Low-noise amplifiers (LNAs) that are small and low in cost, while still maintaining noise figures of typically 0.8 dB at personal-communications-services (PCS) frequencies, are key requirements for cellular base stations. While a variety of modular and monolithic commercial LNAs are currently available, few, if any, offer the performance, size, and cost-effectiveness of a line of balanced amplifiers based on the use of integrated Xinger®-brand hybrid couplers. In addition to achieving low noise figures at low cost, the balanced configuration delivers greater dynamic range than single-ended designs with similar bandwidths and noise levels.
In communications systems, noise is often a limiting factor to received signal quality, especially at the low end of the dynamic range. High-powered transceivers can transmit signals over a distance greater than over which they can receive signals, a discrepancy known as link imbalance. This discrepancy is made worse by channel fading and multipath conditions (due to natural and man-made obstructions, such as buildings), which tend to raise the transceiver requirements for dynamic range. To improve link imbalance, high-performance duplexers and LNAs are placed close to the transceiver's antenna in the tower mast, eliminating about 3 dB of cable loss prior to the transceiver's front-end LNA and thus improving the overall system noise figure.
In a typical base-station transceiver, the first stage LNA is the most critical for setting the overall system noise figure (G1 in Fig. 1). To improve system noise figure, this LNA is typically located either in the tower mast close to the antenna as described above, or as a first stage in the base-station cabinet itself. The LNA portion of a base-station transceiver usually consists of two and sometimes three cascaded amplifier stages, depending upon the system's overall gain requirements. The first-stage amplifier, G1, sets the minimum possible noise figure for the receiver (Rx). Additional functionality is usually also implemented in the LNA circuitry, including the bypass of one or more LNAs to allow for overload or failure, as well as circuitry to compensate for gain variations with temperature and frequency. Variable attenuation is also used to set the absolute gain of the cascaded stages to a desired level of gain, due to inherent process-related performance variations in the transistors used in amplifier stages G2 and G3.
LNA applications such as for this base-station transceiver are usually implemented as balanced configuration (Fig. 2), at least for the first (G1) stage. A balanced amplifier configuration has several advantages over simple, single-ended amplifiers:
- The intercept point is 3 dB higher than for a single stage.
- Inherent 50-Ω input and output match due to the couplers.
- Redundancy, which minimizes a hard failure, i.e., if one of the two amplifiers were to fail—the entire LNA will still be operational, but with degraded performance.
Ensuring Stability
A balanced amplifier configuration ensures good input and output impedance match, and helps ensure stability. However, the splitter/combiner network must exhibit low loss, since insertion loss in front of the LNA will add directly to its noise figure. In single-ended amplifiers, the input matching circuitry is usually a compromise between acceptable noise performance and acceptable return loss. The balanced configuration has an added advantage: it allows the designer to optimize the input match of the transistors for optimum noise performance—since the couplers inherently will ensure good return loss of the balanced stage. The noise added by the loss of the (splitter) coupler will to some extend be made up for by the reduced noise because of the optimum noise match of the transistors.
Hybrid Couplers
Traditional balanced amplifiers are implemented with printed couplers on high-quality, low-loss microstrip circuit boards. More recently, designers have been able to replace printed couplers with Xinger® surface-mount hybrid couplers, with advantages over printed couplers in size, insertion-loss performance, and repeatability. The challenge of designing a small, high-performance, low-loss coupler for the first stage is the reason that balanced amplifiers have not been integrated on either ceramic or semiconductor substrates. To achieve a high-performance coupler in a small real estate, a multilayer design approach is needed, typically as implemented in a softboard backward wave coupler, such as the Xinger® models.
Due to market demands, the company has now developed a line of Xinger® LNAs based on low-loss power splitting and combining, matching circuitry, and a pair of low-noise enhancement-mode pseudomorphic high-electron-mobility transistors (ePHEMTs). The compact layout minimizes insertion loss prior to the active devices, in the process minimizing noise figure (Fig. 3). Since the pair of couplers (for the splitter and combiner) are printed on the same layer, production tolerances tend to balance, improving the overall performance. Since the entire LNA is mounted on low-loss circuit-board material within the Xinger® package, microstrip boards can be eliminated entirely for the LNA and it can be mounted on low-cost FR4 material without penalties in noise figure (assuming input connections are made directly to the LNA). This level of integration offers significant size advantages over conventional microstrip-based LNAs (Fig. 4), even those designed with surface-mount couplers, with significant reduction in cost for low-to-medium-volume manufacturing runs compared to traditional LNA bills of materials (BOMs).
