How to get the most out of your low noise amplifier solution?

灞波儿奔

How to get the most out of your low noise amplifier solution? [Copy link]

As 5G wireless networks continue to evolve, the performance of the radio front end plays an increasingly critical role in the RF receiver signal path, especially for low noise amplifiers (LNAs). With the emergence of new process technologies suitable for LNAs, such as silicon germanium (SiGe), gallium arsenide (GaAs), and silicon-on-insulator (SOI), designers must re-evaluate the performance trade-offs of LNA parameters such as noise, sensitivity, bandwidth, and power to effectively use these process technologies.
　　The importance of the front end cannot be overstated, as it largely determines the ultimate performance of the system in weak signal situations and the achievable bit error rate. If the LNA performance is not up to standard, the remaining design efforts in circuit and receive channel management to meet 5G performance requirements will have little effect.
　　This article will discuss the current state of 5G and its requirements for LNA performance. It will then introduce solutions that use the latest processes to help meet these requirements and how to make the most of them. A
　　brief overview of the current state of 5G
　　There is a long way to go, but solid progress has been made: Although the specifications for 5G have been finalized, they are still being refined. Many of the exciting features of 5G are still to be determined, and more meetings and field trials are needed, as well as input from component vendors and wireless operators.
　　However, some issues are already clear: 5G designs will occupy new blocks of the electromagnetic spectrum, but some initial implementations will still be below 6 gigahertz (GHz). Most 5G systems will operate in the millimeter wave bands, with the 27 to 28 GHz and 37 to 40 GHz bands available in the United States. Some preliminary allocations are even above 50 GHz. Due to technical challenges, the first millimeter wave implementations will use the 27 to 28 GHz band. The
　　specific role of the LNA
　　While the 5G specifications provide a lot of modulation, power, data rate options and other features, most of them are generally not related to the receive channel LNA. This component must do one thing, which is to capture and amplify the weak signal from the antenna that is corrupted by noise, while minimizing the noise added. Therefore, it is wise to take a closer look at the LNA itself rather than focusing too much on the higher-level specification issues that continue to evolve.
　　The primary LNA specification for acceptable operation within a given frequency band is the noise figure (NF), which is the amount of inherent noise added by the LNA. For 5G, especially near the 28 GHz band, the NF typically needs to be between 1 and 3 dB, with 1 to 2 dB more acceptable in some cases. Gain of between 15 and 20 dB is typically required to boost the received signal to a level that can be properly processed by subsequent amplifiers, filters, and digitization.
　　Finally, linearity-related coefficients for the 1 dB output compression point (known as OP1 or P1dB) and the output third-order intercept point (OIP3) need to be at least -20 and -35 dBm, respectively. At lower 5G frequency bands, these requirements for OP1 and OIP3 are less stringent, with OP1 in the -20 dBm range and OIP3 at -10 to -15 dBm. Note that more negative values indicate better performance (-25 dBm is better than -20 dBm), but many data sheets omit the negative sign, which can be confusing.
　　Functionally, LNAs are “simple” amplifiers with a very basic block diagram—usually just an amplifier triangle—and only a few package leads (usually 6 to 8). As a result of this simplified design, their packages are small, measuring about 1 to 2 mm per side, and many are even smaller.
　　New processes push LNAs toward 5G applications
　　Many high-performance LNAs are tailored for low frequencies in the few GHz range (such as the 2.4 GHz and 5 GHz bands), but they do not meet the stringent requirements of 5G front ends. As silicon-based LNAs appear to have reached their performance limits, vendors are rushing to use newer semiconductor materials and processes to meet the stringent requirements of many 5G performance specifications. Even at lower 5G frequency bands, standard silicon does not have low enough noise figures and high OP1/OIP3 levels to meet 5G requirements, as its transmit and receive signal levels are lower than existing wireless standards.
　　For these reasons, vendors have invested heavily in the research and development and mass production of new processes based on SiGe, SOI, and gallium arsenide (GaAs) materials, as these new processes offer higher electron mobility, smaller geometries, and less leakage.
　　For example, Infineon Technologies' BGA8U1BN6 LNA uses a SiGe process with a noise figure of only 1.6 dB, an OP1 between 18 and 22 dBm, and an OIP3 between 10 and 15 dBm. It operates in the 4 to 6 GHz band with a gain of 13.7 dB.
　　In addition, the BGA8U1BN6 offers a power-saving feature that, when activated, allows it to enter bypass mode, passing the input signal to the output with an insertion loss of only 7.5 dB (Figure 1). This feature is useful when received signal strength is high, as it both prevents overloading of the next stage and reduces the LNA supply current from about 20 milliamps (mA) to about 100 microamps (A) on a 2.8-volt supply, resulting in significant power savings. The
　　Infineon Technologies SiGe BGA8U1BN6 LNA includes a bypass mode that removes the LNA from the signal path; this reduces gain, preventing overloading and saturation of subsequent stages, while also reducing current requirements. (Image: Infineon Technologies)
　　Skyworks Solutions’ SKY65806-636LF, which also offers a bypass mode, is an SOI LNA for the 3400 to 3800 MHz band. Its gain is similar to that of the Infineon device, about 13.6 dB, but its noise figure is only 1.2 dB. The supply voltage range is 1.6 to 3.3 volts, and it operates on just 3.85 mA. Like Infineon’s LNA, this 50-Ω LNA includes a user-controlled bypass feature.
　　The ADL5724 LNA from Analog Devices is also SiGe-based and operates in the 12.7-GHz to 15.4-GHz band (Figure 2). Its 100-Ω balanced differential outputs are ideal for driving differential downconverters and analog-to-digital converters. Typical gain is greater than 23.7 dB, and typical noise figures are 2.1 dB at 12.7 GHz and 2.4 dB at 15.4 GHz.
　 Analog Devices’ SiGe ADL5724 offers balanced differential outputs that support enhanced signal integrity between the device and the next stage of the signal chain. (Image: Analog Devices)
