Design of a Low Voltage CMOS Transconductance Amplifier

This project aims to illustrate the design and simulation of a Low Voltage Complementary Metal Oxide Semi Conductor (CMOS) Transconductance Amplifier (OTA) with an improved power consumption, Direct Current (DC) gain and bandwidth.

Two techniques were introduced for linearization: Pseudo Differential Pairs and Source degeneration under nano-scale technology.

The nonlinearity, which is caused by the short length effect due to the small size of transistor can be reduced using linearization techniques, Two techniques were introduced for linearization: Pseudo Differential Pairs and Source degeneration, were both go under nano-scale CMOS technology.

The proposed OTA requires a proper control system, where the common-mode control system has been designed for system stability .

The results of this implementation are: power consumption of (x), DC gain of (x) , a size of (x) µ, number of transistors (x) . All the work was simulated using the Advances Design System ( ADS ) , under 130nm CMOS Technology.

1.1 Motivation

The huge pace in technology and the growing demand of electronic devices makes the Integrated Circuit ( IC ) designers think twice about low power and low voltage with the trade of the Threshold Voltage ( VT ) which doesn’t scale down when compared to the power supply.

Throughout the years, designing analog integrated circuits has been challenging, where the power supply is being reduced due to the demand of technology which requires to scale down the total power.

The Operational Transconductance Amplifier (OTA) is recognized to be one of the most important basic building blocks in analog ,mixed mode circuits, filters, including multipliers, voltage control oscillators, and Very Large Scale Integrations ( VLSI ) applications, where the VLSI technology is the process of creating IC’s by combining several transistors into a single chip. In such applications the OTA is the key circuit to such design, OTA at the input of the block determines the efficiency of the overall system, hence improving the performance of the OTA block is critical for enhancing the overall module performance.

Operational Transconductance Amplifier

An OTA main purpose is to convert its input voltage to the desired output current; i.e. in other words an OTA is a voltage controlled current source, where Gm is the Transconductance with a unit (Ampere/Volt).

In real life circuits, harmonics are introduced, and nonlinearity should be taken into consideration which caused by the short channel effect of the transistors, the expression of the output current with Taylor series expansion can be as follow :

where ai is determined by the implementation of the circuit.

In order to achieve a proper OTA with the specifications mentioned, linear transformation factor should be implemented in the design to reduce harmonics, linearization methods have been developed through the years to solve the problem.

1.2 Objectives

The main goal is to design a low voltage Transconductance CMOS amplifier which converts its input voltage to the desired output current with high linearity, which can be achieved by linearization techniques, Pseudo Differential Pair and Source degeneration techniques.

1.3 Realistic Constraints

The listed below are the main constraints that should be taken into consideration for the design :

1.3.1. Economic Constraints:

The student shall use the available simulation tools such as : Advanced Design System for simulation design, and Synopsys for layout .

As for the design, the main goal is to reduce harmonic distortion, achieve high linearity, and to be able to transform the input voltage to the required output current with minimum size. Since the size of the transistors determine the size of IC , and the size contributes in the cost of the IC.

The designed IC is being manufactured in fabrication laboratories. During the fabrication process, thousands of ICs are being etched onto a single blank wafer. After the testing process ,only percentages of the IC’s are considered useable, and being distributed among electronic stores.

1.3.2. Manufacturability and Sustainability Constraints:

The designed circuit shall be worked across process and temperature corners for improved yield.

1.3.3. Ethical and Safety Constraints:

Documentation should not have more than 30% similarity on Turnitin.

1.3.4. Standardization

All technologies used in this project are 130 nm CMOS technology.

1.4 Design Requirements

The design shall meet the following requirements:

1. The design uses CMOS based technology.
2. The total power consumption will be less than 15mW.
3. The supply voltage will be less than 1 volt.
4. Bandwidth should be larger than 50MHz
5. DC gain should be larger than 20dB

1.5 Design Achieved

The goal of this project was achieved by designing a low voltage CMOS transconductance amplifier using linearization techniques with high linearity, low power consumption of (x), DC gain of (x) , a cost of (x).

1.6 Task Distribution

Work was done as a team; but some work was distributed to ensure every member has their own task. This is shown in the table below:

Table 1.1: division of labor

1.7 Organization

The rest of the documentation illustrates the design characteristics where it goes as follows;

Second chapter discusses the background and literature review of different approaches related to the same design. Third chapter discloses in detail the overall design, including the explanation of each linearization techniques, as well as justifying the region of each transistor. Forth chapter demonstrates the results of the design. To end with, chapter Five conclude the design, along with the future work which can be implemented to improve the design.

2.1 Transconductance Amplifier Topologies

This project aims to design an Amplifier which able to convert its input voltage to the desired output current, with Pseudo Differential Pair and Source Degeneration as linearization techniques

Varies architectures had been developed through the past years to build the basic OTA block

Table 2.1 :comparison between three different papers

3. Design

1. Design Requirements

Referring to chapter 1, the design shall meet the following requirements:

1. The design uses CMOS based technology.
2. The total power consumption will be less than 15mW.
3. The supply voltage will be less than 1 volt.
4. Bandwidth should be larger than 50MHz
5. DC gain should be larger than 20dB

2. Analysis of Requirements and Constraints

In order to attain the design specifications mentioned in section 3.1, the requirements and constraints are clarified below

1. Analysis of Design Requirements

• CMOS based technology

CMOS circuits components are becoming the most desired to be implemented in nowadays technology, due to its low power consumption. Furthermore, its high speed when compared to other used technology.

• Power Consumption

• Supply Voltage
• Bandwidth
• DC gain
  2. Analysis of Design Constraints

• Economic Constraints

• Manufacturability and Sustainability Constraints

The design should meet the proper operating temperature and environmental corners. The circuit topologies have been implemented using an advanced simulation that can foresee the behavior of the circuit under such circumstances.

For instance, a high power circuitry will increase its temperature, thus the degradation of the performance in time, but if the circuit surpass in supreme corner, it is expected to have a longer life time.

• Ethical and Safety Constraints

Documentation shouldn’t exceed 30% similarity, citation should be considered along with stating proper referencing

3. Design Approaches

According to the cited papers in chapter 2 there are four different architectures for the converter design. All of these architectures can’t be used to achieve the requirements of this project. The developed design discussed in the next section has the opportunity to achieve the requirements