RFID Tag Doubt

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truebs

Joined: 21 Jan 2005
Posts: 322
Helped: 19
Location: Asia

31 Mar 2005 18:24 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 !

flatulent

Joined: 19 Jul 2002
Posts: 4877
Helped: 324
Location: Middle Earth

31 Mar 2005 19:00 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.