A clamper circuit consists of a time-varying input signal, a capacitor, and a diode. The output voltage is measured across the diode.

During the negative half-cycle of the square wave input signal, the diode is forward-biased, resulting in zero voltage across the diode.

But at the same time, the capacitor charges to the peak value of the input signal.

During the positive half-cycle of the square wave, the diode becomes reverse-biased. Here, the input voltage and the charged capacitor act as a combined voltage source.

At this instant, the voltage across the diode is twice that of the input voltage. This circuit acts as a positive clamper.

When a load is connected across the diode, the output current flows from the load through the capacitor. This reduces the output voltage exponentially according to the time constant RC. 

As the input voltage decreases, the output voltage follows and charges the capacitor. As the input voltage increases again, the capacitor discharges and the cycle repeats