1. JFET and MOSFET Amplifiers
UNIT III
JFET and MOSFET Amplifiers

2. OUTLINE Small signal Analysis of JFET amplifiers
Small signal Analysis of MOSFET and JFET
BICMOS cascode Amplifier

3. FET ( Field Effect Transistor)
Unipolar device i. e. operation depends on only one type of charge carriers.
Voltage controlled Device (gate voltage controls drain current).
Very high input impedance ( ).
Source and drain are interchangeable in most Low-frequency applications.
Low-power consumption.
Less Noisy as Compared to BJT.
Very small in size, occupies very small space in Ics.

4. Types of Field Effect Transistors (The Classification)
JFET
MOSFET (IGFET)
n-Channel JFET
p-Channel JFET
FET
Enhancement MOSFET
Depletion MOSFET
n-Channel EMOSFET
n-Channel DMOSFET
p-Channel DMOSFET
p-Channel EMOSFET

5. The Junction Field Effect Transistor (JFET)
Figure: n-Channel JFET.

6. JFET
There are two basic configurations of junction field effect transistor, the N-channel JFET and the P-channel JFET.
The N-channel JFET’s channel is doped with donor impurities meaning that the flow of current through the channel is negative (hence the term N-channel) in the form of electrons.

7. SYMBOLS Gate Drain Source Gate Drain Source Gate Drain Source
n-channel JFET
Offset-gate symbol
n-channel JFET
p-channel JFET

9. Small signal Analysis
The coupling capacitors bypass capacitor are short circuitED
Short the DC supply voltage
Replace the FET with the hybrid-p model

10. Common source JFET

11. Small signal -Common source JFET-contd…

12. Small signal -Common source JFET-contd…

13. Source Follower JFET

14. Small signal equivalent-Source Follower JFET
Av= gmRs/(1+gmRs)

15. Common Gate JFET

16. Small signal model-Common Gate JFET
AV= -gmRd
Rin=1/gm

17. Self bias of JFET

18. Small signal equivalent -Self bias of JFET-contd…

19. Enhancement MOSFET showing channel length L and channel width W.

20. MOSFET- SYMBOL

21. Small-signal equivalent circuit for FETs.

22. FET small-signal equivalent circuit that accounts for the dependence of iD on vDS.

23. MOSFET Characteristics

24. Common source Amplifier -MOSFET

25. Small signal -Common source Amplifier -MOSFET
For drawing an a c equivalent circuit of Amp.
Assume all Capacitors C1, C2, Cs as short circuit elements for ac signal
Short circuit the d c supply
Replace the FET by its small signal model

26. Analysis of CS Amplifier
A C Equivalent Circuit
Simplified A C Equivalent Circuit

27. Analysis of CS Amplifier-contd…

28. Small Signal ‘T’ Model : NMOSFET
sedr42021_0439.jpg

29. Small Signal Models
‘T’ Model
sedr42021_tb0402a.jpg

30. Analysis of CS Amplifier with Potential Divider Bias
This is a CS amplifier configuration therefore the
input is on the gate and the output is on the drain.

31. An Amplifier Circuit using MOSFET(CS Amp.)

32. A small signal equivalent circuit of CS Amp.

33. Common Source Amplifier (CS)
Signal ground or an ac earth is at the source through a bypass capacitor
Not to disturb dc bias current & voltages coupling 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

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

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

36. Small-signal analysis performed directly on the amplifier circuit with the MOSFET model implicitly utilized
sedr42021_0443a.jpg

37. Common-source amplifier with a resistance RS in the source lead
sedr42021_0444a.jpg

38. 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

39. Small-signal equivalent circuit with ro neglected.
sedr42021_0444a.jpg

40. Small-signal Analysis.

41. Voltage Gain : CS with RS

42. Small-signal Analysis.
Source Resistance can be used to control the magnitude of the signal vgs& thus ensure that vgs does not become too large to cause non-linear distortion
vgs << 2(VGS-Vt) << 2VOV

