Basic principles of LNA and PA in wireless design

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The need for performance, miniaturization and higher frequencies is challenging the limitations of two key antenna connection components in wireless systems: power amplifiers (PAs) and low-noise amplifiers (LNAs). the development of 5G and the use of PAs and LNAs in microwave radio links, VSATs (satellite communication systems) and phased-array radar systems are contributing to this shift. Requirements for these applications include lower noise (for LNAs) and higher energy efficiency (for PAs) as well as operation at higher frequencies up to or above 10 GHz. To meet these growing demands, LNA and PA manufacturers are shifting from traditional all-silicon processes to gallium arsenide (GaAs) for LNAs and gallium nitride (GaN) for PAs.

This paper will describe the role and requirements of LNAs and PAs and their key characteristics, followed by typical GaAs and GaN devices and design considerations when utilizing these devices.

The sensitive role of LNA

 

The role of the LNA is to acquire an extremely weak and uncertain signal from the antenna, typically on the order of microvolts or below -100 dBm, and then amplify that signal to a more useful level, typically about 0.5 to 1 V (Figure 1). Specifically, 10 μV is -87 dBm in a 50 Ω system, and 100 μV equals -67 dBm.

Such gains can be easily achieved using modern electronics, but the problem is far less straightforward when the LNA adds a variety of noise to a weak input signal. the amplification benefits of the LNA can be completely lost in such noise.

Figure 1: The low-noise amplifier (LNA) for the receive path and the power amplifier (PA) for the transmit path are connected to the antenna via a duplexer, which separates the two signals and prevents the relatively powerful PA output from overloading the sensitive LNA input. (Image credit: Digi-Key Electronics)

Note that LNAs operate in a world of unknowns. As the front end of a transceiver channel, the LNA must be able to capture and amplify the very low-power, low-voltage signals in the relevant bandwidth and the associated random noise caused by the antenna. In signal theory, this situation is called the unknown signal/unknown noise puzzle and is the most difficult part of all signal processing puzzles.

The main parameters of the LNA are noise factor (NF), gain and linearity. Typical values of NF are 0.5 – 1.5 dB for noise from thermal and other noise sources, and typical gain is between 10 – 20 dB for a single-stage amplifier. Some designs use a low gain, low NF stage followed by a cascaded amplifier with a higher gain stage, which may achieve higher NF, although this becomes less important once the initial signal has been “ramped up”. (For more on LNAs, noise, and RF receivers, see the article “Low Noise Amplifiers Maximize Receiver Sensitivity” in TechZone.

Another problem with LNAs is nonlinearity, as synthesized harmonics and intermodulation distortion can deteriorate the quality of the received signal, making signal demodulation and decoding more difficult when the bit error rate (BER) is quite low. The third-order intermodulation point (IP3) is usually used as a linearity characterization parameter to relate the nonlinear product caused by the third-order nonlinear term to the signal amplified in a linear manner; the higher the IP3 value, the better the linearity of the amplifier performance.

Power consumption and energy efficiency are not usually primary issues in LNAs. By nature, most LNAs are fairly low power devices with current consumption between 10 – 100 mA that provide voltage gain to the next stage but do not deliver power to the load. In addition, with only one or two LNAs in the system (the latter often used in multifunction antenna designs for interfaces such as Wi-Fi and 5G), there is little point in saving energy through low-power LNAs.

In addition to operating frequency and bandwidth, the various LNAs are relatively similar in terms of functionality. Some LNAs also have gain control, so they can cope with the wide dynamic range of the input signal without overloading and saturation. In mobile applications with a wide range of loss from the base station to the cellular channel, such a wide range of input signal strength variations is often encountered, even in single connection loops.

The routing of the input signal to the LNA and from its output signal is as important as the specifications of the components themselves. As a result, designers must use sophisticated modeling and layout tools to achieve the full potential performance of the LNA. Quality components may be susceptible to degradation due to poor layout or impedance matching, so it is important to use vendor-supplied Smith’s Circle diagrams (see “Smith’s Circle Diagram: An ‘Old’ Graphic Tool Still Critical in RF Design”), as well as support for reliable circuit models that support simulation and analysis software.

For these reasons, almost all high-performance LNA vendors working in the GHz range provide evaluation boards or proven printed circuit board layouts, because every aspect of the test setup is critical, including layout, connectors, grounding, bypass, and power. Without these resources, designers would need to waste time evaluating the performance of components in their applications.

