A UHF voltage multiplier circuit using a threshold-voltage cancellation technique

The operating range of passive UHF transponder systems is largely determined by the tag current consumption and the rectifier efficiency. Reading ranges of several meters have recently been reported for many state of the art RFID (Radio frequency IDentification) tags [1]. At this distance, the main issue for the rectifier design is the low amplitude of the high frequency antenna signal. Schottky diodes are often used for their low forward voltage drop and high switching speed. As an alternative to Schottky diodes, different circuit techniques for compensating the threshold voltage of standard transistor diodes have been utilized [4]. The transistor gates are biased near the threshold voltage, so that the devices effectively act as diodes with very low forward voltage drop. In the presented rectifier, a secondary diode charge pump is used to generate the DC bias for the threshold voltage compensation. The circuit is implemented in a standard CMOS technology and operates at a minimum available power of −11.3 dBm for an output DC power of 7.5 µW.