TABLE OF CONTENTS In Chapter 4 we considered the realization of second-order or biquadratic filter sections. These filter sections are often used as the building blocks for high-order filter realization. By "high-order" filter we mean a filter whose order is greater than or equal to 4. Since a third-order filter can be realized by simply adding a passive RC network to the active biquadratic section, such a network serves as an additional building block for odd-order filters.
As mentioned in Chapter 4, there are several ways one can realize a high-order filter; however, they all fall into one of the following five realization classes.
In the direct synthesis approach a high-order filter H( s) is realized using a particular configuration comprised of one or more active elements and passive- RC networks.
In the cascade realization approach, a high-order filter is realized by cascading two or more active biquadratic sections (e.g., those described in Chapter 4). This method is widely used in industry because cascade filters are relatively straightforward to design and tune.
In the inductance simulation approach, an inductance is simulated by an active- RC network such as a capacitor-loaded gyrator; thus, classical lossless passive-ladder filter realizations can be transformed directly into active-filter realizations. The transformed active filter retains many of the desirable properties of the passive realization.
In the generalized immittance approach, which closely parallels the inductance simulation method, classical RLC synthesis is followed with a frequency-dependent impedance (or admittance)...
