Harmonic tuned GaN-based power amplifiers with enhanced efficiency, bandwidth and thermal management

Abstract

High efficiency broadband power amplifiers (PAs) are increasingly in demand for the novel wireless communication standards to meet the high spectral efficiency and integration requirements in front-end modules (FEMs) and massive multiple input multiple output (MIMO) active antenna systems. Modern PAs are therefore developed to address extended operational bandwidth requirement under increased peak data rates but still suffer from inefficiencies resulting from complicated matching networks and large power dissipation in the active devices. These can degrade the power efficiency and linearity performance and increase the heat density, particularly for high power PAs (HPAs), which in turn affects the overall performance and reliability of transmitters. To address these challenges, a two-pronged approach is taken in this thesis: (i) novel techniques to increase power efficiency and reduce the thermal load over wide bandwidths are proposed and (ii) efficient thermal management techniques are investigated. First, to enhance power efficiency while increasing bandwidth novel harmonic impedance tuning at the device input are developed for two distinct classes of operation: continuous-mode inverse class GF (CCGF−1) and the continuous class GF (CCGF). For the CCGF-1, we propose new closed-form drain current expression to model the PA current waveforms in the time domain. We then use this analytical expression to exploit second source harmonic impedance manipulation in order to expand the design space of the output matching circuit resistively. This approach allows to diminish the complexity of the design of the load matching network at the fundamental and harmonic frequencies and to achieve wider bandwidth while simultaneously improving drain efficiency across the new optimum admittance points. As a proof of concept, a wideband CCGF−1 PA is designed, fabricated and tested. Results show a drain efficiency of more than 70% from 3.05 GHz to 3.85GHz, a gain between 11 and 12.4 dB with a gain flatness of ± 0.7 dB and an output power at 3-dB gain compression between 39.9 and 41.4 dBm over the same frequency band. For the CCGF, the second source harmonic in CCGF is similarly optimized to flatten the power amplifier’s frequency response over a wideband range. Moreover, a new design space is explored by considering the effects of controlling the input nonlinearity of the gate-source capacitance (Cgs) on the drain current waveforms under continuous mode drain voltage waveforms. Unlike the CCGF-1 mode where current overshoot can occur, the obtained drain current waveforms in the CCGF mode do not exceed the maximum drain current. Since the current overshoot can negatively affect the reliability of the device over time while simultaneously increasing the overlap with the voltage waveform in continuous mode, avoiding it by going to the CCGF mode will alleviate these problems. A second proof of concept prototype of a CCGF PA is also designed, fabricated and tested. Measurements show a flat frequency response from 3.3 GHz to 4.3 GHz with variations less than ± 0.4 dB for 40 dBm output power, and ± 2% for 66% drain efficiency. The efficient thermal management of HPAs is undertaken through the design, fabrication, integration and testing of a 40 W C-band amplifier. Two alternative approaches are investigated: (i) the integration of an existing single MMIC (Monolithic Microwave Integrated Circuit) chip PA design, where matching and power combining are carried out on-chip, on a heat-sink housing and (ii) the hybrid PA design where lower power MMIC power-bars are matched and combined off-chip using low-loss multi-layer LTCC (Low Temperature Cofired Ceramics) technology and mounted in an optimized thermal management structure. It is shown that by using highly thermally conductive interposers with properly matched thermal expansion coefficient, namely copper-graphite, along with cavities in the LTCC and a copper base, the hybrid approach yields significant improvements in thermal management. Indeed, thermal images from the chip surface show 10-15 °C better thermal management of the hybrid HPA compared to MMIC HPA at the same saturated output power level.

Date	10 Dec 2021
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Ammar B. Kouki (Supervisor)