Dual‐amplifier expansion and the design of very‐low‐frequency (VLF) hybrid integrated active filters

The problem of designing very‐low frequency (VLF) active filters in hybrid‐integrated form is discussed. It is shown that a class of single‐amplifier second‐order filters exists with which VLF active filters can be built while at the same time satisfying the constraints imposed by hybrid‐integrated circuit technology (i.e. limited resistor and capacitor values). However, with decreasing pole frequency, the gain required of the single amplifier becomes excessive, the gain‐sensitivity product (GSP) intolerably high. A solution to the problem is to introduce a second amplifier into the circuit and to distribute the required gain between the two amplifiers. This ‘dual‐amplifier expansion’ (DAX) significantly reduces the GSP, enhances the stability of the filter response, and reduces the susceptibility of the filter characteristics to nonideal amplifier effects. On the other hand DAX may not be arbitrarily introduced into a single‐amplifier circuit without taking the effect of the limited gain‐bandwidth product of the two amplifiers into account. It is shown that the degree by which the gain may be distributed between them is limited by the gain‐bandwidth product ratio of the two amplifiers. Much can be improved in this respect by proper frequency compensation of the two amplifiers. It is shown that a combination of the so‐called T‐compensation and the single‐pole single‐capacitor compensation produces results that are close to those obtainable with ideal amplifiers with unlimited bandwidth.