A CMOS OTA for HF Filters with Programmable Transfer Function

EVERAL CMOS OTAs suited for the design of S filters have been reported [ll-[5]. The poles and zeros of these filters are defined by the unity-gain frequencies f U , = G , / ( 2 d L ) of OTA-C integrators (G, is the transconductance of the OTA and C, is its load capacitance). Usually G, is tunable over a sufficient range to compensate for processand temperature-dependent errors. However, the tunability of G, can also be used to implement programmable filters [6], [71, which will be the scope of this paper. It is well known that filter properties are extremely sensitive to integrator phase errors at f u g [l], [8]. Since these phase errors generally depend on processing and temperature variations, a phase correction scheme (“Q control”) is needed [2] . If the integrators in a filter are identical to or are scaled versions of a “master integrator,” one control loop can control the phase of all integrators via a “master-slave’’ concept. However, since the phase error generally also varies when G, is tuned, only simultaneous tuning of f of all integrators is then possible (e.g., [3]: a low-pass filter with programmable cutoff frequency). The OTA presented in this paper has a built-in phase compensation, which allows tuning of G, while maintaining a small phase error. In a filter built with such OTAs, independent programming of f u g of the individual inte-