Design of Two-Stage Operational Amplifier at 180nm Technology

Project Description

This project involves the design of a two-stage operational amplifier at 180nm technology using the GM over Id methodology. The amplifier is designed to meet the specifications: Gain > 60dB, load capacitance Cl = 20fF, gain bandwidth product (GBW) = 1GHz, and Phase margin > 50 degrees.

Table of Specifications

Specification	Value
Gain	> 60dB
Load Capacitance (Cl)	20fF
GBW	1GHz
Phase Margin	> 50°

Design Details

Model and Methodology

• Model File: CMOS180nm
• Design Methodology: gm over ID methodology (Emphasis on optimized W/L ratio and Width/Length multiples of 0.18nm)

Prerequisites

Before designing, the following steps were taken:

• Used Analog Designer Toolbox (ADT) for generating parameter charts using the CMOS180.txt model file.
• ADT allows plotting various parameter charts for NMOS and PMOS devices, considering different specifications such as length, VDS, VSB, etc.

Tools Used

• LTspice: Schematic and simulation purposes
• ADT (Analog Designer Toolbox): Generating lookup tables for CMOS 180nm and parameter charts for GM over Id methodology
• Electric Binary Software: Layout design

Calculations for the First Stage

• Gain = 40 , "CL" =20f , Rout = 93 KΩ (Assumed after Back Calculations)
• Let 𝐺𝑚= 𝐺𝑎𝑖𝑛/𝑅𝑜𝑢𝑡 = 0.43mS
• Taking 𝐺𝑚/𝐼𝑑=12, Id = 0.43𝑚𝑆/12 = 35.83uA
• For NMOS: (From Look Up Tables in ADT)
• For 𝐺𝑚/𝐼𝑑=12, 𝐺𝑚/𝐺𝑑𝑠=77.05, 𝐼𝑑/𝑊=9.913 , Vgs=0.7316
• Hence, Gds= 0.43𝑚𝑆/77.05 = 5.58uS W= 35.83𝑢/9.913 =3.6u and Vg =0.7316+0.45
• and Vg = 1.18
• For PMOS:
• As, Rout = Ro(NMOS) || Ro(PMOS) Gds(PMOS)= 1/𝑅𝑂𝑈𝑇 - Gds(NMOS)
• Hence , GDS(PMOS)= 5.172uS
• Now, as the current will remain the same i.e 35.83uA
• 𝐼𝑑/(𝐺𝑑𝑠(𝑃𝑀𝑂𝑆))= 35.83𝑢/5.17𝑢=6.93 using this From LUT in ADT(Analog Designer Toolbox), Gm/Id is calculated for PMOS
• Hence , 𝐺𝑚/𝐼𝑑=5.487 Gm (PMOS) = 5.487 * 35.83uA = 0.196mS
• For, 𝐺𝑚/𝐼𝑑=5.487 𝐼𝑑/𝑊=10.8 Vsg=0.781
• Hence, W= 35.83𝑢/10.8 = 3.317 (Taken 3.24u)
• And Vb = 1.8 – 0.781 = 1.019
• AS CL=20f, BW (-3dB) = 𝐺𝑚/(2𝜋𝑅𝑜𝑢𝑡 𝐶𝐿)=85.56MHz

Calculations for Next Stage (Common Source using PMOS driver and Current Source Load)

• Gain = 28, CL = 20f
• Various calculations for PMOS and NMOS.
• Gain = 28, "CL" =20f
• For PMOS:
• Taking 𝐺𝑚= 8 * 0.43mS = 3.44 mS
• As, Av=Gm*Rout Rout = Av/Gm=8.139K
• Vb(PMOS) = 1.0296 V Vsg=0.7704
• From ADT, For Vsg =0.7704 Gm/Id is Calculated 𝐺𝑚/𝐼𝑑=5.658 Id=607.8U
• Now, for the above Gm/Id 𝐺𝑚/𝐺𝑑𝑠=47.92, 𝐼𝑑/𝑊=10.51
• Hence , Gds = 71.7uS W = 57.84u
• For NMOS:
• As, Rout = Ro(NMOS) || Ro(PMOS) Gds(NMOS)= 1/𝑅𝑂𝑈𝑇 - Gds(PMOS)
• Hence , GDS(NMOS)= 48.5uS
• Now, as the current will remain the same i.e 607.98uA
• 𝐼𝑑/(𝐺𝑑𝑠(𝑁𝑀𝑂𝑆))=12.534 using this From LUT in ADT, Gm/Id is calculated for NMOS
• Hence, 𝐺𝑚/𝐼𝑑=5.61 Gm (PMOS) = 5.61 * 607.98uA = 3.41mS
• For, 𝐺𝑚/𝐼𝑑=5.61, 𝐼𝑑/𝑊=36.7, Vgs=0.7913
• Hence, W= 16.56u

Schematic Pictures

First Stage Schematic Second Stage Schematic Two-Stage Operational Amplifier Schematic

Simulation Pictures

Transfer Function Transient Analysis Frequency Response

Simulation Results

Layout using Electric binary

References

• "The Gm/Id Design Methodology Demystified" by Dr. Hesham Omran
• "CMOS Analog IC Design Lectures" by Dr. GS Javed
• "The Design of TWO STAGE Miller OpAmp: Final Verdict" by Dr. Hesham Omran
• NPTEL IIT Madras Course on OpAmp (Lecture 1, Lecture 2, Lecture 3)