A Family of Active Switched Capacitor Biquad Building Blocks

Active RC building blocks which realize a biquadratic transfer function1" have played an increasingly important role in supplying filters for the Bell System. Thus, at the present time, S T A R building blocks enjoy a very large volume of production and there is a constant effort directed toward the reduction of their costs.14 Perhaps the most promising recent development in filtering is the emergence of active switched capacitor filters. ' These filters are fully MOS realizable and therefore enjoy the cost advantages of LSI integrated circuit realizations. Further, they can be expected to share in the future cost reductions due to V L S I . Since biquadratic building blocks have played such a dominant role in the active RC field, they are also expected to be of great importance 2235 for active switched capacitor (sc) filter realizations. Many of these basic biquadratic blocks can then be realized on a single chip to implement higher order filter functions. The purpose of this paper is to describe a pair of similar active sc biquadratic filter topologies9 capable of realizing any stable biquadratic 2-domain transfer function. In arriving at these networks, we have been particularly careful to eliminate the effects of parasitic capacitances. Such capacitances arise mainly from two sources. First is a sizable (approximately 10 percent) parasitic capacitance from the bottom plates of the capacitors to the epitaxial layer (ac ground). By using two operational amplifiers, it is possible to arrange matters so that the parasitic capacitors are connected either to a voltage source (operational amplifier output) or ground, thereby eliminating any effects due to them.