Signal Clamper Using Diode
Diode and capacitor can be used to clamp an AC signal, shift the level into positive region for all cycle. Sometimes this condition is needed, to have positive state at all time, for example in some analog-to-digital conversion.
At first of its operation, when the signal is in a direction diode forward, the capacitor is charged until the peak level of the signal. The output of this circuit is almost zero, things like being shorted by the Zener diode. When the signal is reversing until its maximum, now the AC signal source and the capacitor voltage is summed, producing signal with doubled peak.