Hi,
Quote: I am designing a rectifier for passive tag applications. I don't have any voltage source/battery to provide gate voltage to switch ON NMOS
now your question is starting to become more clear.
Quote:So, I selected PMOS switch. Whatever dc voltage I get, it is the rectified dc voltage.
I don't understand why you can use PMOS, but not NMOS. Both require a DC drop from the gate to the source/drain.
Quote:The other topology is sucking some current and I do see some negative voltage at the switch output
The negative voltage is either caused by your rectifier, or the SPDT. It could be due to diode (body diode) turn on in the SPDT. Normally you need to bias the gate/body negative wrt the drain/source in order to properly turn off the switch. Otherwise a large input swing could turn on the switch when its supposed to be off. The same phenomenon could be causing the current draw.
Instead of biasing the gate/body at a negative voltage, some designs ground the gate/body, and pull up the source/drain. If you show your waveforms I might be able to provide better insight into whats happening.
Aaron