Design Of Current-Mode Fully Ballanced High-Order Lowpass Filters

Current-mode circuit is a front subject in microelectronics, circuits and systems because of its high speed, low power and simple structures. Current-mode filter applies widely in communication, electronic measurement, instrument & appearance and cybernation. The paper mainly studies the high-order filter's design method and circuit realizations based on current control fully balanced current conveyor II (CFBCCII). First of all, the paper introduces the development and character of the current-mode circuit, and compares current-mode circuit with voltage-mode circuit simply. Afterwards, it gives a short introduction in the history of filter, and summarizes high-order active filter's design methods. Finally, it proposes three kinds of high-order filters combining with the project supported by National Natural Science Foundation of China (No. 60676021). The main achievements consisit of following several aspects:(1) A current-mode high-order leapfrog filter which based on CFBCCII is presented. The filter has a fully balanced structure, that all of the inputs and outputs adopt differential structure which can restrain even order harmonic distortion and common mode disturbing signal. The filter adopting leapfrog structure has low sensitivity. It has the property of current controllability, and the cutoff frequency will be changed with the changes of the CFBCCII's bias current. The presented circuit contains only a few grounded capacitors and no resistance, so it is easily to be integrated. PSPICE simulation for this filter uses 0.35μm CMOS craftwork, and the result shows that the proposed circuit is valid and effective.(2) The paper presents a systematic method of designing differential nth-order filters with multiple-loop feedback. Different feedback constructs different low-pass filter by this method. This system has a fully balanced structure which can restrain even order harmonic distortion and common mode disturbing signal. The filters have no limitation of the orders. The produced nth-order filters are constructed by n CFBCCIIs and 2n grounded capacitors and have no resistance. Simulation result and the analysis of the property of current controllability show that the proposed system is correct. From the sensitivity curve of MATLAB, it can be seen that the system has low sensitivity.(3)A method of designing current-mode high-order elliptic filter based on CFBCCII is proposed. The realization of the proposed filter is using the signal flow graph to simulate the passive low-pass ladder network, and then using integrator and proportioner which based on CFBCCII to replace the module in the signal flow graph. The designed elliptic filter's cut-off frequency is controlled by the bias current, and it adopt the fully balanced structure which can restrain even order harmonic distortion and common mode disturbing signal. All the RC components are grounded. PSPICE simulation and analysis of the Avc show the elliptic filter is correct.