Top things to keep in mind when designing wireless with GaAs LNAs and GaN PAs

The drive for performance, miniaturization, and higher frequency operation is challenging the limits of two key antenna-connecting components for wireless systems: the power amplifier (PA) and the low-noise amplifier (LNA). Efforts to make 5G a reality and the use of PAs and LNAs in VSAT terminals, microwave radio links, and phased array radar systems have contributed to this transition.

The drive for performance, miniaturization, and higher frequency operation is challenging the limits of two key antenna-connecting components for wireless systems: the power amplifier (PA) and the low-noise amplifier (LNA). Efforts to make 5G a reality and the use of PAs and LNAs in VSAT terminals, microwave radio links, and phased array radar systems have contributed to this transition.

Requirements for these applications include lower noise (for LNAs) and higher energy efficiency (for PAs) and operation at higher frequencies up to and above 10 GHz. To meet these growing demands, LNA and PA manufacturers are switching from traditional all-silicon processes to gallium arsenide (GaAs) for LNA and gallium nitride (GaN) for PA.

This article will describe the role and requirements of LNAs and PAs and their key characteristics, then describe typical GaAs and GaN devices and what to keep in mind when designing with these devices.

Sensitive role of LNA

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

Such gains can be easily achieved with modern electronics, but when the LNA adds all kinds of noise to the weak input signal, the problem is far less simple. The amplification advantage of the LNA is completely lost in such noise.

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

Note that LNA works in a world full of unknowns. As the front end of the transceiver channel, the LNA must be able to capture and amplify very low power low-voltage signals within 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 hardest part of all signal processing puzzles.

The main parameters of the LNA are noise figure (NF), gain and linearity. Noise originates from heat and other sources of noise, and the noise figure is typically 0.5 – 1.5 dB. The typical gain of a single stage amplifier is between 10 – 20 dB. There are designs that use a cascaded amplifier with a higher gain stage followed by a low gain, low NF stage, which may achieve a higher NF, but this becomes less so once the initial signal has “boosted” important. (For more information on LNAs, noise, and RF receivers, see the TechZone article “Low Noise Amplifiers Maximize Receiver Sensitivity.”)

Another problem with LNAs is non-linearity, as synthesized harmonics and intermodulation distortion can degrade 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 characterization parameter for linearity, and the nonlinear product caused by the third-order nonlinear term is associated with the signal amplified in a linear manner; the higher the IP3 value, the linearity of the amplifier performance. the better.

Power consumption and energy efficiency are usually not primary concerns in LNAs. Essentially, most LNAs are fairly low power, 10 – 100 mA devices that provide voltage gain to the next stage, but do not deliver power to the load. In addition, only one or two LNAs are used in the system (the latter is often used in multi-function antenna designs for interfaces such as Wi-Fi and 5G), so saving energy through low-power LNAs is not meaningful.

With the exception of operating frequency and bandwidth, the various LNAs are relatively functionally similar. Some LNAs also feature gain control, so they can handle the wide dynamic range of the input signal without overloading or saturation. In mobile applications with a wide base station to handset channel loss range, input signal strength variations this wide are often encountered, even with a single connection cycle.

The routing of the input signal to the LNA, and the output signal from it, is as important as the specification of the component itself. Therefore, designers must use sophisticated modeling and placement tools to realize the full potential performance of the LNA. Premium components can be prone to degradation due to poor layout or impedance matching, so be sure to use a vendor-supplied Smith chart (see “Smith Chart: An ‘Old’ Graphical Tool Still Crucial in RF Design”), And reliable circuit models that support simulation and analysis software.

For these reasons, almost all suppliers of high-performance LNAs operating in the GHz range offer evaluation boards or proven printed circuit board layouts, as every aspect of the test setup is critical, including layout, connectors, grounding , bypass and power supply. Without these resources, designers waste time evaluating the performance of components in their applications.

A representative of a GaAs-based LNA is HMC519LC4TR. This is an 18 to 31 GHz pHEMT (pseudomorphic high electron mobility transistor) device from Analog Devices (Figure 2). This leadless 4×4 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 can draw 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, ground, or not used. (Image credit: Analog Devices)

Going from a simple functional block diagram to multiple external capacitors of different values ​​and types requires a design process that provides proper RF bypassing with low parasitics on the three power rail feeds, designated Vdd (Figure 3).

