A CMOS Linear-in-dB Variable Gain Amplifier For The RF Front-end Circuits Of Wireless Sensor Network

Wireless sensor network (WSN) is a self-organized network which comprises multitudinous wireless sensors nodes disposed randomly and densely in the monitored area. It can sense, collect and process the sensed information, finally, sent it to the observer collaboratively. WSN is considered as one of the most influential technology in the 21st century. It can be widely used in military security, traffic surveillance, medical treatment, environmental monitoring, etc.With the development of the system on chip (SOC) and RF-CMOS front-end design technologies, the wireless sensor node integrated in a single chip becomes availability gradually. This dissertation focuses on the research on WSN transceiver RF front-end based on RF-CMOS process, aiming to provide a key circuit for WSN SOC chips.The variable gain amplifier embedded in an automatic gain control loop is an essential component of WSN RF front-end circuits.With the feedback loop, it constitutes the automatic gain control circuit for ADC to provide constant power signal.A digital-controlled variable gain amplifier (VGA) is designed for the RF front-end circuits of large scale wireless sensor network. Five serial stages are employed in the VGA and each stage is composed of a programmable attenuator followed by a fixed-gain amplifier that has the function of DC-offset rejection. Variable gain is achieved via two steps. The 6dB gain steps are implemented using the R-2R ladder network and the 0.75dB gain step is implemented by a series ladder linear attenuator. Simulation results show that the VGA provides a 78dB gain control range with 0.75dB step and the overall gain accuracy was 0.7mdB. The equal input noise at the maximum gain is 34.1μV and the OIP3 is 16.1dBm. The VGA is fabricated in SMIC 0.18μm 1P6M CMOS process. The VGA consumes 2.16mV at a single 1.8V supply, and occupies 0.08 mm2 chip area. In brief, the VGA and RF front-end work well and their performance meet the specifications, and the design could be applied in the WSN system completely.