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This integrated technology will be applied to a complete line of LNAs. Initial units cover 1.71 to 2.025 GHz (see table), including all main communication bands from DCS (GSM1800) through the US 1900-MHz PCS band and all third-generation (3G) bands like wideband-code-division-multiple-access (WCDMA) and IMT-2000 cellular uplink frequencies. The major advantages of these integrated LNAs for Rx designers include:
- Integrated low-loss splitting and combining on high-performance materials within the Xinger package, allowing the rest of the Rx front end to be implemented on low-cost FR4 material.
- Superior noise performance, with low noise figure achieved due to the compact design and the elimination of lossy transmission lines prior to the active stages.
- A design well suited for high-volume manufacturing, ideal for use with pick-and-place machines.
- A design that is 100-percent pretested and well suited for use by contract manufacturers.
- Unconditional stability, with input and output ports impedance matched to 50 Ω.
- Reduced BOM and vendor base.
- Reduced time-to-market and minimized design issues.
- Significant reduction in size, at 0.65 × 1 in. (1.651 × 2.54 cm), compared to conventional LNA solutions.
- Increased reliability, with fewer solder joints.
- Improved repeatability and standardization.
- Ease of biasing, with single-voltage-supply operation.
Preliminary test results for these integrated LNAs (Figs. 4 and 5) reveal better than 0.8-dB noise figure in the band from 1.71 to 2.025 GHz, with about 19-dB gain and gain flatness of ±1 dB. In the individual wireless bands, the gain flatness is as good as ±0.2 dB. Preliminary specifications can be found in the table. It should be noted that these amplifiers achieve very high intercept points, due to the specific ePHEMTs used. Normally, the third-order intercept point (IP3) for an LNA is about 7 to 8 dB higher than the 1-dB compression point. In these integrated LNAs, the IP3 is typically 13 to 14 dB higher than the 1-dB compression point.
Normally, standard multilayer printed-circuit-board (PCB) production facilities are not suited for handling electrostatic-discharge (ESD) sensitive devices such as ePHEMTs inside multiple-layer packages. The harsh processes associated with electroplating, in particular, will normally cause an issue. Due to its history in space- and defense-related manufacturing, however, Anaren's production lines are already geared for handling these devices.
One of the key design issues in the development of these integrated LNAs was the need for good grounding on the sources of the devices. More specifically, it is a well-known fact, in dealing with common-source field-effect-transistor (FET) designs, that minimizing source inductance is important to achieve good gain and stability. Using an SM package in the LNA means the ground inside the package is achieved by drilling a hole through the softboard package and plating these to contact the top and bottom of the package; additional vias must therefore be placed right next to the location of the FETs. However, to avoid crushing the embedded components, they are placed inside a cavity, as is normal practice in a multilayer package. Due to the many components needed in the matching and biasing network of the LNA, the total real estate consumed by cavities constitutes more than 50 percent, leaving little room for ground vias. If plated ground vias are to be achieved in close proximity of the FETs and the FETs are inside a cavity, these two factors constitute a challenge: How to create vias inside a cavity? This hurdle has been overcome by proprietary process in Anaren's design, and is being used in the company's new Xinger®-brand LNAs.
REFERENCE
- Ian Piper, Sid Seward, Samir Tozin, and H.P. Ostergaard , "Balanced LNA Suits Cellular Base Stations," Microwaves & RF, April 2002, pp. 70-80.
EDITOR'S NOTE
Anaren Microwave, Inc. recently established an operation in China that includes production, engineering, and sales functions. Known as Anaren Communications Suzhou Co. Ltd., the facility is located in Suzhou, People's Republic of China. For more information, visit the company's website at www.anaren.com.