Wideband Low Noise Amplifiers for 3.3 Volt Operation
By Darrin Walraven, RFMD®

Introduction
When designing a radio receiver, it is important to focus on low noise amplifier design in order to obtain the highest system performance. Critical system parameters such as dynamic range, sensitivity and strong signal handling are governed by the first amplifier in the receiver chain. When developing a product, designers strive to select parts that exhibit a high intercept point and a low noise figure, with current consumption often being critical in battery-powered systems.

Traditionally, designers have chosen transistor stages as the classic solution, but transistor stages can be difficult to get matched and are inherently narrow-banded, with some requiring complicated biasing schemes. Such issues have led to longer product development times and uncertainty that the transistor stage will operate as expected. As an alternative to these challenges, monolithic microwave integrated circuit (MMIC) amplifiers have now been developed. These new MMIC amplifiers are easier to match, or completely internally matched, and are designed to be stable.

Modern-Packaged MMIC Devices
In recent years, with the introduction of modern-packaged MMIC devices, low noise amplifier (LNA) selection has become much easier. Today it is not uncommon to see 50-ohm matched parts with below 1dB noise figures from several gigahertz (GHz) to very low frequencies, leaving the designer simply to select the current draw and linearity to match system requirements.
Figure 1 shows a generic radio front end and the position of the LNA in the system following the antenna and transmit/receive (Tx/Rx) switching. Due to the LNA location near the antenna, high linearity and low noise figure are considered vitally important design parameters.

An LNA for Most Any Design
RFMD® offers a broad selection of LNA products to fit most any design need. Four wideband LNA products are selected for analysis and discussion. Graph 1 and Graph 2 plot gain and noise figures for these wideband LNA products.

Table 1 lists additional data on linearity and current consumption.

The SPF-5122Z is based on gallium arsenide Psueudomorphic High Electron Mobility Transfer (GaAs pHEMT) technology and delivers very wideband performance out to at least 3.8 GHz at a device current of 58 mA at 3.3 volts. The part boasts a quiet output, with less than 1 dB noise performance under 3 GHz. For even higher performance, such as increased P1dB or IP3, the part can be run from a 5-volt supply.

Figure 2 shows the simplicity of biasing the SPF-5122Z with essentially only input and output coupling capacitors and a self-resonant (or high impedance) bias inductor on the output. Low frequency operation can be accomplished by making the coupling capacitors and inductor larger than indicated. The SPF-5122z is available in a 2 x 2 mm QFN style package or in a SOT-89 relabeled as the SPF-5189z.

At less than half the current draw than that of the SPF-5122z, the SPF-5043z is also in the GaAs pHEMT family of wideband matched LNA parts. Noise figure is still an impressive sub-1 dB below 3 GHz at 3.3 volts with a current draw of 25 mA. Simple biasing is accomplished in the exact same manner as in Figure 2, with only coupling caps and a DC bias inductor needed. No matching network is required. Five-volt operation also is possible with the SPF-5043z for increased linearity. The device comes packaged in a tiny 1.24 x 2 mm SOT 343.

With a miniscule 10.5 mA current draw from 3.3 volts, the SGL-0622z is an excellent performer for battery-operated devices. Housed in a 2 x 2 mm QFN package and internally matched to 50 ohms, the biasing is as simple as the previous two parts shown in Figure 2, with only coupling capacitors and an inductor needed for very wideband coverage. The SGL-0622z uses silicon germanium (SiGe) technology to produce high IP3 for lower current consumption. Noise figure performance is good and remains below 2 dB at less than 2.5 GHz.

Rounding out this selection from RFMD’s LNA portfolio is the SGL-0363z in the SiGe process. The current draw at 3.3 volts is a tiny 5.2 mA. Noise figure is around 1 dB to 1 GHz and then below 2 dB out to 2 GHz. Output compression begins at about 1 dBm, but IP3 is reasonably into the teens, making the part a good selection for extremely low-power applications where tight front end filtering reduces the need for strong signal handling. The SGL-0363z does require a simple input and output matching structure. At frequencies below 500 MHz, a feedback structure is recommended to reduce the low end gain and improve stability (see Figure 3).

Conclusion
RFMD’s family of pre-matched wideband LNA products provides a wide selection of linearity and current draw optimized for 3.3 volt systems, which greatly reduce design cycle time. RFMD offers a wide portfolio of LNA products along with application-specific parts that optimize performance over more narrow frequency segments. Additional design data and full specification sheets for each device can be found at www.rfmd.com or you can contact the sales and applications group for more information about RFMD’s complete LNA product family.

About the Author
Darrin Walraven holds a BSEE from Texas A&M University. He has 20 years of experience in the field of RF and microwave circuit design and has worked as a field applications engineering manager at RFMD® for the last five years. He can be reached at dwalraven@rfmd.com with any questions or comments.

RFMD®
www.rfmd.com
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