Class-C PA-VCO Cell for FSK and GFSK Transmitters

In this paper, a Class-C PA-VCO cell tailored to FSK/GFSK transmitters is presented. In the proposed solution, a Class-C VCO and a common-gate stage PA are stacked in a current-reuse architecture operating with 1.2 V power supply. The PA and the VCO efficiencies are maximized by adjusting their voltage headroom without the use of any DC-DC converters. The PA-VCO is inserted in a transmitter based on an open-loop architecture. The presented prototype, fabricated in 0.13 μm CMOS technology, occupies an active area of 0.2 mm2. A maximum TX efficiency of 17.5% is achieved while the TX is delivering an output power of -1 dBm at 2.45 GHz. A phase noise of -129 dBc/Hz at 2.5 MHz frequency offset results in a carrier-frequency drift below 7 Hz/s and an FSK error below 0.7%, which allows the transmitter to operate in open-loop while delivering long data-packets. The transmitter is also compliant to BLE specifications when FSK and GFSK modulations with index of 0.5 are applied.