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In this paper, the protection of CMOS power amplifiers against load mismatch is addressed. To this purpose, a closed-loop protection circuit is proposed, which is based on a novel current-mode detection and comparison technique. The circuit allows a faster protection lock-in, by enabling peak detection and loop frequency compensation to be performed at the same circuit node, thus reducing the number of low-frequency poles and improving loop bandwidth. The effectiveness of the method is demonstrated through the implementation of a monolithic 0.25-mum 2-V CMOS power amplifier for GSM applications, which can deliver a 3-W output power with 55% overall PAE. The amplifier is able to sustain a 20:1 load VSWR at full power. Excellent RF performance and VSWR ruggedness are hence attained simultaneously, despite a simple common-emitter power stage is used. An experimental reliability assessment allowed the cognizant choice of the maximum drain-gate stress that could be tolerated. Device degradation was characterized by operating a power gain cell at RF, under real-world load and power conditions. Analysis of the degradation data enabled the design of an efficient, yet provably reliable, power amplifier.
Date of Publication: Sept. 2008