologies ACMD-7612 is one such duplexer (Figure 1b); it uses Avago’s Film Bulk Acoustic Resonator (FBAR) technology for this function.

Figure 1a: A duplexer prevents power bleed-through from the PA side to the LNA side, so the receiver is not "blinded."

Figure 1b: The ACMD-7612 duplexer from Avago Technologies is a semiconductor device with a footprint compatible with high-density PC board designs.

Receiving starts with the LNA

The function of the LNA is to take the extremely weak and uncertain signal from the antenna, usually on the order of microvolts or under –100 dBm (note that in a 50 Ω system, 10 μV is -87 dBm and 100 μV equals -67 dBm), and amplify it to a more useful level, to about one-half to one volt. While providing this gain itself is not a major challenge with modern electronics, it is severely compromised by any noise which the LNA may add to the weak input signal. This noise can overwhelm any benefits of the amplification that the LNA adds.

For LNAs, the primary parameters are noise figure (NF), gain, and linearity. Noise is due to thermal and other sources, with typical noise figures in the 0.5 to 1.5 dB range. Typical gain is between 10 and 20 dB for a single-stage; some designs use cascaded amplifiers with a low-gain, low-NF stage followed by a higher-gain stage which may have higher NF, but this is less critical once the initial signal has been "gained up." (For more on LNAs, noise, and RF receivers, see the TechZone article “Low-Noise Amplifiers Maximize Receiver Sensitivity.”)

Nonlinearity is another issue for the LNA, as the resultant harmonics and intermodulation distortion corrupt the received signal and make demodulating and decoding it with sufficiently low BER (bit error rate) more difficult. Linearity is usually characterized by the third-order intercept point (IP3), which relates nonlinear products caused by the third-order nonlinear term to the linearly amplified signal; the higher the IP3 value, the more linear the amplifier performance.

Power consumption and efficiency in the LNA are generally not the primary concerns. By their nature, most LNAs are fairly low-power devices with current consumption from 10 to 100 mA, and they are providing voltage gain to subsequent stages, not delivering power to a load. Also, there are only one or two LNA channels in the system (the latter usually in diversity antenna designs for Wi-Fi interfaces), so any savings from using a lower-power LNA would be modest.

Other than operating frequency and bandwidth, there is a relatively large amount of functional similarity among LNAs. Some LNAs also include gain control so the amplifier can handle a wide dynamic range of input signals without overload and saturation. Such widely varying input-signal strength is a common occurrence in mobile applications where the base station-to-phone path loss can have a wide range, even during a single connection cycle.

As low-power basic amplifiers, LNA bare die are usually very small (often under 1 × 1 mm) with few connections, so they are housed in small packages such as SOT-363. Most LNAs just have connections for input, output, power, and ground, plus gain (if available), and a bias operating-point setting in some designs.

LNAs for consumer products are based on CMOS, GaAs (gallium arsenide), or SiGe (silicon germanium) process technology, with some using pHEMT structures (pseudomorphic high-electron-mobility transistors). There is an ongoing, back-and-forth race between CMOS (with its low-cost, high-volume infrastructure), and other process technologies; as a result, the "best" LNA process at any time changes frequently. There are even CMOS-based combined LNA+PA ICs, which have some performance tradeoffs but are still suitable for some applications, and offer smaller size and lower cost.

Although there is no such thing as a typical LNA, a representative one is the Maxim MAX2659, designed for GPS, Galileo, and GLONASS satellite navigation systems (Figure 2, note that this is a receive-only situation). Designed on a SiGe process, it has a noise figure of just 0.8 dB, gain of 20.5 dB, and -5 dBm IP3. The 1.6 to 3.6 V part draws 4.1 mA, and measures just 1.5 × 1.0 × 0.75 mm.

Figure 2: The Maxim MAX2659 is an LNA designed for receive-only systems; in this case, satellite receivers for GPS/Galileo/Glonass designs.

When considering a high-performance LNA that operates in the GHz range, be sure the vendor offers an evaluation board, since every aspect of test setup is critical, including layout, connectors, grounding, bypassing, and power (Figure 3). Otherwise, you will spend time, much of it frustrating and wasted, trying to assess the part's performance in your application.