One representative of a GaAs-based LNA is the HMC519LC4TR, an 18 to 31 GHz pHEMT (pseudocrystalline high electron mobility transistor) device from Analog Devices (Figure 2). This leadless 4 x 4 mm ceramic surface mount package provides 14 dB of small signal gain, along with a low noise figure of 3.5 dB and a high IP3 of + 23 dBm. The device draws 75 mA from a single +3 V supply.

Figure 2: The HMC519LC4TR GaAs LNA provides low noise gain for low-level inputs from 18 to 31 GHz; most package connections are for power rails, grounded, or unused. (Image credit: Analog Devices)

A design process is required for everything from simple functional block diagrams to multiple external capacitors with different values and types, providing proper RF bypass with low parasitic effects on the three power rail feeds, designated as Vdd (Figure 3).

Figure 3: In a real-world application, the HMC519LC4TR LNA requires multiple bypass capacitors of the same voltage rating on its supply rails to provide large capacitance for low-frequency filtering and smaller value capacitance for RF bypass to minimize RF parasitic effects. (Image credit: Analog Devices)

An evaluation board was generated from this enhanced schematic, detailing the layout and BOM, including the use of non-FR4 printed circuit board materials (Figures 4(a) and 4(b)).

Figure 4(a)

Figure 4(b)

Figure 4: Given the high frequency at which these LNA front ends operate and the low-level signals they must capture, a detailed and tested evaluation design is essential. This includes a schematic (not shown), board layout (a) and BOM, and details of passive components and printed circuit board materials (b). (Image credit: Analog Devices)

The MACOM MAAL-011111 is a GaAs LNA for higher frequencies that can support 22 to 38 GHz operation (Figure 5). The device provides 19 dB of small signal gain and a noise figure of 2.5 dB. The LNA is ostensibly a single-stage device, but it actually has three internal cascade stages. The first stage is optimized for the lowest noise and medium gain, and subsequent stages provide additional gain.

Figure 5: To the user, the MAAL-011111 LNA is ostensibly a single-stage amplifier, but internally it uses a series of gain stages designed to maximize input-to-output signal path SNR while adding significant gain at the output. (Image credit: MACOM)

Similar to Analog Devices’ LNAs, the MAAL-011111 requires only a low-voltage power supply and is extremely compact at only 3 x 3 mm in size. Users can tune and trade-off certain performance specifications by setting the bias (supply) voltage to different values between 3.0 and 3.6 V. The suggested board layout shows the critical PCB copper skin dimensions required to maintain proper impedance matching and ground plane performance (Figure 6).

Figure 6: Suggested layout that takes full advantage of MACOM’s MAAL-011111, providing both input and output impedance matching. Note that for impedance-controlled transmission lines as well as low-impedance ground planes, use printed circuit board copper skins (dimensions in millimeters). (Image credit: MACOM)

PA driven antenna

In contrast to the difficult signal capture challenges of LNAs, PAs take a relatively strong signal from the circuit, have a high SNR, and must be used to increase signal power. All common coefficients related to the signal are known, such as amplitude, modulation, waveform, duty cycle, etc. This is the known signal/known noise quadrant of the signal processing diagram and is the easiest to cope with.

The main parameter of a PA is the power output at the frequency of interest, with typical gains between +10 and +30 dB. Energy efficiency is another key PA parameter after gain, but the use of models, modulation, duty cycle, allowable distortion, and other aspects of the driven signal can complicate any evaluation of energy efficiency, which ranges from 30 to 80%, but this is largely determined by a variety of factors. Linearity is also a key parameter for PAs, and is determined by the same IP3 value as in LNAs.

While many PAs use low-power CMOS technology (up to about 1 to 5 W), other technologies have matured and become widely used in recent years, especially at higher power levels where energy efficiency is a key indicator of battery life and heat dissipation. In cases where several watts or more are required, PAs using gallium nitride (GaN) have superior energy efficiency at higher power and frequency (typically 1 GHz). Especially when considering energy efficiency and power dissipation, GaN PAs are extremely cost competitive.

The Cree/Wolfspeed CGHV14800F (1200 to 1400 MHz, 800 W device) is representative of some of the latest GaN-based PAs. The combination of energy efficiency, gain and bandwidth of this HEMT PA is optimized for pulsed L-band radar amplifiers, enabling designers to find many uses in applications such as air traffic control (ATC), weather, anti-missile and target tracking systems. Using a 50 V power supply, it offers typical energy conversion efficiencies of 50 percent and higher and is available in a 10 × 20 mm ceramic package with a metal flange for cooling (Figure 7).

