Amplifiers Break Intercept Barrier

July 14, 2011
These broadband GaAs pHEMT amplifier modules show that high output power is no longer a prerequisite for achieving outstanding amplifier linearity through 2.4 GHz.
1. The high-linearity HXG amplifiers are supplied in sealed, surface-mount ceramic packages with internal impedance-matching circuitry. High linearity, typically measured by the third-order intercept point (IP3), is an increasingly critical parameter in wireless communications systems. This performance parameter grows in importance as new industry communications standards, with ever-higher orders of digital modulation, squeeze multiple carriers and more information into limited frequency bandwidths. For RF and microwave amplifiers, the old "brute force" approach for high linearity involved simply increasing the power supply, and accepting the increased amount of heat generated by the amplifier as an unavoidable tradeoff. But now, Mini-Circuits has introduced a pair of GaAs enhancement-mode pseudomorphic high-electron-mobility-transistor (E-pHEMT) amplifiers that provide outstanding linearity with low power consumption. Model HXG-122+ is designed for use from 0.4 to 1.2 GHz, while model HXG-242+ is aimed at applications from 0.7 to 2.4 GHz.

Mini-Circuits' new HXG amplifiers (Fig. 1) exemplify what can be achieved with load-pull technology and patent-pending Mini-Circuits System in Chip (MSIP) internally matched construction techniques: wideband frequency coverage with industry-leading IP3 performance. Model HXG-122+ spans 0.4 to 1.2 GHz with typical IP3 of +46 dBm from 600 to 800 MHz, while model HXG-242+ operates from 0.7 to 2.4 GHz with IP3 of greater than +46 dBm from 1.5 to 2.4 GHz (Fig. 2). Both amplifiers typically draw only 146 mA from a +5-VDC power supply.

2. The output IP3 performance of the HXG-122+ (top) and HXG-242+ (bottom) amplifiers far exceeds the industry norm relative to the P1dB for these amplifiers.

What makes these amplifiers even more outstanding in terms of linearity are their ratios of IP3 to output power at 1-dB compression (P1dB). An accepted industry rule of thumb is that an amplifier's IP3 will be about 10 dB higher than its P1dB. But in the case of the HXG-122+, with a typical P1dB of +23 dBm from 600 to 800 MHz, its IP3 is typically 23 dB higher. For the HXG-242+, with typical P1dB of +23 dBm or more from 1.5 to 2.4 GHz, the IP3 levels are typically more than 22 dB higherwell beyond the usual 10-dB industry rule of thumb (Fig. 3).

Unlike standard GaAs monolithic-microwave-integrated-circuit (MMIC) amplifiers that require additional external impedance-matching circuitry to reach their advertised performance levels, the HXG-122+ and HXG-242+ MSiP amplifiers include carefully designed impedance-matching circuitry, optimizing performance across their frequency bands. Mini-Circuits has given the HXG amplifiers a balanced blend of performance characteristics, including high gain and low noise figures, as well as high P1dB and IP3 through 2.4 GHz (Fig. 4).

Another factor that sets the compact HXG amplifiers apart from most high-IP3 RF/microwave amplifiers is that they do not sacrifice small-signal performance for large-signal performance. The HXG models are truly wide-dynamic-range amplifiers, with highly linear performance in combination with low noise figures, as might be needed for any application requiring high spurious-free-dynamic-range (SFDR) performance.

The amplifiers are well qualified for use as secondary amplifiers in wide-dynamic-range receivers or as low-noise driver amplifiers for transmit systems where the noise figure is factored in as part of the amplifier signal chain's overall dynamic range. In addition to commercial communications applications, the wide dynamic range of the HXG amplifiers also makes them good candidates for military applications, such as in tactical radios and electronic-intelligence (ELINT) receivers, in which desired signals must be extracted from a wide range of intentional and unintentional jammers and blocking signals.

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3. The output power at 1-dB gain compression for the HXG-122+ (top) and the HXG-242+ (bottom) generally exceeds +23 dBm across both of their frequency ranges. For example, the HXG-122+ achieves noise figures within a 2.0-to-2.3-dB window across its operating frequency range from 400 to 1200 MHz. At 500 MHz, the small-signal gain is 16.1 dB and the noise figure is only 2.1 dB, with P1dB of +23.5 dBm and output IP3 of +44.9 dBm. At 1200 MHz, the small-signal gain is typically 14.8 dB and the noise figure "climbs" to 2.3 dB, while the P1dB is +23.4 dBm and the output IP3 is +40.7 dBm. Even at the upper edge of its design bandwidth, the HXG-122+ delivers an IP3-to-P1dB ratio of 17.3 dB, far above the typical 10-dB industry standard of conventional unoptimized GaAs MMIC amplifiers.

For the higher-frequency model HXG-242+, the noise figure is also quite respectable across the operating frequency range, within a 2.0 to 2.5 dB window from 0.7 to 2.5 GHz. The noise figure for the HXG-242+ at 700 MHz, for example, is 2.0 dB, with small-signal gain of typically 16.1 dB, P1dB of +22.7 dBm, and output IP3 of +40.7 dBm. At 1.5 GHz, the noise figure is 2.3 dB, with a typical gain if 15.1 dB, P1dB of +23.4 dBm, and output IP3 of +44.7 dBm. At the upper bandedge of 2.4 GHz, the noise figure is typically 2.4 dB, the small-signal gain is typically 13.8 dB, the output power at 1-dB gain compression is typically +23.9 dBm, and the output IP3 is typically +48.5 dBm.

