3.3 A transformer-coupled true-RMS power detector in 40nm CMOS

To optimize the power consumption and system performance of battery-supplied devices, it is required to monitor and adjust the transmitted RF power accurately and continuously. This is typically done by an external power detector (PD), which increases area and cost. On the other hand, fully integrated power detectors are typically voltage-based [1-5] and only give the correct RF output power for a fixed load impedance. But in practice, antenna impedance variations will occur, causing VSWR mismatches that introduce an error in these voltage-based RF output power measurements. This paper presents a 5GHz WLAN PA with an on-chip true-RMS Power Detector, without any additional power loss or area overhead. The power detector is based on a magnetically coupled sense winding and takes advantage of transformer-based power combining and impedance transformation that has become common practice in nanometer CMOS RF PAs. The proposed power detector performs both an RF voltage and RF current measurement at the PA output and is therefore capable of performing a True power measurement, even under VSWR mismatches or load variations. This proposed power detector is implemented in 40nm standard CMOS and unlike earlier reported power detectors [1-4], it is integrated together with a 5GHz RF PA targeting the WLAN (IEEE 802.11a) communication standard.