RFMD’s GaN Portfolio Expands to Include High-Linearity, Broadband Power Amplifiers
By Jason Layton, Design Engineer, RFMD®

As OEMs continue the effort to incorporate commercial, off-the-shelf parts into systems provided to commercial and defense markets, the unparalleled performance of high band-gap RF solutions have accelerated as a premier option. Rapid adoption of GaN RF amplifiers due to high performance, ease of integration, and application flexibility continues to drive development in the GaN power amplifier market. Initial GaN products focused on very high peak power and high efficiency, while next generation developments moved forward to target specific linear applications. In multi-channel communication systems where the PA linearity may be a primary figure of merit, RFMD’s RFHA3942 and RFHA3944 high-linearity, unmatched power transistors offer an optimized balance of linearity, drain efficiency, peak power and cost. These transistors were designed using RFMD’s second generation, advanced Gallium Nitride fabrication process (GaN2) as an alternative to the high efficiency transistors, RF3931 and RF3932. Combined with other linearization techniques, such as digital predistortion and feed-forward techniques, RFMD’s family of GaN2 devices can meet even the most stringent of spectral distortion requirements.

Fundamentally, radio frequency power amplifiers display non-linear characteristics when the input power level drives the amplifier into or even past gain compression. While efficiency may be the primary figure of merit for the PA used in a single-channel application, linearity may be the primary figure of merit for the PA used in a multi-channel application. A power amplifier that offers good linearity helps the designer deal with the inconsistent properties of the distortion components. Factors such as thermal effects, channel bandwidth, AM to PM effects, and waveform or envelope requirements will define the performance limits of the system.

Digital transmission standards require a more linear performance from amplifiers in the transmitters to ensure effective transmission of the digital signal. Additionally, these standards require more peak power to maintain the required bit error rates and modulation error ratios. Regardless of the external linearization techniques employed, such as digital predistortion, the fundamental amplifier linearity lies within the power amplifier’s performance characteristics. The PA must minimize both AM-AM conversions (modulation of output signal amplitude by input signal amplitude) and AM-PM conversions (modulation of output signal phase by input signal amplitude). RFMD’s high linearity GaN2 process offers the required solutions to the high-power, high-efficiency and high-linearity requirements faced by designers.

The RFHA3942 is a 48V, unmatched 30W, high power discrete amplifier tunable from DC to 4GHz. Based on RFMD’s GaN2 technology, this device offers an excellent balance of high efficiency, flat gain and power, and excellent linearity. See Figure 1 and Figure 2.

The RFHA3944 also functions with a drain voltage of 48V, but delivers 60W of power and is tunable DC to 4GHz. See Figure 3 and Figure 4.

The RFHA3942 is offered as a high linearity option for the RF3931 and the RFHA3944 is offered as a high linearity option for the RF3932.

The parts are packaged in a hermetic flanged ceramic package that provides the best balance between performance and thermal management. The package also enables ease of integration to allow the optimization of the impedance match for specific applications. The package supports the tuning required to maintain the loading conditions, which affect the linearity of the complete transmitter. The RF360-2 package and specific impedance matches allow the maximum available bandwidth and power.

Specifically, the high gain, high peak power and improved linearity allows OEMs working with UHF SATCOM systems access to a wider range of modulation and multiplexing schemes at higher frequencies. RFMD’s GaN2 process allows the power amplifiers to work with modulation schemes other than FSK and CDMA, which typically maintain a relatively constant amplitude modulation. In military communication systems that are intended for mobile users, such as the Mobile User Objective System (MUOS), the RFHA3942 and RFHA3944 can enable military UHF SATCOM radio systems to utilize the commercial cellular standards that require the envelope and phase components of the digital signal to be accurately combined within the transmitter to produce the correct digital spectrum. To illustrate the advantages offered by the high linearity GaN2 process, Figure 5 shows the adjacent channel power (ACP) of the RF3932 power transistor with a WCDMA (TM1, 7.5dB at 0.01% CCFD) waveform versus the RFHA3944 manufactured using the GaN2 process.

The measurement was made at a 5MHz offset, using an integration bandwidth of 3.84MHz on an application circuit tuned for 2140MHz. The application circuit was designed using a low dielectric, high frequency laminate for the PCB. The components used for the input and output matches were designed in a distributed element simulation using die models constructed in Verilog-A. The die models are combined with package parasitics to offer customers a packaged model for use in their PA designs. Application circuits for other bandwidths within the S-Band are available through RFMD.

To further demonstrate the capability of the GaN2 process, Figure 6 illustrates the ACP of the RF3932 versus the RFHA3944 driven with an OFDM (64QAM, 9.8dB PAR at 0.01% CCFD) waveform. The measurement was again taken on an application circuit tuned for 2140MHz, but at a 10MHz offset using an integration bandwidth of 10MHz. The ability of the high linearity power amplifiers to support the multiple user modulation techniques at higher frequencies adds yet another degree of freedom for the system designers to exploit. In concert with the high peak power, flat gain performance and ease of integration, the RFHA3942 and the RFHA3944 are cost effective options for designers working to extend the performance of their systems.

As command, control, communications, and computer systems continue to account for significant portions of capital expenditures, RFMD will continue to expand its product portfolios. RFMD will continue to develop its GaN processes and product lines to offer the most technologically advanced and cost effective options on the market. RFMD’s product roadmaps and competitive pricing will ensure its GaN products will keep pace with demand for more data at less cost.

RFMD®
www.rfmd.com
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