TABLE OF CONTENTS As we have seen in Section 6.2, the most common types of filter response required are lowpass and bandpass, but occasionally a demand arises for other types; highpass, band-reject and allpass, together with equaliser functions. The components available for filter design depend on the IC technology used; for current designs, pure CMOS is by far the most popular technology, and further discussion will focus on CMOS circuit techniques. Bipolar technologies have been extensively used also, and offer some performance benefits in that bipolar devices have greater transconductance and lower noise for a given bias current. From the designer's viewpoint, BiCMOS would appear to offer the best choice of implementations. All these technologies offer resistors and capacitors with good performance, however inductors are only useable for applications in the gigahertz range. To design lower frequency filters with complex poles and zeros in their response without inductance requires the use of active circuits with gain. The other necessary requirement for integrated filter designs is tunability, which requires at least one type of component to be made variable. This can be achieved by using a component with a value dependent on bias voltage or current. An alternative to electrically tuneable circuit elements is the use of switched arrays, in which component variation in discrete steps is achieved. This is a technique well suited to digital control; it is also useful for extending the tuning range.
At the system level, the required response of a filter may be...
