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Differential Amplifiers

Differential Amplifiers. Outline. Single-Ended Versus Differential Operation. The transitions disturb the differential by equal amounts, leaving the difference in tact. Immunity to Supply Noise. If VDD changes by ∆V, Vout changes by the same amount. Noise in VDD affects VX and VY,

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Differential Amplifiers

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  1. Differential Amplifiers

  2. Outline

  3. Single-Ended Versus Differential Operation The transitions disturb the differential by equal amounts, leaving the difference in tact.

  4. Immunity to Supply Noise If VDD changes by ∆V, Vout changes by the same amount. Noise in VDD affects VX and VY, but not Vx-Vy

  5. Reduction of Coupled Noise Noise coupled from L3 to L1 and L2 to L1 cancel each other.

  6. Sensitivity to the Common mode level Excessive low Vin,CM turns off Devices.

  7. Basic Differential Pair

  8. Schematic of Differential Amplifier

  9. Input/Output Characteristics Minimum Slope Independent of Vin,cm Maximum Slope Thus maximum Gain

  10. Range of Vin,cm Lower bound of Vin,cm: VP should be sufficiently high in order for M3 to act as a current source. Upper bound of Vin, cm M1 and M2 need to remain in saturation.

  11. Sensitivity to Vin, cm M3 in the linear region is modeled as a resistor M1=M2 =On M1=M2 =Off M1=M2 =On M1=M2 =On M1=M2 =Off M1=M2 =Off M3=Linear M3=Linear M3=Linear

  12. Small signal Gain as a function of Vin, CM

  13. Maximum Allowable Output Swing The higher the input CM level, the smaller the allowable output swings.

  14. Transconductance ∆Vin1Represents the maximum differential signal a differential pair can handle.

  15. Linearity W/L increases ISS Constant Constant W/L ISS increases

  16. Determinations of Small Signal Gain • CS with resistive source degeneration • Thevenin Resistance • Cascode • Superposition Principle

  17. CS with resistive source degeneration Interpretation: The resistance at the drain Divided by the resistance in the source path

  18. Treat M1 as a CS stage with resistive source degeneration to find VX/Vin

  19. Replace M1 by its Thevenin Equivalent Circuit If RS is sufficiently large, then the small signal gain of the amplifier can be obtained using thevenin’s equivalent circuit (see hand out)

  20. Gain of CG

  21. Replace M1 by its Thevenin Equivalent Circuit

  22. Small Signal Gain

  23. Half-Circuit Concept

  24. Conversion of Arbitrary inputs to Differential and Common-Mode Components

  25. Superposition Principle

  26. Schematic of Differential Amplifier

  27. Simulation Vin,m=1 mV Vout,m=8.735 mV Av=-8.735 Calculations: Gm=1mS ro=30.53 KOhm RL=12 Kohm Av=-Gm(ro||RL)=-8.615

  28. Common-Mode Response • Sensitivity of Vout,CM due to Vin,CM • In the presence of resistor mismatch • In the presence of transistor mismatch • Common Mood Rejection Ratio (CMRR)

  29. Sensitivity of Vout,CM due to Vin,CM Vin,CM ↑, VP↑, I(RSS)↑,VX,V↓

  30. Output CM Sensitivity due to Vin, CM Vout,m =0.285 mV Vin,cm =1 mV RL=12 K Gm=1.043 mS Gds3=58.29 uS Av, CM(Analytical)=0.343 Av, CM(Simulation)=0.285 (Excluding gmb, ro)

  31. Common-Mode to Differential Conversion at High Frequencies Even if the output resistance of the current source is high, the common-mode to differential conversion becomes significant at high frequencies.

  32. Resistor Mismatch (from CS with resistive source degeneration)

  33. Common Mode to Differential Mode Conversion

  34. Voutp-Voutn Differential Mode signal at the output: 1.176 uV

  35. Effect of CM Noise in the Presence of Resistor Mismatch Common Mode to Differential Conversion

  36. Transistor Mismatch

  37. Supply Noise Sensitivity

  38. CMRR

  39. Diode Connected Load Problem: Difficult to decrease (W/L)P without dropping the common mode voltage of Vout.

  40. Addition of Current Source to Increase Voltage Gain Reduce gm by reducing current rather than the aspect ratio. Reduce I(M3) and I(M4).

  41. Variable Gain Amplifier

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