Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
The DC level in an AC signal is just a DC level added to an AC signal, typically a sine wave.
Clamping circuits very often use a diode to clamp the positive or negative, or both, excursions of an AC signal to a particular voltage level.
A capacitor and a diode can be used to to clamp the positive or negative excursion of an AC signal to a particular voltage. This will add a DC offset to the signal, as shown in the schematic below.
In the schematic below the top circuit is a positive clamp and the bottom circuit is a negative clamp.
Assume a 10V peak to peak sine wave input with no DC offset, so the input sine wave positive peak is 5V and the negative peak is -5V. Also assume that the clamp voltage is 0V in each circuit. Assume that the diodes are perfect and have no forward voltage drop:
(1) In the positive clamp circuit the diode will stop the output sine wave from ever going more positive than 0V. Thus the output sine wave positive peak will be 0V and the negative peak will be -10V.
(2) In the negative clamp circuit the diode will stop the output sine wave from ever going more negative than 0V. Thus the output sine wave negative peak will be 0V and the positive peak will be 10V.
On the other hand, if you passed an AC signal with a DC offset (positive or negative) through a capacitor, the AC signal would appear at the other side of the capacitor without a DC offset.