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