POSITIVE CLAMPER

V1 at +v eterminal decides the voltage for which the capacitor gets charged almost instantaneously when the Diode conducts Vc = Vref - Vin, for positive clamper this happens when Vin is negative and is effective only till Vref > -ve peak of Vin. The output resistor R3 decides the phase shift, rate of charge decay in the capacitor when diode is OFF. Although, the rate of voltage decay can be predominantly controlled by the capacitor value.