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A comprehensive analysis of a wideband MOSFET amplitude detector for built-in-self-test is presented under multiple operating conditions. A simplified RF-to-DC conversion equation for detection in the saturation region is derived. Two conversion modes are uncovered in the subthreshold operation region. The first mode, which applies to relatively large input signal levels, has linear RF-to-DC proportionality. The second subthreshold mode, for small signal levels, has square-law proportionality. The body effect was investigated in the saturation region and subthreshold's two modes. Detection in the triode is also discussed. A bandwidth equation accounting for the detector's loading effects on the circuit-under-test is introduced.