Analysis And Application Of MOSFET Mismatch

Identically designed MOS transistor in a chip often display variations in device characteristics,such as the threshold voltage and drain current,due to nonuniformity and random factors in processing,such variations are generally categorized as MOS transistor mismatch.As the developing of CMOS technologies,the minimum features of MOS transistor have came in to the sub-micron range, the effect of MOS transistor mismatch becomes more prominent in circuit design,which reduces the performance of analog circuit. In the HDTV tuner chip,in order to improve the performance of the frequency synthesizer,the charge pump need to be designed with perfect current matching,so it's needed to choose a mismatch model and utilize the corresponding technology mismatch parameters to evaluate the mismatch performance of the charge pump.In this paper,the causes of current mismatch of MOS transistors and the mismatch parameter extraction are firstly introduced.Secondly,some analysis of the existing mismatch models are given and the Pelgrom mismatch model are adopted in the charge pump design.Then,the design of high performance charge pump used in the frequency synthesizer are characterized,including the fundamental of charge pump,design of low voltage cascade CMOS charge pump,design of bias circuit,unit gain buffer and test circuit design.The mismatch model can't directly used in the conventional electrical circuit simulators to evaluate the mismatch performance,so a subcircuit call is specially designed,as long as substitute each transistor with the subcircuit call in the netlist,the current mismatch performance can be predicted by carry out the Monte carlo analysis using the Hspice simulator,based on the proposed method of simulating the mismatch performance, simulation of the charge pump are executed to evaluate it's current mismatch.In the end, the circuit design and layout design of the charge pump are finished based on the Chartered 0.25 standard CMOS technology,and the measurement are carried out based on the proposed measurement method.The measured result indicate that the current mismatch of the charge pump is less than one percent which can satisfy the system spec(2%), considering the error induced by the Pelgrom mismatch model,the measured current mismatch and simulated data are in good agreement.