In any rectifier, the switches (diode/FET/BJT etc.) that are used to rectify the incoming ac have a small forward voltage when conducting or ON, usually around 0.2 to 1 V. If I want to rectify a low voltage AC signal(say from ambient RF/microwave) so that I can get a small DC voltage to use for powering some device, the incoming AC will not be able to surpass the forward voltage. How do I overcome this?

Please note that I want to use the low voltage signal for power and not as a signal for information/control, so something like Op-amps to amplify the incoming AC defeats the purpose as it requires biasing power.

 

usually around 0.2 to 1 V.

"usually" is the key word. there are exceptions.

 

What you need is an ultra-low voltage energy harvester such as LTC3108.

 

Thank you MrChips......great circuit..that's a keeper.

 

That circuit requires DC at the input so you still need a rectifier.
The lowest forward drop diode to rectify low-level, RF/microwave signals is a back (tunnel) diode.