All Pass Filter

Problem:
What is an All Pass Filter?

Solution:
An all pass filter is a filter whose amplitude response is flat (within the bandwidth limitation of the amplifier), and which has a 90 degree phase shift at the "corner" frequency:



All pass filters are most often used for matching phase in systems where phase is important, for producing delays in circuits that need to delay a signal, and for creating 90^o shifts for quadrature modulators.