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  MID SEMESTER REPORT

ON

MODIFIED CMOS OPERARATIONAL AMPLIFIER WORKING ON LOW VOLTAGE

                                                             

BY

GOKUL RAMASAMY 2016H123012U

UNDER THE ABLE GUIDANCE OF

DR. Vilas H Gaidhane

BITS PILANI, DUBAI CAMPUS

Dubai International Academic City, Dubai

UAE

(JANUARY 2017– MAY 2017)

 MID SEMESTER REPORT

ON

MODIFIED CMOS OPERARATIONAL AMPLIFIER WORKING ON LOW VOLTAGE

                                                             

BY

GOKUL RAMASAMY                                2016H123012U

UNDER THE ABLE GUIDANCE OF

DR. Vilas H Gaidhane

BITS PILANI, DUBAI CAMPUS

Dubai International Academic City, Dubai

UAE

(JANUARY 2017– MAY 2017)

ACKNOWLEDGEMENTS

I would like to thank Prof. Dr. Vilas H Gaidhane, for suggesting me this topic and who has given us an opportunity to apply and understand our engineering concepts in a practical atmosphere.

GOKUL RAMASAMY

2016H123012U

CONTENTS

Abstract…………………………………………………………………   III       

Acknowledgement……………………………………...……………….   IV

                                                       

Chapter 1    INTRODUCTION………………………………………………...……...1 1.1 Objective............................................................................................... 1    1.2 Overview............................................................................................... 1

Chapter 2    

DESIGN AND CONSTRUCTION…..………………………………….3

    2.1 CONVENTIONAL CMOS…………………..………………….....3

   2.2 THE BULK-DRIVEN MOSFET…………………………………...5

2.3 DIFFERENTIAL AMPLIFIER……………………………………....8

Chapter 3

PROPOSED WORK……………….……..……………………………..11

Chapter 4

INFERENCE…………………………….……………………................11

Reference……………………………………………………………….12

CHAPTER 1-INTRODUCTION

1.1 Objectives

• To study the CMOS the technology and its characteristic.

• To study the characteristics of bulk-driven CMOS technology.

• To compare bulk-driven and conventional Mosfet.

• To observe the characteristics of bulk-driven Differential amplifier pair.

• To study the working of Cherry-Hopper Operational amplifier and then to observe the difference between Cherry-Hooper and the conventional operational amplifier.

• To modify the Cherry-Hooper Operational amplifier and to make it work on low voltage.

• To simulate the modified operational amplifier using Multisim.

• To analyze the simulated results.

1.2 Overview

CMOS stands for Complementary Metal Oxide Semiconductor. Complimentary refers to how they produce either a positive or negative charge. Because CMOS based transistors acts as an inverter. It converts 1’s to 0’s and 0’s to 1’s, they run efficiently, using up very little power. This is because the charges can stay in one state for a long period of time, allowing the transistor to use little or no power except when needed. Because of their efficiency, processors that use CMOS-based transistors can run at extremely high speeds without getting too hot. The low power consumption of CMOS[1],[2] allows the memory to be powered by a simple Lithium battery for many years.

When it comes to CMOS technology VLSI design automatically comes into picture. Very-large-scale integration (VLSI) is the process of creating an integrated circuit (IC) by combining thousands of transistors into a single chip. According to Moore’s law the number of transistor used in integrated circuit doubles approximately every two years. But this law was surpassed a decade back and the number of transistors that can be place on a chip is increased exponentially. VLSI helps to reduce the size, power consumption[4], gate delays and to increase speed of complex operations, switching time.

The main focus is on developing analog circuit techniques that are compatible with future CMOS technologies. These circuit techniques which permit low voltage operation with large thresholds offer the potential for more thoroughly utilizing the technology at any voltage range even if low threshold voltage technologies become standard. Analog building block circuits such as differential amplifiers and current mirrors which achieve 1-V operation will be described in detail. Some of the blocks will then be used to design and implement a 1-V CMOS[6] rail-to-rail input/output op amp that has been fabricated in standard 2um CMOS technology having threshold voltages in the range of ± 0.8V

Using this MOSFET an operational amplifier  is designed. This differential amplifier is made in such a way that it operates in a very low voltage of 1 volt. To achieve lower threshold voltage the technique used is bulk-driven Mosfet. The improved characteristics is shown in differential amplifier that uses bulk driven differential pair (BDDP).

CHAPTER 2-DESIGN AND CONSTRUCTION

2.1 CONVENTIONAL CMOS:

    CMOS semiconductors use both NMOS and PMOS circuits. Since only one of the circuit types is on at any given time, CMOS chips require less power than chips using just one type of transistor [3].

All CMOS gates are arranged in two parts: the pull-up network (PUN), built from p-type transistors and connect to source; and the pull-down network (PDN), built from n-type transistors and connected to ground (also called drain). The two parts are logical duals of each other, so that if the PUN is active, then the PDN is inactive, and vice-versa. In this way there can never be a direct path between source and ground (in any steady state).

