Integrated Low-Noise Amplifiers Set New Noise Figure Benchmarks
By Tuan Nguyen and Mark Andrews, TriQuint Semiconductor

Located in the receive chain very close to where the signal is captured, low-noise amplifiers (LNAs) are key determinants of overall system performance. Their importance continues to increase as the spectral environment becomes congested and carriers must achieve the best possible reception under all conditions in order to maintain performance at the data rates that they advertise.

Even though most smartphone subscribers can’t distinguish between WCDMA and LTE, or fully appreciate one advertised data rate compared to another, consumers do take to heart subjects like dropped calls, digital fade, and upload / download speeds. For the carriers, fast is good, slow is bad and dropped connections send the customer next door. Everyone with a piece of the value chain including manufacturers, RF solutions providers and operators have a reason to ‘hit the number’ since it’s more critical to another number—their bottom line—than ever before.

While not long ago a noise figure below 1 dB and output third-order intercept point (OIP3) of 33 dBm would have generally been acceptable, greater performance is now required. The noise figure of a base station’s first stage LNA directly impacts the receiver’s sensitivity while the third order linearity affects the system’s dynamic range with respect to channel interference. Achieving ultra-low noise performance was previously not possible through integrated, packaged LNAs. But that has changed. Two new integrated, surface mount GaAs E-pHEMT LNAs from TriQuint Semiconductor meet these requirements from 400 to 1500 MHz and 1500 to 2700 MHz with noise figures of 0.45 dB or less, OIP3 of +35 dBm, and gain of at least 19 dB. These figures currently represent the best performance of any commercially available integrated LNAs covering this frequency range. Since key functions are integrated into an industry-standard package, TriQuint’s devices have simplified the RF design process and achieved better low-noise performance at the same time.

The new LNAs (Figure 1) are fabricated with TriQuint’s 0.35-µm enhancement-mode pHEMT process and together cover 400 to 2700 MHz, addressing most of the licensed and many key unregulated wireless bands used throughout the world. Accordingly, the new devices are well suited for infrastructure applications ranging from cellular base stations to tower-mounted amplifiers (TMAs), small cell wireless networks, repeaters, LTE networks operating in bands at 700 MHz, and emerging wireless systems using “white spaces” in the UHF spectrum.

The TQP3M9036 LNA covers 400 MHz to 1500 MHz with a noise figure of 0.45 dB, 19 dB of gain at 900 MHz, and high linearity (+35 dBm OIP3). The TQP3M9037 operates from 1.5 to 2.7 GHz with a noise figure of 0.4 dB and 20 dB of gain at 1900 MHz, and an OIP3 of +36 dBm. The excellent noise performance of the two devices is best shown graphically. For example, Figure 2 shows the noise figure of the TQP3M9037 and TQP3M9036. From about 1700 to about 2300 MHz, the noise figure of the TQP3M9037 is actually less than 0.4 dB ranging as low as 0.33 dB around 1900 MHz. The noise figure of the TQP3M9036 (between 700 and 900 MHz) is typically below its rated 0.45 dB, never rising higher than 0.47 dB throughout the band and dropping to only 0.41 dB at 800 MHz.

Both devices are very rugged and can withstand high-power input signals from blocking interferers or transmit power leakages of greater than +22 dBm. They are unconditionally stable, internally matched to 50 ohms, and incorporate an active bias circuit to ensure optimum performance over temperature. Both LNAs also address the growing TDD-LTE market by implementing a digital shut-down biasing capability. The LNAs are flexible to provide good performance from bias voltages from +3 to +5 VDC without the need for a negative supply voltage. Both parts are pin compatible with each other and are housed in industry-standard, RoHS-compliant 2x2-mm, 8-lead DFN packages. Only four external components – an inductor (choke) and bypass/blocking capacitors are required for operation. Detailed specifications for both devices are shown in Table 1.

Design Challenges
Designing an LNA to meet the challenges described at the beginning of this article is a tall order. It requires attempting to juggle the conflicting requirements for high gain, low noise figure, high linearity, 50-ohm matched input and output ports, and unconditional stability at the lowest possible current. Every type of electronic design inevitably involves balancing interdependent performance objectives. In the case of these products, noise match versus gain and input return loss, stability versus gain, noise figure, linearity, and of course—cost-effectiveness versus performance—were driving considerations.

TriQuint’s best choice for satisfying these requirements was its GaAs enhancement-mode pHEMT process, with its 45-GHz transition frequency (Ft), maximum current of 325 mA/mm, and high transconductance of 600 ms/mm. Enhancement mode is becoming more and more popular for very-low-noise devices as it eliminates the need for negative voltage supplies. A cascode topology was chosen for its inherently high gain, ability to achieve broad bandwidths, and high stability. The LNA is internally biased to operate between 15% and 20% of its maximum drain current for best noise figure, linearity, and reliability. TriQuint has simplified RF connectivity and design for base station manufacturers through high performance and cost-effective integration.

Most LNAs with very low noise figures require an external high-Q RF choke for gate bias in order to maintain the device’s performance. In the case of TriQuint’s TQP3M9036 and TQP3M9037, the choke is implemented on-chip, which reduces the complexity of an LNA design for RF engineers. The biasing network maintains stability over temperature through a current mirror and resistive feedback as well as providing the switching circuit for the digital power-down function.

First stage LNAs are typically surrounded by filters which are highly reflective out-of-band. As such, the products’ design also ensures unconditional stability to eliminate potential oscillations that may occur with other LNAs that are offered for these applications. Design techniques that were utilized include output resistive loading that trades-off OIP3 and gain, an input-to-output feedback network that improves stability at lower frequencies, output filter-matching at a demonstrated unstable frequency range, and source degeneration to improve stability at higher frequencies.

The TQP3M9036 and TQP3M9037 are currently sampling; production levels are expected in September. TriQuint provides design resources including evaluation boards, application notes, and other technical documentation. Visit www.triquint.com for more information, or send an e-mail to info-networks@tqs.com.

TriQuint Semiconductor
www.triquint.com
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