Research And Design For High Linearity OP-AMP Applicated In Analog Radio Frequency Receiver

With the development of wireless technology and the popularization of personal wireless terminals,more and more radio communication protocol standards are used.Multi-mode,multi-standard radio communication systems will be the development tendency of wireless terminal communication system.Because of the approach of these communication protocol frequency spectrums,the useful signal will be disturbed by large signal at adjacent frequency bands.In order to make sure that the useful signal could be transferred correctly to the ADC input,the linearity of receiver will have a higher requirement.Baseband filter is the key module of the receiver and the op-amp of the filter is the main source of the nonlinearity.The purpose of this paper is to design a high linearity op-amp applied in filter in TSMC 0.18 um.Firstly,this paper introduces and analyzes some amplifier structures,and indicates the characteristic and applications of different structures.And then,we study the nonlinearity of the MOS transistor,the differential input pairs,the operational amplifier,and the op-amp applied in close loop and cascade system.In next part,we study the influence between op-amp and the stability of filter.We design an op-amp model with ideal component,and research the relationship between the phase margin of the op-amp and the stability of filter,with the help of the op-amp model and the pole positions of the system transfer function.We do a research about the minimum phase margin of the op-amp in biquad low-pass filter and I/Q couple band-pass filter in Cadence software,and verify the availability of this method which is a criterion for the stability of filter.In the design of the three-stage feedforward op-amp,we infer the mathematical model of the three-stage feedforward op-amp and the nonlinearity output current of the op-amp applied in close loop system.This nonlinearity current equation is utilized to guide the detail parameters design of the op-amp.On account of the improvement of the op-amp's linearity,we design an auxiliary linearity-improved circuit which could generate inverse nonlinearity current to cancel the distortion from main op-amp.We design a constant-gm bias circuit to supply the bias current of the op-amp.And we also design an auxiliary start-up circuit to solve the probable problem that op-amp applied in close loop system runs into deadlock state.At last,the three-stage feedforward op-amp with an auxiliary linearity-improved circuit shows that DC gain is over 72 dB,phase margin is over 49.7°,power consumption is 3.83 mW,GBW is 436 MHz.We put the op-amp into a 4th order active-RC low-pass filter with 10 MHz bandwidth,the simulation result of the in-band IIP3 of the filter in a twotone test input signal at 4.5MHz and 4.6MHz is 50.2dBm.