Hi Sanghamitra Debroy ,

I feel you should provide more information:

Do the voltages carry information or just energy? (In the latter case, a (bridge) rectifier would do the trick.)

In the first case: Is the voltage interpreted as analog or as digital signal?

Which voltage range and bandwidth do you have in mind?

In the case of an analog signal: Is the generation of harmonics permitted?

A simple solution in the digital case or if some harmonics are tolerated doesn't even need a CMOS structure: Just connect a resistor to the positive supply rail, in series with a Zener diode (cathode at resistor); connect the input voltage to the anode, and pick up the voltage at the cathode. If VZ is larger than the lowest input voltage, the output is always positive.

The catch: The source of the input voltage must be able to sink the current through Z-diode and R.

Another simple solution which works perfectly, provided the source of the signal and the receiver of the signal must not share a certain potential: Just supply the receiver in such a way that the potential of its negative supply rail is always below the signal voltage.

I think use of rectifier gives only positive voltage

You can use a differential input single-ended output amplifier.