LNAs Lower Noise In Wireless Base Stations

April 11, 2011
This pair of GaAs pHEMT-based low-noise amplifiers feature internal active bias circuitry for stable performance and high linearity in wireless infrastructure applications through 3 GHz.

LOW-NOISE AMPLIFIERS (LNAs) are critical components in most receivers, helping to boost sensitivity by adding signal gain with minimal noise. Although wireless infrastructure equipment, such as base stations, is often associated with large-signal power amplifiers, it can also benefit from small-signal LNAs.

The SKY67100-396LF and SKY67101-396LF LNAs from Skyworks Solutions incorporate advanced GaAs pseudomorphic high-electron-mobility-transistor (pHEMT) technology with active bias circuitry. Thus, they are able to achieve the high gain, low noise figure, and outstanding linearity. These new LNA improve the performance of wireless infrastructure receivers, especially in frequency bands of 700 to 1000 MHz and 1700 to 2000 MHz. Forthcoming LNAs within this product family will optimize performance at even higher frequencies. All of the amplifiers use a common package and layout.

These amplifiers are based on an enhancement-mode process for excellent linearity with a single positive supply voltage. With their internal active bias circuitry, circuit designers can adjust the supply current to fine-tune small-signal gain without affecting the noise figure. The LNAs are useful in a growing number of wireless infrastructure applications over their frequency ranges, from highly visible base stations near cellular towers to less-visible repeaters and smaller nanocells that are used to extend wireless service inside public buildings (such as in shopping malls).

Both amplifiers are supplied in miniature 2 x 2 mm, 8-pin dual-flat-no-lead (DFN), pin-compatible packages (Fig. 1). The lower-frequency SKY67101-396LF can be used over a full range of frequencies from 0.4 to 1.2 GHz, but provides optimum performance from 0.7 to 1.0 GHz with a single matching circuit. The amplifier provides usable broadband gain (Fig. 2), although with better gain flatness over the typical bandwidths found in wireless communications channels (Fig. 3).

With optimum performance in the 800-MHz band for cellular systems operating in that range, it features 0.63 dB typical noise figure at 0.80 GHz, 0.59 dB typical noise figure at 0.85 GHz, and 0.57 dB typical noise figure at 0.90 GHz (Fig. 4). The typical small-signal gain is 18.7 dB at 0.80 GHz, 18.0 dB at 0.85 GHz, and 17.6 dB at 0.90 GHz.

It exhibits input return loss of better than 17.2 dB at 0.80 GHz, 18.8 dB at 0.85 GHz, and 18.2 dB at 0.90 GHz. The output return loss is typically better than 19.8 dB at 0.80 GHz, better than 33 dB at 0.85 GHz, and better than 20 dB at 0.90 GHz. The reverse isolation is typically better than 30 dB from 800 to 900 MHz. Judging by a Rollet's stability factor of greater than 1 through 18 GHz, the SKY67101-396LF is extremely stable under a wide range of load conditions. It consumes typically 56 mA current from a +4-VDC supply, but can run at +5 VDC with higher current for improved linearity.

Amplifier linearity is vital to newer communications systems, especially those built around some form of amplitude or phase modulation, such as quadrature amplitude modulation (QAM). Because information is carried in the form of the amplitude and/or phase of a modulated signal's envelope, signal variations induced by component nonlinearitieseven the passive intermodulation (PIM) distortion of cables and connectorscan result in the loss of the information carried by the modulation.

In an ideal amplifier, a x-y plot of input power on the x axis versus output power on the y axis would show a straight line, with the slope of the line equal to the gain of the amplifier. That is, for every increase of input power, the relationship of the signal at the output of the amplifier would always increase by the same amount, with the gain being constant for all input power levels.

In practical amplifiers, however, some amount of nonlinearity exists due to active device variations, process variations, temperature-dependent variations, and other factors. The nonlinearity of an amplifier is typically judged in terms of the input and output power levels at where the gain starts to deviate from that ideal straight line, and where a given increase in input power yields somewhat less of a proportional increase in output power. At this point, the gain is said to compress.

One of the common figures of merit to compare amplifiers in terms of gain compression is the 1-dB compression point, which can be measured for both input and output signal levels. The SKY67101-396LF LNA has an input 1-dB compression point of +2.6 dBm at 0.9 GHz and an output 1-dB compression point of +19.2 dBm at 0.9 GHz.

Another measure of amplifier linearity is a parameter known as the third-order intercept point, which is essentially a measure of intermodulation distortion (IMD). Transistors in amplifiers are not ideal, and can generate harmonically related tones as well as IMD, which are the sum and difference products of two or more tones mixing in a transistor or transistor amplifier. The IMD for an amplifier is usually specified in terms of the output power at a given intercept point, such as the third-order intercept point.

For the SKY67101-396LF LNA, the input third-order intercept point is +16.2 dBm measured at 900 MHz, with two test tones each at -20 dBm and set 5 MHz apart. The output third-order intercept point is typically +33.8 dBm at 900 MHz under the same test tone conditions. The test conditions for third-order intercept point can vary widely from manufacturer to manufacturer, so it is important to note the test conditions (number of test tones, frequencies, power levels, and separation between tones) when comparing amplifiers from different suppliers.

The higher-frequency amplifier of the pair, SKY67100-396LF, can be used over a total frequency range of 1.2 to 3.0 GHz but provides optimum performance from 1.7 to 2.0 GHz. It delivers 18.3 dB typical small-signal gain at 1.75 GHz, 18.0 dB typical small-signal gain at 1.85 GHz, and 17.6 dB typical small-signal gain at 1.95 GHz. The typical noise figure is 0.62 dB at 1.75 GHz, 0.65 dB at 1.85 GHz, and 0.71 dB at 1.95 GHz (Fig. 5).

The LNA features better than 30 dB input return loss at 1.75 GHz, better than 25 dB input return loss at 1.85 GHz, and better than 20 dB input return loss at 1.95 GHz. The output return loss is typically better than 11 dB at 1.75 GHz, better than 12 dB at 1.85 GHz, and better than 12 dB at 1.95 GHz. At test frequencies of 1.75, 1.85, and 1.95 GHz, reverse isolation is typically 43 dB or better.

Like its lower-frequency counterpart, this LNA is designed for linear operation, but with low-level input signals. It has a 1-dB input compression point of +0.47 dBm at 1.75 GHz, +0.65 dBm at 1.85 GHz, and +0.80 dBm at 1.95 GHz. The 1-dB output compression-point performance is what one would expect from a small-signal linear LNA, with output levels of +18.7 dBm at 1.75 GHz, +18.4 dBm at 1.85 GHz, and +18.4 dBm at 1.95 GHz.

The input third-order intercept point performance levels are typically +15.7 dBm at 1.75 GHz, +16.1 dBm at 1.85 GHz, and +16.4 dBm at 1.95 GHz with two -20-dBm tones spaced 5 MHz apart. The typical output third-order output intercept-point performance levels are +34.0 dBm at 1.75 GHz, +34.3 dBm at 1.85 GHz, and +34.0 dBm at 1.95 GHz.

Both amplifiers are rated for maximum input levels to +20 dBm and offer active bias circuitry that allows supply current to be varied for gain adjustments without adversely affecting noise figure. Both are suitable for all parts of a wireless infrastructure.

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