Figure 3: A vendor evaluation board and reference design is essential to credible assessment of the performance of the LNA or PA; here, the board for the MAX2569 shows the associated components surrounding the tiny IC.

The PA drives the antenna


While the LNA interfaces with a miniscule, noisy, relatively unknown signal, and therefore has a more difficult technical challenge than the PA, the PA gets more scrutiny and attention in many designs. This is primarily because its performance involves relatively significant power and battery life, as well as achievable transit data rate and signal fidelity. To meet diverse transmitter needs, there is a wider range of available PAs than LNAs; a PA for 100 mW output is very different in design and process than one for 2 W, 5 W, or more, at a given frequency.

The primary parameter for the PA is gain at the frequency of interest, with typical PA gain spanning between +10 and +30 dB. The PA's output impedance must also be fully characterized by the vendor, so the OEM can match it properly to the antenna for maximum power transfer and to keep SWR (standing wave ratio) as close to unity as possible.

Often, an impedance-matching circuit is needed, composed primarily of capacitors and inductors, which may be discrete devices or fabricated as part of the PC board or packaging. Tools such as the venerable Smith chart are essential to understand and implement the needed impedance matching.

Along with gain, efficiency is the next critical PA parameter, but any assessment of efficiency is complicated by the usage model, modulation, duty cycle, allowable distortion, and other aspects of the signal to be boosted. PA efficiencies are in the 30 to 80 percent range, but this is very dependent on many factors. PA linearity, also critical, is judged by IP3, just as for the LNA.

While many PAs use CMOS technology at lower power levels (up to about 1 W), in recent years other technologies have matured and are also in widespread use, especially at higher power levels where efficiency is critical both for battery life and thermal dissipation. PAs using gallium arsenide (GaAs) and gallium nitride (GaN) offer better efficiency at higher power levels and higher frequencies (typically above 1 GHz), where several watts or more are needed. Other PA technologies include LDMOS, BJT, and SiGe HBT. As with LNAs, there is a rapid turnover in the leading process technology at any given time for a given set of performance priorities.

Due to the PA die size and power levels, packaging is a critical issue for PAs. Many PAs enable heatsinking via wide, heat-dissipating package leads using a thermal slug under the package to act as a path to the PC board copper, or top-mounted heatsinks, depending on power levels and operating modes. At higher power levels (above about 5 to 10 W) the heat sinks usually are mounted on top, and fans or other advanced cooling techniques may be needed.

An example of a wideband PA is the Skyworks SKY65015-70LF (Figure 4a), for 250 to 2,500 MHz operation, fabricated using a GaAs HBT process. This device has gain of 22 dB at 450 MHz, 17 dB at 900 MHz, and 12 dB at 1,960 MHz, with corresponding efficiencies of 38.5, 33, and 47 percent.

Figure 4a: The Skyworks SKY65015-70LF wideband PA is a good choice when a single circuit design must satisfy needs of different application bands with minimum BOM and layout changes.

This highly linear component has an IP3 of 40 dBm. Output power is 25 dBm, corresponding to approximately 0.3 W; note the extended tab on the package used for heatsinking and thermal management as well as ground (Figure 4b).

Figure 4b: Although under half-watt in output, the SKY65015-70LF needs a small thermal tab for heat sinking, as do nearly all PAs in this power range.

Of course, other PAs are available from this vendor and many others that, in contrast, are optimized for specific, narrow bands rather than wideband operation. Their performance specifications are therefore carefully tailored to specific applications in a relatively small part of the RF spectrum.

Summary

RF-based devices ranging from smartphones to niche wireless links face difficult demands for higher-frequency operation, greater power efficiency, and improved overall performance in their critical parameters. This article has examined some of the issues and basic performance parameters for the components which connect directly to the system antenna, the front-end low-noise amplifier (LNA) and the output power amplifier (PA), as well as representative specifications. For more information on the products mentioned here, use the links provided to access product pages on the Hotenda website.
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