Portland State University. Department of Electrical and Computer Engineering
Date of Publication
Master of Science (M.S.) in Electrical and Computer Engineering
Electrical and Computer Engineering
Power amplifiers -- Design and construction, Gallium nitride, Silicon carbide, Microwave amplifiers
1 online resource (viii, 84 pages)
This manuscript describes the design, development, and implementation of a linear high efficiency power amplifier. The symmetrical Doherty power amplifier utilizes TriQuint's 2nd Generation Gallium Nitride (GaN) on Silicon Carbide (SiC) High Electron Mobility Transistor (HEMT) devices (T1G6001032-SM) for a specified design frequency of 3.6 GHz and saturated output power of 40 dBm. Advanced Design Systems (ADS) simulation software, in conjunction with Modelithic's active and passive device models, were used during the design process and will be evaluated against the final measured results. The use of these device models demonstrate a successful first-pass design, putting less dependence on classical load pull analysis, thereby decreasing the design-cycle time.
The Doherty power amplifier is a load modulated amplifier containing two individual amplifiers and a combiner network which provides an impedance inversion on the path between the two amplifiers. The carrier amplifier is biased for Class-AB operation and works as a conventional linear amplifier. The second amplifier is biased for Class-C operation, and acts as the peaking amplifier that turns on after a certain instantaneous power has been reached. When this power transition is met the carrier amplifier's drain voltage is already approaching saturation. If the input power is further increased, the peaking amplifier modulates the load seen by the carrier amplifier, such that the output power can increase while maintaining a constant drain voltage on the carrier amplifier.
The Doherty power amplifier can improve the efficiency of a power amplifier when the input power is backed-off, making this architecture particularly attractive for high peak-to-average ratio (PAR) environments. The design presented in this manuscript is tuned to achieve maximum linearity at the compromise of the 6dB back-off efficiency in order to maintain a carrier-to- intermodulation ratio greater than 30 dB under a two-tone intermodulation distortion test with 5 MHz tone spacing. Other key figures of merit (FOM) used to evaluate the performance of this design include the power added efficiency (PAE), transducer power gain, scattering parameters, and stability. The final design is tested with a 20 MHz LTE waveform without digital pre-distortion (DPD) to evaluate its linearity reported by its adjacent channel leakage ratio (ACLR).
The dielectric substrate selected for this design is 15 mil Taconic RF35A2 and was selected based on its low losses and performance at microwave frequencies. The dielectric substrate and printed circuit board (PCB) design were also modeled using ADS simulation software, to accurately predict the performance of the Doherty power amplifier. The PCB layout was designed so that it can be mounted to an existing 4" x 4" aluminum heat sink to dissipate the heat generated by the transistors while the part is being driven. The performance of the 3.6 GHz symmetrical Doherty power amplifier was measured in the lab and reported a maximum PAE of 55.1%, and a PAE of 48.5% with the input power backed-off by 6dB. These measured results closely match those reported by design simulations and demonstrate the models' effectiveness for creating a first-pass functional design.
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Baker, Bryant, "A 3.6 GHz Doherty Power Amplifier with a 40 dBm Saturated Output Power using GaN on SiC HEMT Devices" (2014). Dissertations and Theses. Paper 1781.