White Paper: Low-Noise Amplifiers Drop Below 1-dB Noise-Figure Mark

Sponsored by Richardson Electronics

Low-noise amplifiers (LNAs) can be found in the front end of virtually all communications receiver designs, especially those where signal sensitivity is critical. The first-stage LNA in a receiver is a critical component since it essentially sets the limit for how low the system noise figure can reach. While LNA noise figures approaching as low as 1 dB have been available commercially, few devices ever reach noise figures of less than 1 dB. Given this as a reference, the performance of the SuperLNA family of LNAs from Richardson Electronics is even more remarkable, considering the LNAs operate over typical bandwidths of 500 and 600 MHz with noise figures of 0.6 dB or less.

The SuperLNA amplifiers (Fig. 1) benefit from the use of discrete devices and precise device characterization performed with well-calibrated microwave vector network analyzers. The characterization, which provides an accurate set of S-parameters for the devices, leads to the development of the input, output, and interstage matching networks that are so critical to achieving the lowest possible noise figures from these GaAs FET devices.

In synthesizing a matching network for an active device, be it for connection to a source, load, or another active device, a compromise must be reached between an LNA's two main performance parameters: noise figure and gain, since it is generally not possible to match a device for simultaneous maximum gain and lowest noise figure. Constant gain and noise-figure circles are typically used to select a usable trade-off between gain and noise figure.

In the SuperLNA amplifier family, the matching networks defy traditional constraints, working the FETs to achieve extremely high gain while dropping to new low levels of noise figure. For the lowestfrequency amplifier, the UHF model RLAS0205A, the combination of 0.5-dB noise figure and 43-dB associated gain from 200 to 550 MHz is a rare combination of low noise with exceptionally high gain. In spite of the generous gain, which sometimes leads to a tendency for amplifiers to regenerate reflected signals from their output ports and oscillate, the SuperLNA amplifiers show a Rollet's stability factor (K) of greater than 1, indicating unconditional stability.

All of the SuperLNA amplifiers measure 0.50 × 0.35 × 0.08 in. (12.5 × 9 × 2 mm) in a surfacemount-technology (SMT) package; connectorized versions are also available for lab and test requirements. Matched to 50 ? at the input and output ports, they include input and output DC blocks along with temperaturecompensation circuitry. They are projected to have a mean time before failure (MTBF) of more than 600,000 hrs. (68 years) when tested according to TR-NWT-000332, Issue 3, September 1990 at +40°C. These are rugged amplifiers constructed for commercial, industrial, and military environments, designed to meet the requirements of MILSTD-202, MIL-STD-883, and MIL-STD-810F.

The RLAS0205A exhibits maximum noise figure of 0.65 dB at room temperature (Fig. 2). Its internal impedance matching results in a very low input/output VSWR of nominally 1.18:1 and no higher than 1.22:1 (an input/ output return loss of 22 dB). The gain flatness is tightly controlled, reaching a maximum of ±1 dB across the full 200-to-550-MHz bandwidth and a nominal value of ±0.75 dB. Although such low-noise performance normally implies fairly low output power and intercept point, the RLAS0205A shines in both regards, with nominal +18 dBm output power at 1-dB compression and nominal two-tone (both a 0-dBm) output third-order intercept point of +35 dBm. The output power and output thirdorder intercept point define the upper limit of an amplifier's dynamic range, while the noise figure defines the lower limit of that dynamic range. Together, these performance parameters for the RLAS0205A characterize an amplifier that brings expanded dynamic range to a receiver design. The reverse isolation is an impressive 45 dB. The amplifier draws 90 mA from a +5-VDC supply.

The other members of the SuperLNA amplifier family bring similar levels of performance to higher frequencies (see table). Model RLAS0510A features nominal gain of 38 dB and ±1.2-dB gain flatness over the octave band from 500 to 1000 MHz (Fig. 3), with nominal noise figure of 0.4 dB. The maximum noise figure over that band is 0.55 dB, remaining consistently low below 900 MHz (Fig. 4). The amplifier delivers +19 dBm typical output power with an output third-order intercept point of +30 dBm. The nominal input/output VSWR is 1.22:1 (input/output return loss of 20 dB). The RLAS0510A exhibits reverse isolation of at least 40 dB and typically 45 dB. The amplifier draws 100 mA from a +5-VDC supply.

The RLAS1216A is ideally suited for Global Positioning Systems (GPS) within the 1200-to-1600-MHz, with 31-dB typical gain and ±1-dB typical gain flatness across that frequency range. The typical noise figure is 0.5 dB, with a maximum value of 0.65 dB. The amplifier generates output power at 1-dB compression of typically +10 dBm and shows reverse isolation of typically 37 dB. It features input/output VSWR of 1.20:1 (input/output return loss of 20 dB) and draws 40 mA typical current from a +3.3-VDC supply.

The RLA1722A is designed for use from 1700 to 2200 MHz, a frequency range that includes DCS, PCS, UMTS, and other 3G cellular communications receivers and transceivers. The tiny surfacemount-amplifier boasts typical noise figure of 0.55 dB, with gain of 30 dB and gain flatness across the band of ±1 dB. It achieves +19 dBm output power at 1-dB compression with output third-order intercept point of +32 dBm (Fig. 5). The reverse isolation is typically 43 dB and the input/output VSWR is 1.17:1 (return loss of 20 dB). The amplifier draws 90 mA current from a +5-VDC supply.

The highest-frequency member of the SuperLNA family, the RLAS2026A, provides 3G and Industrial-Scientific-Medical (ISM) band coverage from 2000 to 2600 MHz with 26-dB typical gain and 0.60-dB typical noise figure. The maximum noise figure is 0.8 dB and the gain flatness is nominally ±1 dB. The output power at 1-dB compression is typically +12.5 dBm. The amplifier, which draws 55 mA current from a +5-VDC supply, has input/output VSWR of 1.17:1 (20-dB return loss) and 43-dB reverse isolation.

It should be noted for all of the SuperLNA amplifiers that the noise figures given above are at room temperature (about +25°C). A characteristic-of GaAs FET amplifiers is temperature-dependent noisefigure performance. A GaAs transistor or amplifier will exhibit a noise figure that increases with increasing temperature. When cooled, as in many satellite-communications receivers, GaAsbased LNAs can achieve extremely low noise figures. For the 2000- to-2600-MHz model RLAS2026A, for example, the noise figure is typically about 0.6 dB from 1900 to 2700 MHz (Fig. 6) at room temperature, dropping to about 0.5 dB across that frequency range at ?40°C and rising to 0.7 dB across that frequency range at +85°C.

The SuperLNA amplifiers provide designers with broadband solutions through 2.6 GHz that allow a receiver signal chain to begin with a noise figure below 1 dB for best possible SNR and, with the LNAs' high intercept points, enhanced dynamic range. The new amplifiers boast cost advantages over similar-frequency amplifiers, especially considering that they have eliminated the need to add matching circuitry and DC blocking capacitors. The amplifiers are suitable for commercial and military receivers, wireless data communications, and test and measurement applications. The RoHS-compliant amplifiers are designed to MIL-STD-202, MILSTD-883, and MIL-STD-810F, and can be supplied as drop-in modules, with connectors, or in multifunction modules. Custom frequencies are also available.

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