Output of diode clamper circuit for a sinusoidal wave input
that until input waveform goes above zero at 2π and diode becomes forward biased again.


If the capacitor is charged with Vm the diode does not become forward biased again until almost (3/2)π, to replace the charge that has leaked off through the resistor. If the RC time constant is much longer than the period then that will be close to (3/2)π and the diode only conducts briefly at the positive peaks after initially charging.

Note that this is assuming an ideal diode - a real diode has a significant voltage drop relative to your 2V peak signal.