Analysis of output power variation under mismatched load in Power Amplifier FEM with directional coupler

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.