AWR Application Notes

APPLICATION NOTE AWR Software for the Design of a High- Efficiency Broadband GaN HEMT Doherty Amplifier for Cellular Transmitters Next-generation 4G/5G telecommunication systems require power amplifiers (PAs) to operate with high efficiency over a wide frequency range to provide multi-band and multi-standard concurrent operation. In these systems with increased bandwidth and high data rates, the transmitting signal is characterized by high peak-to-average power ratio (PAPR) due to wide and rapid variations of the instantaneous transmitting power. Therefore, it is important to provide high efficiency at maximum output power and at lower power levels typically ranging from 6dB backoff and less over a wide frequency bandwidth. Design Overview This application note describes the design of an innovative Doherty amplifier architecture using 200W high-efficiency broadband 1.8-2.7GHz gallium arsenide (GaN) high-electron mobility transistor (HEMT) technology, which achieved average efficiencies of 50-60 percent for output powers up to 100W and significantly reduced the cost, size, and power consumption of the transmitters. The designers used Cadence ® AWR ® Microwave Office ® circuit design software within the Cadence AWR Design Environment ® platform for the development of this amplifier. Previously published work in this area includes a conventional Doherty amplifier with a quarter-wave impedance trans- former and a quarter-wave output combiner. The measured power-added efficiency (PAE) of 31 percent at backoff power levels of 6-7dB from the saturated output power of about 43dBm has been achieved across the frequency range of 1.5-2.14GHz. 1 To improve the broadband performance of a conventional Doherty amplifier, an output network can be composed of two quarter-wave impedance inverters with reduced impedance transformation ratios. 2 For broadband combining, an output quarter-wave transmission line with fixed-characteristic impedance can be replaced by a multi- section transmission line consisting of different characteristic impedances and electrical lengths in order to cover the frequency range from 2.2-2.96GHz. 3 In this case, the broadband matching was realized by applying the simplified real-frequency technique with the desired frequency-dependent optimum impedances. However, nonlinear optimization of the entire Doherty amplifier system made the design more complicated in terms of circuit simulation and results in a sufficiently large size of the final board imple- mentation. Another example includes a PA design with high-peak power of 350W that was achieved across the lower frequency band of 760-960MHz using a modified combining scheme with two quarter-wave lines in the peaking path. 4 Using an asymmetric Doherty architecture, the saturated power of more than 270W and linear gain of more than 13dB with a drain efficiency of more than 45 percent at 8dB backoff points was achieved across the frequency range of 2.5-2.7GHz. 5

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