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Lecture 6

Lecture 6. Active Load Amplifier Multistage Amplifier Circuit. Differential to Single-Ended Conversion. Reason for converting the signal from differential to single ended—to drive an offchip load. A Simple Circuit for Differential to Single-Ended Conversion.

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Lecture 6

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  1. Lecture 6 Active Load Amplifier Multistage Amplifier Circuit

  2. Differential to Single-Ended Conversion Reason for converting the signal from differential to single ended—to drive an offchip load.

  3. A Simple Circuit for Differential to Single-Ended Conversion Problem: the drain signal current of Q1 is wasted!

  4. If Q4 is perfectly matched to Q3 I(Q3)=I(Q4)=I/2 Vo=VDD-VSG3

  5. If Q4 is NOT perfectly matched to Q3 • If I(Q3) is I/2+ and I(Q2) is I/2 • will flow into the high output resistances of Q2 and Q4. • The DC output voltage will deviate significantly from VDD-VSG3 • Need a common mode Feedback circuit

  6. Small Signal Operation See next slide

  7. Virtual Ground High Res into the drain of Q4 Low Res into the drain of Q3 Not a virtual ground (Strictly Speaking) Common Drain Gain: (Drain Resistance)/(1/gm+Source Resistance)

  8. Analysis Technique Output resistance Short circuit transconductance

  9. Short Circuit Transcondutance • Approximations: • ro1>>1/gm3 • ro3>>1/gm3 • Implications of the short at the output: • ro4 is shorted out. • ro2 is shorted out. Virtual Ground Justification: The drain of Q4 is grounded! The resistance into Q3 is low! (e.g. 1/gm3) Therefore, we have an approximate virtual ground.

  10. Output Resistance

  11. A 2-Stage CMOS Op-Amp

  12. A Bias Circuit that Stabilizes gm

  13. Materials Not Covered in Class

  14. CMRR Circuit

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