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Broadband Design of a High Efficiency 200-W GaN HEMT Doherty Amplifier

Parent Category: 2020 HFE

Introduction

LTE and 5G base stations require power amplifiers (PAs) that operate over increasingly wide frequency ranges, while maintaining high linearity and power-added efficiency (PAE) to provide multi-band and multi-standard operation. These communication systems employ transmitting signals characterized by high peak-to-average power ratio (PAPR), which require amplifiers to operate at lower input power levels ranging from 6-dB back-off and less in order to minimize nonlinearities that lead to spectral re-growth and increased error vector magnitude (EVM).

However, operating at these lower power levels also reduces amplifier efficiency. Through load modulation and impedance inversion, Doherty PA architectures improve efficiency at lower power levels, enabling PAs to operate in their linear region. Doherty PAs, however, are often band limited due to the frequency-dependence of the impedance-matching/inversion networks and phase-delay lines between the peak and carrier (main) branches. A multi-band Doherty amplifier can be achieved when all of its parts are carefully designed using broadband matching techniques and careful circuit simulation of the individual amplifier components.

Impedance Matching/Inverters Drive Broadband Response

For instance, the output network can be composed of two quarter-wave impedance inverters with reduced impedance transformation ratios to improve the broadband performance of a conventional Doherty amplifier. The load network generally comprises a low-pass lumped or transmission-line structure with two or three matching sections. Therefore, it is advantageous for matching circuits to be partly implemented inside the device package for devices with an average output power of 40 W and higher, given the very low device impedance of these high-power devices. Providing a higher impedance at the package terminal will make it easier for the designer to achieve an acceptable match across the required frequency range.

An equivalent circuit of the device with input-matching elements inside the package (including the lead frame) and the small-signal frequency response is shown in Figure 1.

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Figure 1 •  Equivalent circuit of the packaged GaN device (a) and S11 frequency response (b).

The 50-V device, fabricated by Sumitomo Electric Device Innovations, has six basic 15-W gallium nitride (GaN) high-electron mobility transistor (HEMT) cells connected in parallel and capable of providing a combined saturated output power of more than 80 W across the entire band from 1.8 to 2.7 GHz. Outside the packaged device, the three-section microstrip transformer, designed using the Microwave Office circuit simulator within the AWR Design Environment software platform (now part of Cadence Design Systems, Inc.), is implemented using an alumina substrate with a high permittivity of 250 and a thickness of 0.16 mm, yielding a compact structure transforming the device input impedance to 10 ohms, with an S11 less than −25 dB.

The carrier amplifier and frequency response are shown in Figure 2 as a simplified schematic consisting of a single-ended, 80-W GaN HEMT PA operating in Class AB mode with external input and output matching circuits, which operates from 1.7 to 2.7 GHz. The classic two-stage Doherty amplifier has limited bandwidth in the low-power region, since it is necessary to provide a quarter-wave impedance transformation from 25 to 100 ohms when the peaking amplifier is turned off. Given the bandwidth limitations of the conventional structure, the Doherty effect is not strong across the full band.

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Figure 2 • Class AB (carrier) PA with broadband conjugate matching (a) and measured performance (b).

Design for Packaged Device Behavior

The inverted Doherty architecture shown in Figure 3 can be helpful in achieving broadband results if, in the low-power region and depending upon the characteristics of the transistor, it is easier to provide a short circuit rather than an open circuit at the output of the peaking amplifier. In this case, a line is used to transform a very low output impedance after the offset line to a high impedance seen from the load junction. Taking into account the device package parasitics of the peaking amplifier, an optimized output-matching circuit and a proper offset line can be designed to maximize the output power from the peaking device in the high-power region and approximate a short circuit in the low-power region.

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Figure 3 • Broadband inverted Doherty PA.

The large-signal power gain and drain efficiency of a tri-band inverted Doherty amplifier (Vgc = −2.5 V, Vgp = −5.5 V and VDD = 50 V) was simulated using the APLAC harmonic balance (HB) engine in Microwave Office software. The simulation resulted in an output power greater than 53 dBm and a linear power gain greater than 10 dB across the entire 1.8 to 2.7 GHz range. Drain efficiencies greater than 50% at saturation and 7 dB backoff were simulated at the center of the three bands—1.85, 2.15, and 2.65 GHz—with maximum drain efficiency greater than 70 % at the lower frequencies and peak efficiency at maximum backoff output power of around 6 dB over the entire frequency range (Figure 4a).

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Figure 4 • Tri-band inverted Doherty PA with simulated drain efficiency and power gain at three different bands (a) and measured drain efficiency and power gain at five different bands (b).

The design approach and simulation results of the tri-band inverted Doherty amplifier were validated with measured results on a test board. A power gain greater than 9 dB was achieved from 1.8 to 2.7 GHz and drain efficiencies greater than 55% at saturation (P3dB) and around 50% at 7 dB backoff were measured across the entire band, with the maximum drain efficiency greater than 70% at the frequencies below 1.95 GHz and peak efficiency points at maximum backoff power around 6 dB over the entire frequency range (Figure 4b).

Conclusion

This multi-band Doherty amplifier design was achieved using broadband matching techniques and circuit simulation of the individual amplifier components. Careful attention to the frequency response of the in-package and on-board transformation networks, as well as application of the most appropriate Doherty architecture for the available transistor/package characteristics, enabled the design to achieve its excellent broadband performance.

Acknowledgements

Thank you to James Wong, RF Specialist at Sumitomo Electric Device Innovations, for his inspirational work https://www.awr.com/customer-stories/sumitomo-electric-industries and support associated with this article.

May 2020

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