Questions about RFID Tag working at microwatts

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RFID Tag Doubt

It is said that RFID tag (passive ) works at microwatts.

The voltages anywhere are in micro or atmost millivolts ,but it actually comprises of a rectifier circuit (for power generation in passive tags) which would use a diode.

My doubt is diodes usually work above 0.7 V (si ) or 0.3 V (Ge) then how is it possible to use diode in passive tags at micro or millivolts ?

Are special diode used ? or With VLSI technology it is possible to make such low power diodes in CHIP itself ?

or any other reason ?

Thanks !
 

Re: RFID Tag Doubt

The diode numbers you give are for high power levels. Their I-V curves pass through 0 with curvature. This nonlinearity is what does the rectifying at low signal levels.

In the early days of broadcasting the audience used "crystal sets' which were just the antenna, a tuned circuit, a sort of diode made from tungsten wire pressed on lead sulfide, and a crystal headphone. There was enough power rectified by the crude diode to power the headphones for stations miles away.
 

RFID Tag Doubt

(1) Use higher Zo.

(2) Use diode and capacitor to form a Voltage doubler for further DC power recovery(or even higher order).
 

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