Figure 7: CGHV14800F 1200 to 1400 MHz, 800 W, GaN PA A 10 × 20 mm ceramic package with a metal flange must meet both difficult RF and thermal requirements. For mechanical and thermal integrity reasons, be careful to mount the flange with the package screwed (not soldered) to the printed circuit board. (Image credit: Cree/Wolfspeed)

The CGHV14800F is powered by a 50 V supply and typically provides 14 dB of power gain with > 65% energy conversion efficiency. As with the LNA, evaluation circuitry and reference designs are critical (Figure 8).

Figure 8: The demo circuit provided for the CGHV14800F PA requires very few components other than the device itself, but physical layout and thermal considerations are critical; considering installation integrity and thermal goals, the PA is secured to the board with screws and nuts (on the bottom, not visible) through the package flange. (Image credit: Cree/Wolfspeed)

Equally important among the many specification sheets and performance curves is the power dissipation derating curve (Figure 9). This curve shows the available power output rating versus case temperature, indicating that the maximum allowable power is a constant 115°C and then decreases linearly to a maximum rating of 150°C.

Figure 9: Because of its role in delivering power, the PA derating curve is needed to show the designer that the allowable output power decreases as the case temperature increases. Here, the rated power drops rapidly after 115⁰C. (Image credit: Cree/Wolfspeed)

MACOM also offers GaN-based PAs, such as the NPT1007 GaN transistor (Figure 10). Its DC to 1200 MHz frequency span is suitable for both broadband and narrowband RF applications. The device typically operates from a single supply between 14 and 28 V and provides 18 dB of small signal gain at 900 MHz. The design is designed to withstand a 10:1 SWR (standing wave ratio) mismatch without device degradation.

Figure 10: MACOM’s NPT1007 GaN PA spans the DC to 1200 MHz range for broadband and narrowband RF applications. Designers get additional support with a variety of load stretch charts. (Image credit: MACOM)

In addition to plots showing the performance basis at 500, 900 and 1200 MHz, the NPT1007 supports a variety of “load stretch” plots to assist circuit and system designers striving to ensure a stable product (Figure 11). The load stretch test is performed using paired signal sources and signal analyzers (spectrum analyzer, power meter or vector receiver).

The test requires seeing the impedance change of the device under test (DUT) to evaluate the performance of the PA (including factors such as output power, gain and energy efficiency), as all relevant component values may change due to temperature changes or due to variations within the tolerance band around their nominal values.

Figure 11: The NPT1007 PA’s load stretch graph exceeds the min/max/typical specification criteria table to show PA performance when its load impedance deviates from its nominal value (initial production tolerances as well as thermal drift can cause this in actual use). (Image credit: MACOM)

Regardless of the PA process used, the output impedance of the device must be fully characterized by the supplier to allow the designer to properly match the device to the antenna for maximum power transfer and to maintain as consistent a SWR as possible. The matching circuitry consists primarily of capacitors and inductors and can be implemented as discrete devices or manufactured as part of a printed circuit board or even a product package. The design must also maintain PA power levels. Again, the use of tools such as Smith’s circle diagram is key to understanding and performing the necessary impedance matching.

Given the smaller chip size and higher power levels of PAs, packaging is a critical issue for PAs. As mentioned earlier, many PAs dissipate heat through wide heat sink package leads and flange supports as well as heat sinks under the package as a path to the printed circuit board copper skin. At higher power levels (above about 5 to 10 W), PAs can have copper caps that allow the heat sink to be mounted on top and may require fans or other advanced cooling techniques.

The power rating and small size associated with GaN PAs means that modeling the thermal environment is critical. Of course, keeping the PA itself within the allowable conditions or junction temperature range is not enough. The heat dissipated from the PA must not create problems for the rest of the circuit and system. The entire thermal path must be addressed and resolved with consideration.

Summary

RF-based systems from smartphones to VSAT terminals and phased-array radar systems are pushing the limits of LNA and PA performance. This is causing device manufacturers to move beyond silicon and explore GaAs and GaN to deliver the required performance.

These new process technologies are providing designers with devices with wider bandwidths, smaller packages, and higher energy efficiency. However, designers need to understand the basics of LNA and PA operation in order to effectively apply these new technologies.

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