High linearity is essential for maintaining the integrity of the complex modulated waveforms used in wireless telecommunications systems. Quadrature amplitude modulation (QAM), wideband-code-division-multiple-access (WCDMA), and orthogonal-frequency-division-multiplex (OFDM) modulation schemes can all cause amplitude levels to fluctuate wildly. Whenever multiple carriers hit simultaneous peaks, the amplitude of the combined waveform spikes, resulting in extremely high peak-to-average power ratios (PAPRs).

4. The noise figure (top plots) and gain (bottom plots) of the HXG-122+ (left) and the HXG-242+ (right) amplifiers belies their P1dB and IP3 performance levels.

To accommodate these occurrences, a common practice at the system level is to "back off" the signal strength of the system's amplifiers, thus avoiding saturation at the upper end of the dynamic range and operating under conditions that favor a more linear translation of signals from the input to the output ports. While this practice does provide improved linearity, it does not make the most efficient use of the amplifier power supplies. The high IP3 and wide dynamic range of the HXG-122+ and HXG-242+ amplifiers reduce the need for operation under "backed-off" conditions, helping to improve the efficiency along the entire signal path.

To better understand the practical benefits of the HXG series amplifiers in use with high PAPR signals typically found in modern wireless communications systems, Mini-Circuits engineers evaluated the amplifiers in terms of another parameter commonly used to measure linearity: adjacent-channel power ratio (ACPR). This is a logical extension of the classic two-tone test for IP3, using actual modulated waveforms as the input signals instead of sinewave test signals. These waveforms cover a specific frequency range (the "main channel"), and ACPR is defined as the ratio of the output power in adjacent channels (undesired signals, primarily intermodulation and other distortion products) to output power in the main channel (the desired signals). ACPR directly correlates to the linearity of any device, and side-by-side characterization of the HXG-242+ and a typical GaAs MMIC amplifier demonstrates a full 3-dB improvement in ACPR performance even when operating in the "backed-off" region (Fig. 5). This outstanding linearity makes the HXG series amplifiers ideal candidates for transmit-path applications in third-generation (3G) and fourth-generation (4G) wireless communications systems alike, including in the Universal Mobile Telecommunications System (UMTS), WiMAX, and Long-Term-Evolution (LTE) cellular communications systems.

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5. These analyzer scans show the results of ACPR testing with an LTE 10-MHz-wide, 64QAM signal at 1.8 GHz, for a model HXG-242+ MSiP amplifier on the right and "backed-off" GaAs MMIC amplifier on the left. In high-power amplifiers, ACPR performance is often improved by "off-device" linearization techniques such as digital or analog predistortion, feedback, or feedforward circuits. However, in small-signal amplifiers, the complexity and cost of these linearization circuits makes them impractical. The HXG amplifiers provide a much more affordable solution for increasing system ACPR performance in terms of size and cost.

In addition to the established linearization methods already mentioned, high-power linearized amplifiers will often make use of different techniques to improve efficiency, like Dohery combining techniques, envelope elimination and restoration (EER), and envelop tracking. In contrast, the HXG series amplifiers achieve linearity without sacrificing efficiency, as evidenced by their relatively low current consumption.

Both models draw 110 to 180 mA current from a single +5-VDC supply, with typical current draw of 146 mA per amplifier. The advantages of high-IP3 performance at low power levels are clear, considering the potential impact on reliability of increased heat dissipation within a small package space, component size, and overall cost. These benefits equip the HXG series amplifiers for applications in rugged outdoor environments, or wherever a small-signal amplifier or groups of components are being pushed to their thermal maxima.

6. A new parameter, IP3 efficiency (IP3e), demonstrates that the HXG amplifiers can deliver high linearity without sacrificing DC power consumption. The bar graph compares an HXG-242+ MSiP amplifier with various GaAs MMIC amplifiers.

The high IP3 performance of the HXG series amplifiers breaks with tradition, and may require a further break with tradition to properly characterize these amplifiers. For example, one additional means of evaluating the efficiency of the amplifiers could be through a parameter defined as the ratio of output IP3 to DC power consumed, or "IP3 efficiency." Using IP3 efficiency (IP3e) as a yardstick, a number of different commercially available, high-linearity GaAs MMIC amplifiers was compared with the HXG amplifiers (Fig. 6). The results clearly show the higher IP3e performance of the HXG amplifiers. This new parameter offers a fast means of comparing amplifiers for linearity while also evaluating the DC power efficiency.

7. This trace of 20 typical production units shows the repeatability of the HXG amplifiers.

Supporting this level of performance in the marketplace with integrity requires strong verification processes and calibrated, accurate test tools. The HXG amplifiers are 100% tested for IP3, ensuring that the advertised performance is the actual performance. Data compiled from a typical example of 20 production units (Fig. 7) shows that the performance of the HXG amplifiers is highly repeatable, and they are backed by an IP3 performance guarantee for every unit sold.

The HXG-122+ and HXG-242+ amplifiers are the first two units in the series. Recent customer feedback from the IMS2011 exhibition in Baltimore indicates that in addition to requirements served by these first two amplifier models, additional needs exist at higher frequencies. As a result, work is in progress to extend the amplifier module family to 5 GHz and beyond.

Each of the RoHS-compliant HXG amplifiers is supplied in a sealed ceramic package with input and output ports internally matched to 50 O. Each MSiP module measures 0.25 x 0.27 x 0.09 in. (6.4 x 6.4 x 2.4 mm), including gold-plated nickel terminations for good solderability. The HXG series amplifiers are designed for operating temperatures from -40 to +85C. The only external components required are DC blocks for input and output ports and a bias choke.

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