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Chapter 16 CMOS Amplifiers

Chapter 16 CMOS Amplifiers. 16.1 General Considerations 16.2 Operating Point Analysis and Design 16.3 CMOS Amplifier Topologies 16.4 Common-Source Topology 16.5 Summary and Additional Examples 16.6 Chapter Summary. Chapter Outline. Example: Desired I/O Impedances.

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Chapter 16 CMOS Amplifiers

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  1. Chapter 16 CMOS Amplifiers • 16.1 General Considerations • 16.2 Operating Point Analysis and Design • 16.3 CMOS Amplifier Topologies • 16.4 Common-Source Topology • 16.5 Summary and Additional Examples • 16.6 Chapter Summary

  2. Chapter Outline CH 16 CMOS Amplifiers

  3. Example: Desired I/O Impedances CH 16 CMOS Amplifiers

  4. Method to Measure the I/O Impedances • To measure Rin(Rout), deactivate all the other independent sources in the circuit and find the ratio of vX/iX. CH 16 CMOS Amplifiers 4

  5. Example: Input Impedance of a Simple Amplifier CH 16 CMOS Amplifiers 5

  6. The Concept of Impedance at a Node • When the other node of a port is grounded, it is more convenient to use the concept of impedance at a node. CH 16 CMOS Amplifiers 6

  7. Example: Impedance Seen at Drain CH 16 CMOS Amplifiers 7

  8. Example: Impedance Seen at Source CH 16 CMOS Amplifiers 8

  9. Impedance Summary • Looking into the gate, we see infinity. • Looking into the drain, we see rO if the source is (ac) grounded. • Looking into the source, we see 1/gm if the gate is (ac) grounded and rO is neglected. CH 16 CMOS Amplifiers 9

  10. Bias and Signal Levels for a MOS Transistor • Bias point analysis establishes the region of operation and the small-signal parameters. • On top of the bias point, small signals are applied to the circuit. CH 16 CMOS Amplifiers 10

  11. General Steps in Circuit Analysis • First, the effects of constant voltage/current sources are analyzed when signal sources are deactivated. • Second, small-signal analysis is done when constant sources are set to zero. CH 16 CMOS Amplifiers 11

  12. Simplification of Supply Voltage Notation CH 16 CMOS Amplifiers 12

  13. Example: Amplifier Driven by a Microphone • Since the DC (average) value is at zero, and 20mV is not sufficient to turn on M1, M1 is off and Vout is at VDD. CH 16 CMOS Amplifiers 13

  14. Example: Amplifier with Gate Tied to VDD • Since the gate voltage level is fixed at VDD, no signal current will be produced my M1, leading to no amplification. CH 16 CMOS Amplifiers 14

  15. Example: Amplifier with Gate Bias • With proper value of VB, M1 can operate in the desired saturation region and amplify the incoming voice signal. CH 16 CMOS Amplifiers 15

  16. Simple Biasing • In (a), VGS=VDD, whereas in (b) VGS equals to a fraction of VDD. CH 16 CMOS Amplifiers 16

  17. Example: Bias Current and Maximum RD CH 16 CMOS Amplifiers 17

  18. Capacitive Coupling • Capacitive coupling is used to block the zero DC output value of the microphone and pass the voice signal to the amplifier. CH 16 CMOS Amplifiers 18

  19. Biasing with Source Degeneration CH 16 CMOS Amplifiers 19

  20. Example: ID and Maximum RD for Source Degeneration Biasing CH 16 CMOS Amplifiers 20

  21. Example: Maximum W/L and Minimum RS CH 16 CMOS Amplifiers 21

  22. Self-Biased MOS Stage • The gate voltage is provided by the drain with no voltage drop across RG and M1 is always in saturation. CH 16 CMOS Amplifiers 22

  23. Example: Self-Biased MOS Stage CH 16 CMOS Amplifiers 23

  24. Example: PMOS Stage with Biasing CH 16 CMOS Amplifiers 24

  25. Example: PMOS Stage with Self-Biasing CH 16 CMOS Amplifiers 25

  26. Good Example of Current Source • As long as a MOS transistor is in saturation region and λ=0, the current is independent of the drain voltage and it behaves as an ideal current source seen from the drain terminal. CH 16 CMOS Amplifiers 26

  27. Bad Example of Current Source • Since the variation of the source voltage directly affects the current of a MOS transistor, it does not operate as a good current source if seen from the source terminal CH 16 CMOS Amplifiers 27

  28. Possible I/O Connections to a MOS Transistor • Of all the possible I/O connections to a MOS transistor, only (a,d), (a,e) and (b,d) are functional. CH 16 CMOS Amplifiers 28

  29. Common Source (CS) Stage • If the input is applied to the gate and the output is sensed at the drain, the circuit is called a “common-source” (CS) stage. CH 16 CMOS Amplifiers 29

  30. Small-Signal Model of CS Stage CH 16 CMOS Amplifiers 30

  31. Example: CS Stage CH 16 CMOS Amplifiers 31

  32. Example: Faulty CS Stage Design • However, no solution exists since M1 is out of the saturation region (VDD-IDRD<VGS-VTH). CH 16 CMOS Amplifiers 32

  33. CS Stage I/O Impedance Calculation CH 16 CMOS Amplifiers 33

  34. CS Stage Including Channel-Length Modulation CH 16 CMOS Amplifiers 34

  35. Example: ½ Gain No Channel-Length Modulation With Channel-Length Modulation CH 16 CMOS Amplifiers 35

  36. Example: RD → ∞ CH 16 CMOS Amplifiers 36

  37. CS Stage with Current Source Load CH 16 CMOS Amplifiers 37

  38. Example: CS Stage with Current Source Load CH 16 CMOS Amplifiers 38

  39. CS Stage with Diode-Connected Load CH 16 CMOS Amplifiers 39

  40. Example: CS Stage with Diode-Connected PMOS CH 16 CMOS Amplifiers 40

  41. CS Stage with Source Degeneration CH 16 CMOS Amplifiers 41

  42. Example: CS Stage with Source Degeneration CH 16 CMOS Amplifiers 42

  43. Example: Degeneration Resistor Without Degeneration With Degeneration CH 16 CMOS Amplifiers 43

  44. Effective Transconductance CH 16 CMOS Amplifiers 44

  45. Effect of Transistor Output Resistance CH 16 CMOS Amplifiers 45

  46. Stage with Explicit Depiction of rO • Sometimes, the transistor’s output resistance is explicitly drawn to emphasize its significance. CH 16 CMOS Amplifiers 46

  47. Example: NMOS Current Source Design CH 16 CMOS Amplifiers 47

  48. Example: Output Resistance of CS Stage with Degeneration I CH 16 CMOS Amplifiers 48

  49. Example: Output Resistance of CS Stage with Degeneration II CH 16 CMOS Amplifiers 49

  50. Example: Failing Microphone Amplifier No Amplification!! • Because of the microphone’s small low-frequency output resistance (100Ω), the bias voltage at the gate is not sufficient to turn on M1. CH 16 CMOS Amplifiers 50

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