Monolithic fully balanced OTA based high frequency filters

Abstract

This research involved the development of a design approach and the necessary circuit elements for high-frequency continuous-time monolithic filters to be implemented in a standard CMOS process. The investigation began by comparing the commonly used monolithic gain blocks and their topologies to infer which are best suited for high frequency filtering applications. The transconductance amplifier topology is shown to be superior to traditional operational (voltage) amplifier topologies in bandwidth and excess phase. The open loop (transconductance amplifier based) integrator is also shown to provide better characteristics than the closed loop (Miller) integrator. The reported advantages of fully balanced structures led to choice of a fully balanced OTA based filter topology, which is derived from the familiar (op amp based) Tow-Thomas biquad. To improve the linear input range of the transconductance amplifier, a voltage source terminated input stage is utilized. The OTA design is limited in several aspects (CMRR, PSRR, linearity) by device matching. The effects of mismatches in the input pair, current mirror gains, input signals and load capacitors is examined extensively. The average OTA shows measured output current nonlinearity of.88% over a 4V[subscript PP] input range. The OTA transconductance is adjusted with a novel adjustable CMOS voltage source for which a design and experimental results are presented. Filters have been fabricated and tested in the 300-400 kHz range. All filters tested were post-fabrication adjustable to nominal specifications (f[subscript o] = 375 kHz, Q = 3.25). The 1% THD input amplitude was over 1.75 V[subscript PP], and the corresponding dynamic range was approximately 70 dB. The low frequency PSRR was -60 dB, and the critical frequency PSRR was -40 to -50 dB. Since the OTA linearity is achieved through device matching, the linearity is not directly frequency limited, and shows potential for filter applications above 1 MHz. Next generation filters will use better layout techniques to improve the distortion performance.