Filters are often called for in circuits that are to be implemented in integrated circuit (IC) form. In some cases, it has been necessary to implement such circuits with discrete inductors and capacitors located off the integrated circuit. This arrangement requires expensive hybrid circuit techniques and often additional interfacing circuits on the integrated circuit itself. For active filters that include an operational amplifier located either on the IC or external to it, a further drawback is the limited bandwidth of the operational amplifier.
In some circuits, a number of filters may be included that are selectively switched into the signal path upon command. An additional problem with switchable filters has been the difference in their low-frequency gain. As one filter replaces another in the signal path, the change in gain can change the output signal amplitude within the pass band.
USPTO Backgrounds
USPTO Backgrounds