AWR Application Notes

AWR Software for the Design of a High-Efficiency Broadband GaN HEMT Doherty Amplifier for Cellular Transmitters 2 www.cadence.com/go/awr Packaged Device Multi-band Doherty amplifier capability can be achieved when all of its components are designed to provide their corre- sponding characteristics over the required bandwidth of operation. In this case, the carrier and peaking amplifiers should provide broadband high-efficiency performance when, for example, their input matching circuits are designed as broadband and the load network generally can represent a low-pass lumped or transmission-line structure with two or three matching sections. Therefore, it was very important for matching circuits to be partly implemented inside the device package to achieve an average output power of 40W and higher, especially for input matching circuit in view of very low device impedance across the required frequency bandwidth. Figure 1 shows the equivalent circuit of the device inside the package with input matching elements and the small-signal S 11 parameters at the input of the internal input matching circuit including package lead-frame. Here, the Sumitomo 50V device represents six basic 15W GaN HEMT cells connected in parallel and capable of providing more than 80W of saturated output power across the entire frequency bandwidth of 1.8-2.7GHz. The three-section microstrip transformer was implemented using a 0.16mm thick alumina substrate with high permittivity of 250 to achieve a compact structure. Figure 1: Equivalent circuit of packaged devices and its input return loss performance(dB(|S11|) Through this structure, the impedance of the (bare transistor die) gate terminal was transformed to ~10ohm at the reference plane of the package input. When referenced to an environment with a 10ohm characteristic impedance, a return loss better than 25dB was achieved across the band of interests. Broadband Performance Generally, the multi-band impedance transformer required for broadband operation can represent a configuration with N (N ≥ 2) cascade-connected transmission lines with different characteristic impedances. 6 As an example, in order to match the output impedance of 25ohms with the load impedance of 50ohms, the broadband output transformer can be realized using a two-section microstrip line, where the characteristic impedance of the first quarter-wave section is equal to 30ohms and the characteristic impedance of the second quarter-wave section is set to 42ohms. In this case, the magnitude variations of ±0.5ohms and phase variations of ±1° of the input impedance was achieved across the frequency range from 2.0 to 2.8GHz covering simultaneously 2.1GHz (2.11-2.17GHz) and 2.6GHz (2.62-2.69GHz) wideband code-division multiple-access (WCDMA)/ long-term evolution (LTE) bands. 7 At the same time, the magnitude variations of ±1.0ohms and phase variations of ±2 degrees were achieved with a 1GHz bandwidth from 1.9 to 2.9GHz, which meant that reducing the mid-band frequency to 2.3GHz resulted in a simultaneous tri-band operation with inclusion of an additional 1.8GHz (1805-1880MHz) digital cellular system (DCS)/WCDMA/LTE bandwidth.

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