Analysis of Output Power Variation under Mismatched Load in Power Amplifier FEM with Directional Coupler

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  • Author: Xiaofang Mu, Ziv Alon, Gary Zhang and Shiaw Chang

Directional couplers are used for output power detection and control in both WCDMA and GSM/EDGE Power Amplifier (PA) Front End Modules (FEM). At high values of load VSWR, delivered power shows significant variations with the phase. Back-of-envelope calculations are used to estimate output power variations. The objective of this analysis is to define a unified platform for understanding the power control mechanism in PA FEM with directional coupler, explain known-best practices and provide insight into minimizing output power variation. It illustrates how a complex load at isolation port of directional coupler is used to compensate certain non-ideal factors in PA FEM. A practical approach to minimize delivered power variations systematically was implemented and verified in a PA FEM design.