Ripple

Filters used with rectifiers allow the rectified direct current to pass through to the load without appreciable loss, but ripple in the rectified output is attenuated to the point where it is not objectionable. Filtering sometimes must be carried out to a high degree. From the microphone to the antenna of a high-power broadcast station, there may be a power amplification of 2·10¹⁵. The introduction of a ripple as great as 0.005 per cent of output voltage at the microphone would produce a noise in the received wave loud enough to spoil the transmitted program. A rectifier used at the low-power levels must be unusually well filtered to prevent noticeable hum from being transmitted.

Different types of rectifiers have differing output voltage waves, which affect the filter design to a large extent. Certain assumptions, generally permissible from the standpoint of the filter, will be made in order to simplify the discussion. These assumptions are:

1.  The alternating voltage to be rectified is a sine wave.

2.  The rectifying device passes current in one direction but prevents any current flow in the other direction.

3.  Transformer and rectifier voltage drops are negligibly small.

4.  Filter condenser and reactor losses are negligible.