W2: Basic Amplifiers and Differential Amplifier

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W2 Basic Amplifiers and Differential Amplifier
Fall 2005 EE/CSE 596 Individual Study

• Insoo Kim
• Mixed Signal CHIP Design Lab.
• Department of Computer Science Engineering
• The Penn State University

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Dont let the computer think for you
In todays analog design, simulation of circuits
is essential because the behavior of
short-channel MOSFETs cannot be predicted
accurately by hand calculations. Nonetheless, if
the designer avoids a simple and intuitive
analysis of the circuits and hence skips the task
of gaining insight, then he/she cannot interpret
the simulation results intelligently. For this
reason, we say, Dont let the computer think for
you. - Behzad Razavi
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Contents

• Fundamentals
• Basic Amplifiers Low Frequency Analysis
• Basic Amplifiers High Frequency Analysis
• Differential Amplifier
• Feedback

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Fundamentals

• Definitions
• DC Operating Point Load line
• Large Signal Analysis vs. Small Signal Analysis
• MOSFET intrinsic Capacitances

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Definitions
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DC Operating Point Load Line
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Large Signal Analysis vs. Small Signal Analysis

• Large Signal Analysis

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Large Signal Analysis vs. Small Signal Analysis

• Small Signal Analysis

How convenient !!
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MOSFET Intrinsic Capacitances
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(contd) MOSFET Intrinsic Capacitances
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Basic Amplifiers Low Frequency Analysis

• Single Stage Amplifiers
• Multi Stage Amplifiers

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Single Stage Amplifiers CS, CD, and CG Stage
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Common Source Stage Voltage Gain
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Common Drain Stage Output Resistance
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Common Gate Stage Input Resistance
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Summary
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Quiz

• CD stage amplifier is suitable for output stage
  of OPAmp due to its low output impedance and
  large bandwidth. However, in CMOS analog IC, CS
  stage is more widely used for output stage OPAmp
  than CD stage. Why?

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Loads for basic amplifiers
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(contd) Loads for basic amplifiers

• Diode Connected Load

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(contd) Loads for basic amplifiers

• Source degeneration

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Cascode Stage

• Small Signal Analysis
• Rout

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Folded Cascode Stage
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(contd) Folded Cascode Stage

• What are the advantages of folded cascode
  amplifier?
• Disadvantages
• Limited Output swing
• Large Voltage Headroom
• Large Power Consumption

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Basic Amplifiers High Frequency Analysis

• Frequency Analysis
• Dominant Pole Approach

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Frequency Analysis
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(contd) Frequency Analysis

• Bode Plot

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Dominant Pole Approach
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BW Estimation by Dominant Pole Approach
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Bandwidth Comparison
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Quiz

• Design an amplifier which satisfy following
  features using basic single-stage amplifiers.
• High gain
• Large Bandwidth
• High input impedance
• Low output impedance

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Differential Amplifier

• Single Stage Amplifiers
• Multi Stage Amplifiers

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Why differential Amplifier?

• Single Ended Signal can be easily contaminated
• A Differential Signal can be cleaned up
• Power Supply noise can be reduced

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Differential Amplifier Analysis

• Classic Diff Amp

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(contd) Differential Amplifier Analysis
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Diff Amp with Current Mirror Load
Common Mode Input Voltage Range
VSSVTN1VDSAT5VDSAT1 VTN1
1. What is CM Input Voltage? 2. How do we prove
this equation?
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(Std. Library) Design Exercise

• Design Flow
• Determine Specifications
• Power Consumption (ex. 1mW)
• Voltage Gain (ex. 30)
• Active Common Mode Input range (as large as
  possible)
• Others slew rate, CMRR, PSRR, etc.
• Determine minimum channel length
• Determine channel width
• Determine W1,2 from voltage gain spec.
• Determine W5 Bias Voltage from power
  consumption CM min.
• Determine W3,4 from CM max.
• Determine Bias Level of current source tr.
• Check other specifications

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Feedback

• Feedback Stability
• Voltage Amplifier Model
• Common Mode Feedback

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Feedback Stability
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Voltage Amplifier Model

• Models

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(contd) Voltage Amplifier Model

• 1st Order Model

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(contd) Voltage Amplifier Model

• 2nd Order Model

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(contd) Voltage Amplifier Model

• Time Response of the 2nd Order Model

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(contd) Voltage Amplifier Model
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Feedback Characteristics

• Gain desensitization
• Band width extension

• Noise Reduction
• Non-linearity Reduction

• w/o feedback
• w feedback

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Common Mode Feedback

• Why is CMFB circuit needed?
• Due to TR mismatch, TRs may not be in saturation
  region at operating point. ? DM Gain decreases
  and CM gain increases
• Since output CM level is sensitive to device
  properties and mismatches, it cannot be
  stabilized by means of differential feedback.
• General Topology of CMFB Circuit

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(contd) Common Mode Feedback

• Examples of CMFB

Useful for low gain applications
Folded cascode amplifier with CMFB
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References

• Joongho Choi, CMOS analog IC Design, IDEC
  Lecture Note, Mar. 1999.
• B. Razavi, Design of Analog CMOS Integrated
  Circuits, McGraw-Hill, 2001.
• Hongjun Park, CMOS Analog Integrated Circuits
  Design, Sigma Press, 1999.