Figure 3: In practice, the HMC519LC4TR LNA requires multiple bypass capacitors of the same voltage rating on its power rails to provide large capacitors for low frequency filtering and smaller value capacitors for RF bypassing to maximize ground to reduce RF parasitics. (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 frequencies at which these LNA front ends operate and the low-level signals they must capture, a detailed and tested evaluation design is critical. It 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 higher frequency GaAs LNA that supports 22 to 38 GHz operation (Figure 5). The device offers 19 dB of small-signal gain and 2.5 dB of noise figure. This LNA appears to be a single-stage device, but it actually has three cascaded stages inside. The first stage is optimized for lowest noise and medium gain, and subsequent stages provide additional gain.

Figure 5: To the user, the MAAL-011111 LNA appears to be 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’ LNA, the MAAL-011111 requires only one low-voltage power supply and is extremely small in size at only 3 x 3 mm. The user can adjust and trade off certain performance specifications by setting the bias (supply) voltage to various values ​​between 3.0 and 3.6 V. The recommended board layout shows the critical printed circuit board copper dimensions required to maintain proper impedance matching and ground plane performance (Figure 6).

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

PA driven antenna

In contrast to the difficult signal acquisition challenges of an LNA, a PA acquires a relatively strong signal from the circuit, has a high SNR, and must be used to increase the 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 in the signal processing diagram and is the easiest to deal with.

The main parameter of the PA is the power output at the relevant frequency, and its typical gain is between +10 and +30 dB. Energy efficiency is another critical PA parameter after gain, but using models, modulation, duty cycle, allowable distortion, and other aspects of the driven signal can complicate any energy efficiency assessment. The energy efficiency of a PA is between 30 and 80%, but this is largely determined by a variety of factors. Linearity is also a key parameter for PA, and is determined by IP3 value as in LNA.

While many PAs use low-power CMOS technology (up to about 1 to 5 W), other technologies have matured and been widely used in recent years, considering energy efficiency as a key indicator of battery life and heat dissipation. This is especially true at high power levels. Where several watts or more of power are required, PAs using gallium nitride (GaN) have better energy efficiency at higher powers and frequencies (typically 1 GHz). Especially when considering energy efficiency and power dissipation, GaN PAs are very cost competitive.

The Cree/Wolfspeed CGHV14800F (1200 to 1400 MHz, 800 W device) is the latest representative of some of the GaN-based PAs. This HEMT PA’s combination of energy efficiency, gain and bandwidth 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. Offers typical energy conversion efficiencies of 50% and higher using a 50 V power supply, and is housed in a 10 × 20 mm ceramic package with metal flanges for cooling (Figure 7).

Figure 7: CGHV14800F 1200 to 1400 MHz, 800 W, GaN PA A 10 × 20 mm ceramic package with metal flange must meet both difficult RF and thermal requirements. For mechanical and thermal integrity, take care to screw (not solder) the package to the printed circuit board when mounting the flange. (Image credit: Cree/Wolfspeed)

The CGHV14800F operates from a 50 V supply and typically provides a power gain of 14 dB with >65% energy conversion efficiency. As with LNAs, evaluation circuits 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 mounting integrity and thermal objectives, the PA is secured with screws and nuts ( at the bottom, not visible) to the board. (Image credit: Cree/Wolfspeed)

Equally important in many spec 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 constant at 115°C, then decreases linearly to the maximum rating of 150°C.

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

MACOM also offers GaN-based PAs such as the NPT1007 GaN transistor (Figure 10). Its frequency span from DC to 1200 MHz is suitable for wideband 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 tolerate 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 various load-stretch charts. (Image credit: MACOM)

In addition to graphs showing the basis of performance at 500, 900, and 1200 MHz, the NPT1007 supports various “load stretch” graphs to assist circuit and system designers trying to ensure stable products (Figure 11). Load pull testing is done using a paired signal source and signal analyzer (spectrum analyzer, power meter, or vector receiver).

This test requires seeing changes in the impedance 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 changes within the tolerance band of values.

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

Regardless of the PA process used, the device’s output impedance must be sufficiently characterized by the supplier to allow the designer to properly match the device to the antenna for maximum power transfer and SWR as consistent as possible. Matching circuits consist primarily of capacitors and inductors and can be implemented as discrete devices or fabricated as part of a printed circuit board or even a product package. Its design must also maintain PA power levels. Again, the use of tools such as the Smith chart is key to understanding and making the necessary impedance matching.

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

The power ratings and small size associated with GaN PAs mean that modeling the thermal environment is critical. Of course, keeping the PA itself within the allowable conditions or junction temperature is not enough. The heat dissipated from the PA should not cause problems for the circuit and other parts of the system. The entire thermal path must be considered and addressed.


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

These new process technologies offer designers higher bandwidth, smaller packages, and more energy-efficient devices. However, designers need to understand the basics of LNA and PA operation in order to effectively apply these new technologies.

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