Editing Rectifier (section)

=== Higher order and cascade filters ===
To further reduce ripple, the initial filter element may be followed by additional alternating series and shunt filter components, or by a voltage regulator.  Series filter components may be resistors or chokes; shunt elements may be resistors or capacitors.  The filter may raise DC voltage as
well as reduce ripple.  Filters are often constructed from pairs of series/shunt components called RC (series resistor, shunt capacitor) or LC (series choke, shunt capacitor) sections.  Two common filter geometries are known as Pi (capacitor, choke, capacitor) and T (choke, capacitor, choke) filters.
Sometimes the series elements are resistors - because resistors are smaller and cheaper - when a lower DC output is desirable or permissible.  Another
kind of special filter geometry is a [[series resonant choke]] or tuned choke filter.  Unlike the other filter geometries which are low-pass filters, a resonant choke filter is a band-stop filter: it is a parallel combination of choke and capacitor which resonates at the frequency of the ripple voltage, presenting a very high impedance to the ripple.  It may be followed by a shunt capacitor to complete the filter.