Design Of High Order Tunable Filter Employing Rail To Rail Operational Amplifier

The study of fully integrated continous-time filters based on R-MOSFET-C is one of the cutting-edge projects in the fields of microelectronics, circuits and systematology. Continous-time filters have been widely applied in communications, electronic surveying, instrumentation, auto control and so on, especially in the analog and digital mixed signal processing and integratd communication circuits. R-MOSFET-C filters have become an active subject in the field of modern filters because of their good dynamic range and electric control ability and their easy compatibility with CMOS VLSI technology.In this thesis, we mainly study the design principle and method of a fully differential rail to rail mult-stage high-order filter based on R-MOSFET-C and its actual circuits. Firstly, we introduce the study's significace of the project, the basic structure, working principle and model circuit of MOS transistors and some basic composing units of the integrated analog circuit comprised of MOS transistors:the MOS active resistance, the common-source and common-gate current mirror and others. Secondly, we propose a novel fully differential rail to rail mult-stage operational amplifier, and analyse and study the circuit structure, working principle and actual circuit of its input, middle and output stages. Using the operational amplifier, signals can be input to full amplitude and ouput with low harmonic distortion(THD). We also do simulations to examine and test the circuit with the software pspice and the simulation results show the circuit works correctly. We draw on knowledge of well-known RC filters to design the MOSFET-C filter because they are quite similar except that the resistances are replaced by the MOS transistors working in the triode region as active resistances. Finally, we propose adjustion with switching capacitors of the gate voltage of the MOS transistor to change the values of the active transistor resistance,by which the cutoff frequency of the MOSFET-C filter can be accurately achieved. And then we design a high-order low-pass Butterworth filter based on R-MOSFET-C using the operational amplifer and switching capacitor tuning circuit, and realize accurate adjustability of its frequency. The simulation results show the filter we design works correctly and effectively.