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Small Signal Model MOS Field-Effect Transistors (MOSFETs)

Small Signal Model MOS Field-Effect Transistors (MOSFETs). 1. Quiz No 3 DE 27 (CE). 20-03-07. R out. Draw small signal model (4) Find expression for R out (2) Prove v o /v sig = ( β 1 α 2 R C )/(R sig +r π ) (4).

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Small Signal Model MOS Field-Effect Transistors (MOSFETs)

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  1. Small Signal Model MOS Field-Effect Transistors (MOSFETs) 1

  2. Quiz No 3 DE 27 (CE) 20-03-07 Rout. • Draw small signal model (4) • Find expression for Rout(2) • Prove vo/vsig = (β1α2RC)/(Rsig+rπ)(4).

  3. Figure 4.2 The enhancement-type NMOS transistor with a positive voltage applied to the gate. An n channel is induced at the top of the substrate beneath the gate.

  4. Enhancement-type NMOS transistor:

  5. MOSFET Analysis iD = iS, iG = 0

  6. Large-signal equivalent-circuit model of an n-channel MOSFET : Operating in the saturation region.

  7. Large-signal equivalent-circuit model of an p-channel MOSFET : Operating in the saturation region.

  8. Large Signal Model : MOSFET

  9. Transfer characteristic of an amplifier

  10. Conceptual circuit utilized to study the operation of the MOSFET as a small-signal amplifier. The DC BIAS POINT To Ensure Saturation-region Operation

  11. Signal Current in Drain Terminal

  12. Total instantaneous voltages vGSand vD

  13. Small-signal ‘π’ models for the MOSFET

  14. Common Source amplifier circuit Example 4-10

  15. Small Signal ‘T’ Model : NMOSFET

  16. Small Signal Models ‘T’ Model

  17. Single Stage MOS Amplifier

  18. Amplifiers Configurations

  19. Common Source Amplifier (CS) :Configuration

  20. Common Source Amplifier (CS) • Most widely used • Signal ground or an ac earth is at the source through a bypass capacitor • Not to disturb dc bias current & voltagescoupling capacitors are used to pass the signal voltages to the input terminal of the amplifier or to the Load Resistance • CS circuit is unilateral – • Rin does not depend on RL and vice versa

  21. Small Signal Hybrid “π” Model (CS)

  22. Small Signal Hybrid “π” Model : (CS)

  23. Small-signal analysis performed directly on the amplifier circuit with the MOSFET model implicitly utilized.

  24. Common Source Amplifier (CS) Summary • Input Resistance is infinite (Ri=∞) • Output Resistance = RD • Voltage Gain is substantial

  25. Common-source amplifier with a resistance RSin the source lead

  26. The Common Source Amplifier with a Source Resistance • The ‘T’ Model is preferred, whenever a resistance is connected to the source terminal. • ro(output resistance due to Early Effect) is not included, as it would make the amplifier non unilateral & effect of using ro in model would be studied in Chapter ‘6’

  27. Small-signal equivalent circuit with ro neglected.

  28. Small-signal Analysis.

  29. Voltage Gain : CS with RS

  30. Common Source Configuration with Rs • Rs causes a negative feedback thus improving the stability of drain current of the circuit but at the cost of voltage gain • Rs reduces id by the factor • (1+gmRs) = Amount of feedback • Rs is called Source degeneration resistance as it reduces the gain

  31. Small-signal equivalent circuit directly on Circuit

  32. A common-gate amplifier based on the circuit

  33. Common Gate (CG) Amplifier • The input signal is applied to the source • Output is taken from the drain • The gate is formed as a common input & output port. • ‘T’ Model is more Convenient • ro is neglected

  34. A small-signal equivalent circuit

  35. A small-signal Analusis : CG

  36. A small-signal Analusis : CG

  37. Small signal analysis directly on circuit

  38. The common-gate amplifier fed with a current-signal input.

  39. Summary : CG • 4. CG has much higher output Resistance • CG is unity current Gain amplifier or a Current Buffer • CG has superior High Frequency Response.

  40. A common-drain or source-follower amplifier.

  41. Small-signal equivalent-circuit model

  42. Small-signal Analysis : CD

  43. (a) A common-drain or source-follower amplifier :output resistance Rout of the source follower.

  44. (a) A common-drain or source-follower amplifier. : Small-signal analysis performed directly on the circuit.

  45. Common Source Circuit (CS)

  46. Common Source Circuit (CS) With RS

  47. Common Gate Circuit (CG) Current Follower

  48. Common Drain Circuit (CD) Source Follower

  49. Summary & Comparison

  50. Quiz No 4 27-03-07 • Draw/Write the Following:

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