　　Given that many LNAs are not typically deployed in a stable temperature environment, the ADL5724 data sheet includes a graph of key performance factors versus temperature (Figure 3).
　 The dependence of LNA performance on temperature is shown by plotting (a) gain and (b) noise figure versus frequency at -40°C, +25°C, and +85°C. Note how gain decreases while noise figure increases with temperature. (Image: Analog Devices)
　　For the ADL5724, gain decreases slightly with temperature, while noise figure increases with temperature. This is typical of LNAs and is independent of process. Designers need to account for these variations in worst-case modeling and signal chain performance simulations.
　　To achieve high dynamic range and low noise, MACOM Technology Solutions Holdings (MACOM) introduced the MAAL-011078, a GaAs single-stage LNA with high dynamic range and ultra-low noise figure of only 0.5 dB at 2.6 GHz. It also offers 22 dB of gain and high linearity of 33 dBm (OIP3) and 17.5 dBm (P1dB). The IC covers the 700 MHz to 6 GHz band and has an additional feature: an integrated active bias circuit so users can set their own bias (operating point) current via an external resistor. This allows users to tailor power consumption to meet application needs. For example, choosing lower performance for lower operating current (Figure 4).
　With MACOM’s MAAL-011078, users can set the LNA bias current and operating point via external resistors, achieving a change in OIP3 vs. frequency (left) and a decrease in P1dB performance vs. frequency (right) by reducing operating current. (Image: MACOM)
　　Getting the Most
　　Out of a 5G LNA After selecting the right LNA for 5G, there are some considerations and compromises to make the most of the LNA when implementing a 5G front-end design. As operating frequencies span 5 GHz and 10 GHz, there are five important factors to consider beyond the LNA itself.
　　1: PC Board Material Selection - In the gigahertz range, transmission line losses at the LNA input and output are an important factor. This is especially true on the input side, where transmission line losses reduce the maximum achievable signal-to-noise ratio and also increase the output noise of the LNA. Since the transmission lines in most designs are fabricated as striplines to the PC board itself, the board must be made of a low-loss dielectric material.
　　Simply using a generic FR4 PCB laminate is not sufficient to ensure this, so suppliers offer a variety of alternative materials and laminates. One of the most widely used boards has a special laminate placed on an FR4 core that gives the transmission lines a stable loss factor and the basic strength of an FR4 stiffener.
　　Remember that at these frequencies, the PC board must be considered as just another passive "component" in the circuit design, with the same parasitic effects as all other passive components. In addition, details such as the temperature coefficients of the board's key characteristics and its parasitic effects must be considered. Suppliers of high-performance PC board materials provide this data.
　　2: Capacitor Selection – For input and output matching circuits, high-Q capacitors must be used to reduce the noise figure into and out of the LNA. Low-Q components can degrade the noise figure by anywhere from 0.2 dB to 1 dB. Widely used NPO capacitors have lower Q values and higher losses, so they should be avoided. Ceramic capacitors have the highest Q values, but they are expensive. A satisfactory compromise can be found by performance and cost analysis.
　　3: Power Supply Bypassing – This is a well-known point, but it is often overlooked, so it is worth repeating. Careful and thoughtful DC power supply bypassing in the IC and other locations must be implemented to ensure stable and consistent high-frequency performance. The bypass capacitors selected should have the lowest impedance at the required frequency to achieve the highest decoupling performance.
　　For example, a 1000 picofarad (pF) capacitor is not a suitable choice for high-frequency decoupling. At 5 GHz, the self-resonant frequency of a 1000 pF capacitor will make it look like an inductor, so it may actually defeat the purpose of decoupling. Instead, a capacitor with a smaller capacitance (typically less than 10 pF) should be placed close to the LNA. In addition, the design should include traditional low-frequency decoupling using a 1000 pF and 0.01 F capacitor in parallel. These capacitors do not need to be placed close to the LNA.
　　4: Input and Output Matching - Although many LNAs have 50 Ω inputs and outputs, some do not. Even if they do, the circuit driving the LNA and the circuit driven by the LNA output may not have 50 Ω impedance. Therefore, it is necessary to create matched circuits using Smith charts and establish appropriate matching options using S-parameters. Similarly, the reactive passive components (inductors and capacitors) used at 5G frequencies will inevitably generate various types of parasitic effects: internally, on nearby components, and on the PC board.
　　Designers should do three things: select matching components designed to suppress parasitics at these frequencies; ensure that the unavoidable parasitics are fully characterized when placing the components; and use these values to model the matching circuits and adjust the nominal values accordingly.
　　5: Cable Interconnects - Some 5G systems require interconnects beyond the PC board and its stripline transmission lines, so physical cables are required. If differential interfaces are used (usually to maintain circuit balance and improve noise immunity), these cable interconnects may require the use of delay-matched cable pairs, and it is best if both cables have identical propagation characteristics.
　　Therefore, high-performance cables used for 5G to 40 GHz and higher frequencies often have their delays matched to 1 psec (picosecond). They are sold and used in pairs, and because they cannot be installed or replaced separately, the two physical cables are "bandaged" to keep them paired at all times. With these cables, differential circuits can achieve the performance of high-end LNAs when driving the next stage of the signal chain.
　　Conclusion
　　5G wireless standards are pushing operating frequencies higher, into the multi-GHz and tens of GHz ranges. It also requires lower noise/lower distortion performance from analog circuits, especially low noise amplifiers. New IC process technologies such as SiGe, SOI, and GaAs can meet these needs. But the performance of a good LNA will not be achieved without attention to the realities that RF encounters at these higher frequencies.