The Study Of Current Mode Filters Based On Current Mirrors

Differing with the Voltage Mode circuit, current is selected as the main parameter, and determines the function in the Current Mode circuits. This will enormously optimize the circuit in speed, bandwidth, voltage swing and power comsumption. Current mode technology has been deeply developed and the circuits have been well integrated by the latest developments of the technic of integration. Continuous-time filters have several advantages, such as high processing speed, simple circuit configuration, and low power consumption. They have been widely used in the realm of communication, electronic measurement, instruments, autocontrol, etc., and especially used in digital and analog mixed signal processing and integrated circuits.Current mode continuous-time filters combine the advantages of both current mode circuits and continuous-time filters, so they can be well integrated. Typical current-mode continuous-time filters include current-mode filters based on current conveyors, current-mode filters based on OTA-C and current-mode filters based on current mirrors. They are rapidly developed in bandwidth, speed and precision etc., and they bring the filter theories to noble times of fully integrated system development.Above all circuit elements in current mode continuous-time filter, current mirror is the simplest and the most flexible one, and can be easily formed as integrator, differentiator, and adder for filter design.Filters based on current mirrors, which are the rigorous current mode filters and use only 3V supply, are made up of MOS current mirror and capacitor so that to suit CMOS digital circuit.This thesis studies current mode continuous-time filter based on basic current mirrors and cascode current mirrors. By analysing the basic theories and characteristics of both mirrors, we can easily form adders by them. Then studies lossy intergrator, lossless integrator and lossless differentiator based on basic current mirrors, and thus advance a lossy integrator and a lossless integrator based on cascode current mirrors. After simulation and checking up the correctness of the elements, several kinds of filters have been designed by them, including filters based on double lossless intergrators, filters based on a lossy intergrator and a lossless intergrator, filters based on the former structure and a lossless differentiator,filters based on double lossless differentiators, a fourth-order butterworth lowpass filter based on integrators and a fourth-order butterworth highpass filter based on differentiators. Simulations demonstrate the correctness, and the sensitivities have been enduced.The design methods of this thesis include the signal flow graph method, the configuration graph method and the pattern LC ladder filter.The main works and contributions of this thesis are as following:Firstly introduce the classification of filter. Then the developments of current mode in the past twenty years, as well as the developments of analog integrated filter and the research on current mirrors are described.The DC model, AC model and the Pspice model for simulation of MOS field-effect transistor are introduced. Then basic theories of filter and its various classifications, including the classification of order which is important in the following chapter. Present the ideal filter and three familiar method of approximating which was simulated by MATLAB. And discuss several methods of designing current mode continuous-time filter.Then study on basic mirrors and cascode mirrors and analyse their character. Analyse the circuits of basic current mirrors after introduce their basic concept, and discuss their character. Discuss cascode current mirrors and enduce their advantage of high output impedance. Then simulate expectively on both mirrors and figure their character of amplitude-frequency response, phase-frequency response, input impedance and output impedance.Then the thesis foucuses particularly on current mode integrator and differentiator based on current mirrors. Analyse one-port lossy and lossless integrator, and get their transfer function through small signal model. After simulation, make a briefness comparison between them. Then discuss differential lossy integrator and lossless integrator made up of one-port lossy intergrator and lossless differentiator based on basic current mirrors. By analysing their transfer function and simulation, proper results are gained.A lossy intergrator and a lossless intergrator based on cascode current mirrors are advanced, and are used to form filters based on double lossless integrators, and filters based on a lossy intergrator and a lossless intergrator. The high output impedance is the main advantage of cascode structure, which gains more ideal current following characterristic. This works in filter designing that the output current verses on the input current with the same energy consumption. A lossydifferentiator is advanced in the following context by the signal flow graph method, and is used in designing of fourth-order highpass butterworth filter in the next chapter.The most important part of the thesis is to form current mode continuous-time filter based on the integrators and differentiators listed above by different kinds of design methods. After analysing the transfer function of the second-order filter, use the signal flow graph method and the configuration graph method to design filters expectively based on double integrators, double differentiators, fully differential integrators, and double intergrators with a differentiator. At last form high-order filters with pattern LC ladder filter method on the analysis of LC ladder structure. Simulations show resonable and calculations get the result of low sensitivity.Another filter is advanced by adding a lossless differentiator to the filter structure of a lossy integrator and a lossless intergrator. Since the transfer function of the lossless differentiator is simple asαs,so other kinds of filters can be designed by adding a lossless differentiator on lowpass filter and bandpass filter. Although these filters have some shorcomings of small current transfer ratio and limted bandwith because of its goodish characteristics, the method enrichs the design methods of continuous time filter.The thesis has also pay some attention to low voltage current mirror and except to introduce the character of low voltage into the design of current mode continuous-time filter.