1. Basic Amplifiers and Differential Amplifier
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CSE598A/EE597G Spring 2006
Basic Amplifiers and Differential Amplifier
Insoo Kim, Kyusun Choi
Mixed Signal CHIP Design Lab.
Department of Computer Science & Engineering
The Penn State University

2. Don’t let the computer think for you
In today’s 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, “Don’t let the computer think for you.”
- Behzad Razavi
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3. Contents Fundamentals Basic Amplifiers: Low Frequency Analysis
Basic Amplifiers: High Frequency Analysis
Differential Amplifier
Feedback
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4. Fundamentals Definitions DC Operating Point & Load line
Large Signal Analysis vs. Small Signal Analysis
MOSFET intrinsic Capacitances

5. Definitions mb
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6. DC Operating Point & Load Line
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7. Large Signal Analysis vs. Small Signal Analysis
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8. Large Signal Analysis vs. Small Signal Analysis
How convenient !!
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9. MOSFET Intrinsic Capacitances
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10. (cont’d) MOSFET Intrinsic Capacitances
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11. Basic Amplifiers: Low Frequency Analysis
Single Stage Amplifiers
Multi Stage Amplifiers

12. Single Stage Amplifiers: CS, CD, and CG Stage
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13. Common Source Stage : Voltage Gain
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14. Common Drain Stage: Output Resistance
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15. Common Gate Stage : Input Resistance
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16. Summary
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17. 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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18. Loads for basic amplifiers
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19. (cont’d) Loads for basic amplifiers
Diode Connected Load
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20. (cont’d) Loads for basic amplifiers
Source degeneration
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21. Cascode Stage
Small Signal Analysis
Rout
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22. Folded Cascode Stage
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23. (cont’d) 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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24. Basic Amplifiers: High Frequency Analysis
Dominant Pole Approach

25. Frequency Analysis
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26. (cont’d) Frequency Analysis
Bode Plot
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27. Dominant Pole Approach
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28. BW Estimation by Dominant Pole Approach
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29. Bandwidth Comparison
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30. 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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31. Differential Amplifier
Single Stage Amplifiers
Multi Stage Amplifiers

32. 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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33. Differential Amplifier Analysis
Classic Diff Amp
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34. (cont’d) Differential Amplifier Analysis
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35. Diff Amp with Current Mirror Load
Common Mode Input Voltage Range
VSS+VTN1+VDSAT5+VDSAT1 < VIC < VDD–|VDSAT3|–|VTP3|+| VTN1|
1. What is CM Input Voltage?
2. How do we prove this equation?
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36. (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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37. Feedback & Stability Voltage Amplifier Model Common Mode Feedback

38. Feedback & Stability
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39. Voltage Amplifier Model
Models
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40. (cont’d) Voltage Amplifier Model
1st Order Model
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41. (cont’d) Voltage Amplifier Model
2nd Order Model
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42. (cont’d) Voltage Amplifier Model
Time Response of the 2nd Order Model
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43. (cont’d) Voltage Amplifier Model
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44. Feedback Characteristics
Gain desensitization
Band width extension
Noise Reduction
Non-linearity Reduction
w/o feedback
w feedback
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45. 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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46. (cont’d) Common Mode Feedback
Examples of CMFB
Folded cascode amplifier with CMFB
Useful for low gain applications
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47. References
Joongho Choi, “CMOS analog IC Design,” IDEC Lecture Note, Mar
B. Razavi, “Design of Analog CMOS Integrated Circuits,” McGraw-Hill, 2001.
Hongjun Park, “CMOS Analog Integrated Circuits Design,” Sigma Press, 1999.
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