Stability of highpass and bandreject analog continuous-time OTA-C filters

In this paper we address the high frequency stability problem of highpass and bandreject OTA-C filters simulating doubly loaded passive ladder networks. These filters usually have floating nodes where the OTA excess-phase acting over the nodal parasitic capacitance can introduce unstable poles at high frequencies. Fortunately, the filters discussed here can be conveniently modeled at high frequencies in order to clearly explain how the OTA excess-phase acts over the floating nodes. Based on the understanding of this mechanism, a very simple modeling approach allows us to propose an appropriate and very simple stabilization procedure. The stability problem is fairly the same for highpass and bandreject OTA-C filters based on approximations with and without finite zeros. A novel technique for OTA-C filter with finite zeros realizations is used. Only grounded capacitors are needed; all floating capacitors are substituted using active simulation, eliminating bottom plate parasitic capacitor effects and non-observable poles