Design of High Performance Low Noise Amplifier

Abstract

The abundance of wireless communication standards has saturated the RF spectrum. The wireless channels are closely spaced in the latest generations of mobile communication. The decreasing gap between wireless channels demands the higher linearity of the receiver circuits. The linearity of the receiver greatly depends on the linearity of the low noise amplifier (LNA). It is mandatory for an LNA to display excellent linearity properties. LNA being the first block of the receiver decides the overall noise performance of the system. The wireless systems working with high frequency are susceptible to the environmental noise and are not in a position to afford the additional noise produced by the receiver system. The LNA must offer the least possible noise from itself to satisfy the overall noise requirements of the receiver. Apart from high linearity and low noise requirements, an LNA is expected to provide excellent reverse isolation properties to prevent the local oscillator radiations. LNA is also responsible for the sensitivity of the receiver systems, which demands an appropriate amount of gain from the LNA. The latest battery operated mobile phones, equipped with RF receiver, imposes the stringent low power requirements on the receiver systems. The thesis presents a high performance LNA architecture to cater the requirements of the modern day RF receivers. The thesis focuses on the design of an LNA that provides higher second and third order linearity. The detailed literature survey of various linearity improvement techniques is carried out and presented in the thesis. The mathematical analysis of different linearity improvement techniques is also described in the thesis. The thesis highlights the issues involved with each of the linearity improvement technique presented. The source inductive degenerated, resistive load single stage CS LNA is chosen for the improvement in linearity. The linearity improvement using derivative superposition (DS) and complementary derivative superposition (CDS) are applied on the CS LNA to improve the third order nonlinearity. The simultaneous improvement in second and third order linearity is very challenging. Enhanced derivative superposition (EDS) method for the simultaneous improvement in second and third order linearity improvement is proposed. The simulations of DS, CDS and EDS are carried out in the Cadence Virtuoso environment using 180 nm GDPK. Significant improvement in second and third order linearity is observed using EDS as compared to DS and CDS methods. An inductive load effectively filters out the second order intermodulation terms and improves the second order linearity of the LNA. To incorporate the inductor in the load, the inductive load LNA is implemented and simulated. The simulation of the inductive load LNA is carried out and an unmatched improvement in second order linearity is observed. The derivative superposition is applied to the inductive load LNA for the improvement of third order linearity. The IIP3 of almost 10 dBm is achieved using inductive load CS LNA. The inductive load LNA does not fit for low power applications. Moreover, CS LNA fails to display appropriate isolation and input/output reflection properties and the DS, CDS and EDS methods are not suitable for low noise requirements. An inductive load CS LNA based on cascode configuration is designed to accomplish the isolation and reflection properties. The input matching is achieved using inductive source degeneration. The inductive load cascode LNA offers superior isolation and reflection properties. Further, it is found that it is capable of providing ample gain at much less power requirement as compared to single stage CS LNA. Use of inductive load helps the LNA to achieve IIP2 in order of 100 dBm. The IIP3 achieved in case of cascode LNA is -13.34 dBm. A third order linearity improvement is required for the cascode LNA. The post distortion (PD) method for the improvement of third order linearity is applied to the cascode LNA. The simulation results obtained are satisfactory in all the aspects. The lacuna is observed in the range of stability of IIP3 with variations in the input voltage. The value of IIP3 remains more than 0 dBm for almost 70 mV variations in the input voltage. The range of input voltage variation is one of the foremost requirements of a high performance LNA. The post layout simulations for resistive load DS, EDS and inductive load PD applied LNA are carried out and compared. A new linearity improvement technique is proposed in the thesis for the simultaneous improvement of second and third order linearity of the LNA. The mathematical analysis for linearity improvement for the proposed LNA is carried out using the Voltera series and the cancellation of the third order nonlinearity term is proved. The simulations for gain, noise figure and s – parameters are performed and the results are compared with the implemented DS, CDS, EDS and PD applied LNA. The comparison of the proposed LNA with the state of art reported LNA is also presented in the thesis. The proposed LNA is designed in the Cadence Virtuoso environment with 180 nm GPDK. The post layout simulations for the proposed LNA are carried out. The proposed LNA offers IIP3 as high as 6.47 dBm, whereas 108 dBm value of IIP2 is achieved. The noise figure of 6 dB is offered by the proposed LNA. The proposed LNA displays excellent isolation properties. The values of S11, S22 and S12 are obtained as -13.84 dB, -3.9 dB and -25.7 dB, respectively. The proposed LNA provides a gain of 17.4 dB and consumes almost 18 mW of power from the supply of 1.8 V. Above all, the value of IIP3 is achieved more than 0 dBm for almost 200 mV variations in the input voltage. The Monte Carlo analysis, for process and mismatch variations, in IIP3, IIP2 and gain are performed for 1000 samples each. The proposed LNA is found quite stable over the process and mismatch variations. The comparison of the proposed LNA with the state of art reported LNA is carried out by defining Figure of Merit (FoM). The proposed LNA offers value of FoM, comparable to that of the other reported LNAs. The proposed LNA displays optimum performance for all the parameters of interest for terms as a true high performance LNA.