Can anybody tell me how the clamper circuits work? In a circuit with a resistor and capacitor in series(with ac source),i observed the output across capacitor on CRO.
At high frequencies, the voltage across capacitor was always positive, and the amplitude was decreased. How to explain this?

 

Sounds like a low pass filter. A clamp circuit (DC restorer) also has a diode. Are you sure it is just an RC circuit? If so, when measuring the voltage cross the capacitor, you will see an attenuation at higher frequencies because the impedance of the capacitor will decrease.

 

A clamper circuit clamps a signal to some potential via a low impedance path. A rectifier diode or a zener diode is often used for this purpose. The clamp potential can be ground, positive or negative.

By contrast a DC restorer adds a DC component to an AC signal using a process that sums the two components.