TRANSPONDER BACK SCATTER MODULATOR WITH REGULATED MODULATION DEPTH

An RFID transponder includes an antenna and modulation circuitry for back scatter modulation at an local voltage rail connected to the antenna such that a voltage of the antenna is maintained within a predetermined range. The modulation circuitry includes a voltage regulation loop including a rectifier connected between the antenna and the local voltage rail (Vlocal) for rectifying a voltage from the antenna so as to load the local voltage rail (Vlocal) with the rectified voltage from the antenna, an error amplifier (A1) for comparing a voltage at the local voltage rail (Vlocal) with a modulation voltage (Vmod) and producing an output signal (Vout), a switch for switching the modulation voltage (Vmod) between a first reference voltage level (Vref) and a second reference voltage level, wherein a regulated load is coupled between the output of the error amplifier (A1) and the antenna, which is varied in response to the output signal (Vout).