a diode clamp

A clamp (or DC restorer) shifts an entire waveform up or down so that one of its edges sits at a chosen level — without changing its shape. If a signal swings +/-5 V around zero and you want it to swing between 0 and +10 V instead, a clamp slides the whole thing up. Think of lifting a wave's floor to a fixed height while the wave keeps its full size and wiggle.

It is built from a capacitor in series with the signal and a diode to a reference. On the first swing that would push the output past the reference, the diode conducts and charges the capacitor to make up the difference; after that the capacitor holds that DC offset like a battery, and the signal rides on top of it. So a sine that used to be centred on zero now sits with its negative peak pinned at (reference minus ~0.7 V), shifted bodily upward.

Clamps appear wherever you must re-establish a DC level that capacitive coupling threw away — old video sync, level-shifting a signal into an ADC's input range, voltage-multiplier stages. The catch is that the clamp needs the signal to keep swinging to refresh the capacitor's charge; a slow load or leakage lets the clamped level drift, so the time constant must be long compared with the signal period.