Electronic Circuits and Projects Forum

What frequency and waveform is it?

You could try using schottky diodes, and if you half-wave rectify you will only have the loss of one diode, although that wastes the other half of the cycle.

http://www.onsemi.com/pub_link/Collateral/BAT54CWT1-D.PDF shows a loss of about 0.15 V at 300 μA, but that is much more if it gets cold.

Other than that, use a transformer or more turns on your generator to increase the voltage.

I think that a synchronous rectifier could improve the efficiency, but you have so little power to start with that the design would have to be very clever to improve on schottky diodes. There is not enough voltage to turn on a MOSFET, which makes things more difficult.

You may want to start giving some thought to Precision Rectifier Circuits. A Google will bring up plenty of additional circuits along the lines of the link.

Ron

Precision rectifiers need powered op-amps to work. I was under the impression from the OP that the 0.8 Vac was the only power source.

If he is trying to accurately measure the 0.8 Vac, and has other power available, then the precision rectifier is a good way to go.