43. 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

44. Small-signal equivalent circuit directly on Circuit
sedr42021_0444a.jpg

45. BJT / MOSFET

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

47. MOSFET -Source follower.

48. Small-signal ac equivalent circuit for the source follower.

49. Equivalent circuit used to find the output resistance of the source follower.

50. Common-gate amplifier.

51. Small signal-Common Gate

52. Small-signal ‘π’ models for the MOSFET
sedr42021_0437a.jpg

53. Voltage swing limitation
Up swing limited by transistor going in to cut off
Vout (max)= VDD
Lower swing limited by MOSFET entering in to linear region
Vou(min)-VGG-VT

54. A common-gate amplifier based on the circuit
sedr42021_0445a.jpg

55. 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

56. A small-signal equivalent circuit
sedr42021_0445a.jpg

57. A small-signal Analusis : CG
sedr42021_0445a.jpg

58. A small-signal Analusis : CG
sedr42021_0445a.jpg

59. Small signal analysis directly on circuit
sedr42021_0445a.jpg

60. The common-gate amplifier fed with a current-signal input.
sedr42021_0445a.jpg

61. Common Gate
Rin in independent of RL & Rin = 1/gm & gm in order of mA/V.
Input resistance of the CG Amplifier is relatively low (in order of 1kv) than CS Amplifier
Loss of signal
CG is acts as Unity gain current amplifier current buffer – useful for a Cascade circuitry

62. Summary-CG CG has much higher output Resistance
CG is unity current Gain amplifier or a Current Buffer
CG has superior High Frequency Response.

63. A common-drain or source-follower amplifier.
A common-drain or source-follower amplifier
sedr42021_0446a.jpg

64. Small-signal equivalent-circuit model
sedr42021_0446a.jpg

65. Small-signal Analysis : CD
sedr42021_0446a.jpg

66. A common-drain or source-follower amplifier :output resistance Rout of the source follower.
sedr42021_0446a.jpg

67. A common-drain or source-follower amplifier
A common-drain or source-follower amplifier. : Small-signal analysis performed directly on the circuit.
sedr42021_0446a.jpg

68. Common Source Circuit (CS)
sedr42021_tb0404a.jpg

69. Common Source Circuit (CS) With RS
sedr42021_tb0404a.jpg

70. Common Gate Circuit (CG)
Current Follower
sedr42021_tb0404c.jpg

71. Small Signal Model MOSFET : CD
sedr42021_p05144.jpg

72. Small Signal Analysis CD
1/gm
gmvsg D
sedr42021_p05144.jpg

73. Solution Small Signal Analysis : Input Resistance
1/gm
gmvsg D
Ig=0
Rin

74. Solution Small Signal Analysis : Output Resistance
Itest
1/gm
gmvsg D ID
IRD
0 V
Vtest
IG=0
Rout

75. Solution Small Signal Analysis : Voltage Gain+
1/gm
gmvsg D
vsg - - + vo vi + -

76. Solution Small Signal Analysis : Voltage gain
1/gm
gmvsg D +
vsg - + vi -

77. Solution Small Signal Analysis : Voltage Gain
1/gm
gmvsg D + vi -

78. Solution Small Signal Analysis : Voltage Gain
Then

79. Solution Small Signal Analysis : Voltage Gain
1/gm
gmvsg D - + vi + -

80. BICMOS
sedr42021_0646a.jpg

81. BICMOS -contd…
Combining the high gain of BJT and infinite impedance of MOSFET will lead to BiCMOS differential amplifier design.
Rs = typical 100 KW
BICMOS cascode amplifier has overall voltage gain of C-S, but with frequency response comparable to CB Amplifier.

82. BICMOS -contd…
The basic idea is to combine the high Rin and large transconductance (g m) of a commonsource (common-emitter) amplifier with the current-buffering property and superior high-frequency response of the common-gate (common-base) circuit

83. BICMOS -contd…
In response to input signal voltage vi, the CS transistor Q1 conducts a current signal gm1 vi in its drain terminal and feeds it to the source of the CG transistor Q 2 (cascode transistor).
Q 2 passes signal to its drain and to the load RL. Q 2 acts as a buffer, presenting low Rin to the drain of Q1 and providing high Rout at output.