Fig 2.1.1:cross section of CMOS.

Fig 2.1.2: CMOS TRANSISTOR

Fig 2.1.3: LAYOUT AND STICK DIAGRAM OF CMOS

2.2 THE BULK-DRIVEN MOSFET:

The efficient way to lower the threshold voltage is by using the bulk-driven technique[5]. The drain is connected normally and the signal is applied between the bulk and the source. The current flowing from the source to drain is modulated by the reverse bias on the bulk–channel junction. This is a junction Field-effect transistor with the bulk as the signal input (gate).the result is high input impedance depletion device.

Fig 2.2.1:bulk-driven Mosfet.

Drain characteristics of conventional Mosfet in both linear and saturation region is given as

Now the drain characteristics of the bulk-driven mosfet is given as ,

Comparing theoretically  to the existing Mosfet ,bulk-driven mosfet turns on at very low threshold which is because of the short between bulk and source. The nominal threshold voltage of a MOSFET is 0.7V but the turn-on(threshold) of the Bulk-driven Mosfet is as low as -3V but when observed practically the difference is nearly ±0.8V.

Fig 2.2.2: Difference in characteristics of bulk-driven and gate-driven Mosfet.

The bulk-driven MOSFET transconductance can exceed the gate-driven MOSFET transconductance if

The advantage is the depletion characteristic which allows zero, negative, and even small positive values of bias voltage to achieve the desired dc currents. This will lead to larger input common-mode ranges that could not otherwise be achieved at low power supply voltages. The bulk-driven MOSFET is the use of the poly gate to modulate the bulk-driven MOSFET. Because the gate can totally shutoff the channel, the on/off ratio of the bulk-driven MOSFET modulated by the gate is very large. Furthermore, throughout extensive experimental investigation of bulk-driving the MOSFET, latch-up has not appeared to be a problem.

2.3 DIFFERENTIAL AMPLIFIER:

The bulk-driven differential pair (BDDP) is designed with two nMOS with  their gates of both devices are tied to so that an inversion layer channel is formed within each MOSFET. Because the source-coupled MOSFET’s have isolated individual wells, a differential voltage signal is applied between the bulk terminals of M1 and M2

Fig:2.3.1(BDDP-BULK-DRIVEN DIFFERETIAL PAIR).

Using VCM=0 V (Common mode voltage) value of transconductance of each tail current case as the nominal value, the bulk-driven differential pair’s transconductance at VCM=VSS is 16.3% below the nominal value for the 40 A case and 16.5% below nominal for the 50- A case. The BDDP’s transconductance is 28% above nominal for the 40- A case, and 30% above nominal for the 50- A case at VCM=VDD

Fig;2.3.2

Measured common-mode voltage influence on bulk-driven differential pair transconductance

for the nMOS pair (p-well CMOS technology), the threshold voltage reduces as the VCM approaches VDD , allowing the source-coupled node voltage to  rise

Fig:2.3.3

The basic property of differential amplifier is to amplify the voltage difference of the two Mosfets.Since the bulk-driven technique is used here gradual decrease in turn on time is observed.

CHAPTER 3- PROPOSED WORK

• Simluation of differential amplifier using opamp in HSPICE

• Observing the difference in I-V characteristics between gate-driven and bulk-dirven MOSFETS

• Analysising the effect of capacitance in both kinds of the MOSFETS

• Switching time of both the transistor  types are to be tabulated

CHAPTER 4- INFERENCE

The bulk-driving technique removes the MOSFET’s threshold voltage or turn-on requirement from the signal path and a device with depletion characteristics is obtained. Consequently the bulk-driven MOSFET provides a practical solution to enhancing input common-mode range

 REFERENCE

[1] A. P. Chandrakasan, S. Sheng, and R. W. Brodersen, “Low-power CMOS digital design,” IEEE J. Solid-State Circuits, vol. 27, pp. 473–484, Apr. 1992.

[2] M. Nagata, “Limitations, innovations, and challenges of circuits and devices into a half micrometer and beyond,” IEEE J. Solid-State Circuits, vol. 27, pp. 465–472, Apr. 1992.

[3]Context referred from Wikipedia-https://en.wikibooks.org/wiki/Electronics/CMOS

[4] C. Hu, “Future CMOS scaling and reliability,” Proc. IEEE, vol. 81, pp. 682–652, May 1993.

[5] P. E. Allen, B. J. Blalock, and G. A. Rincon, “A 1 V CMOS op amp using bulk-driven MOSFET’s,” in Proc. 1995 ISSCC, Feb. 1995, pp. 192–193.

[6]  B. J. Blalock, “A 1-volt CMOS wide dynamic range operational amplifier,” Ph.D. dissertation, School Elect. Comput. Eng., Georgia Inst. Technol., Atlanta, GA, 1996

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