Reversible Operation of a CMOS Colpitts VCO as a W-band Passive Detector and Energy Harvesting Nir Weissman, Samuel Jameson and Eran Socher School of Electrical Engineering, Tel-Aviv University Tel-Aviv, Israel nirerism@post.tau.ac.il Abstract— An innovative topology for detecting and harvesting energy in the W-band has been realized in 90 nm CMOS process. The circuit uses a differential Colpitts topology in reverse mode to convert an input W-band signal into a DC voltage, current or power output. The circuit achieves a voltage responsivity of 2.1 kV/W at 95 GHz, current responsivity of 2.9 A/W at 99 GHz and NEP of 11.9 pW√Hz without DC power consumption. In power harvesting mode an efficiency of 5% is achieved for an 11.4 dBm at 99GHz input signal. The design occupies only 0.15 mm 2 including pads. Keywords—CMOS, W-band, Passive Detector, Harvester, Differential Colpitts topology, Responsivity, NEP, Efficiency. I. INTRODUCTION The millimeter-wave frequency range from 30-300 GHz has been an active area of research in the fields of high rate short-long range wireless communication, sensing and active/passive imaging. The low attenuation atmospheric windows centered at 35, 94, 140 and 220 GHz provide opportunities for longer range communication, imaging and power transmission. A key element in such systems is a detector that directly converts mm-wave power into DC or zero-IF signals. To date, most W-band passive detectors employ diodes and use either compound semiconductors or non-standard CMOS elements such as Schottky diodes. Integrated designs in CMOS and SiGe mostly use square-law active circuits with additional DC power consumption and inherent flicker noise (1/f) noise [2], [3], [4], [5]. Overcoming sensitivity issues require the use of a Dicke switch and adding another power and area consuming LNA. In this work, we propose using a standard CMOS based Colpitts VCO in reverse mode to achieve a simple, small, passive yet sensitive W-band detector and energy harvester [1]. Fig. 1 illustrates theses different modes of operation. II. CIRCUIT DESIGN A. Topology The circuit, shown schematically in Fig.2, was primarily designed as a wide tuning range and high output power W- band VCO [1]. The design is based on a buffer-less Fig. 1 Different modes of operation Colpitts architecture with a fixed high Q finger capacitor. In order to sustain oscillations with high enough Q the gate capacitor of the transistor composed of the gate-source and gate-drain capacitors plays here a varactor role changing its capacitance in function of the bias voltages and consequently the frequency of the LC tank [1]. Since no output buffer is used, the VCO output port could be used to inject W-band signals into the circuit transistor core. The transformer could thus be used for input matching in either detector or harvester modes. The basic principle of operation is shown in Fig.2. By preserving a differential behavior in the drain, gate and source of the transistors one side consumes current while the other side supplies current to the output at every cycle. Since it is an NMOS transistor the drain and source alternate at every half cycle. The drain and source voltages preserve the same phase while the gate voltage undergoes a 180 degree phase shift. Thus at every half cycle the supplying current is greater than the consuming current. B. Schematics Unlike traditional W-band power detectors that operate solely in one region and consume power this passive detector has no power consumption, and it operates both in the subthreshold and triode regions [3]-[5]. NMOS low power regular threshold voltage transistors were used, all with channel length of 90 nm. The transistors have 10 fingers of 2 um width each. 978-2-87487-031-6  2013 EuMA 7 -10 Oct 2013, Nuremberg, Germany Proceedings of the 43rd European Microwave Conference 830