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2008 EDC Lesson 14- " , Raj Kamal, 1
EDC Transistor and FET Characteristics
LessonLesson--14: 14: Small Signal Model of Small Signal Model of JFETJFET
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2008 EDC Lesson 14- " , Raj Kamal, 2
1. Revision1. Revision
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2008 EDC Lesson 14- " , Raj Kamal, 3
ModelModel
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2008 EDC Lesson 14- " , Raj Kamal, 4
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2008 EDC Lesson 14- " , Raj Kamal, 5
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2008 EDC Lesson 14- " , Raj Kamal, 6
2. Small Signal Model2. Small Signal Model
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2008 EDC Lesson 14- " , Raj Kamal, 7
Gate Source Voltage control of drainGate Source Voltage control of drain--source source current current
ID = gm VGS [Change in drain source current is proportional to change in gate-source voltage. Ratio is called trans-conductance. Trans- refers to output change on the input.]
Conductance is reciprocal of resistance
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2008 EDC Lesson 14- " , Raj Kamal, 8
Definition of TransDefinition of Trans--conductance from slopeconductance from slope
VGS
ID
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2008 EDC Lesson 14- " , Raj Kamal, 9
Example Example
Find gm when change in ID is 2.1 mA on change in VGS by 0.6 V.
2.1 mA÷ 0.6 V = 3.5 mS [ S= 1]
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2008 EDC Lesson 14- " , Raj Kamal, 10
Example Example
Find gm when change in ID is 1.8 mA on change in VGS by 0.7 V.
1.8 mA÷ 0.7 V = 2.57 mS [ S = 1] S stands for Siemens
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2008 EDC Lesson 14- " , Raj Kamal, 11
Mathematical Formula Mathematical Formula
gm = gm0 [ 1 VGS/ Vp ] gm0 = 2 IDSS/|VP/ Vp |
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2008 EDC Lesson 14- " , Raj Kamal, 12
Example Example
Find gm0 when IDSS is 8 mA and VP = 4 V.
gm0 = 2 IDSS/|VP/ Vp | 2 8 mA÷ 4 V = 4 mS [ S= 1]
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2008 EDC Lesson 14- " , Raj Kamal, 13
Example Example
Find gm when gm0 = 4 mS when VP = 4 V and VGS = 0.5V.
gm = gm0 [ 1 VGS/ Vp ] gm = 4 mS [1 ( 0.5V) / ( 4V)] = 3.5
mS [ S = 1] S stands for Siemens
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2008 EDC Lesson 14- " , Raj Kamal, 14
FET INPUT IMPEDENCE FET INPUT IMPEDENCE
Typical Zi = 109 |
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2008 EDC Lesson 14- " , Raj Kamal, 15
FET OUTPUT IMPEDENCE FET OUTPUT IMPEDENCE
rd = 103 -105 | = [VGS ÷ ID ] at constant VGS
ZO = rd = 1/yOS
Admittance yOS 10 S to 50 S = 20 k─1 t0 100 k─1
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2008 EDC Lesson 14- " , Raj Kamal, 16
rrdd from characteristics slope of line from characteristics slope of line
ID
VGS
Q
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2008 EDC Lesson 14- " , Raj Kamal, 17
Example Example
For VGS = 0, if a tangent is drawn in output characteristics, when ID = 0.2 mA and VDS = + 5V.
rd = [VDS /ID ]| VGS= 0V
= + 5V/0.2 mA = 25 k
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2008 EDC Lesson 14- " , Raj Kamal, 18
Example Example
For VGS = ─ 2V , if a tangent is drawn in output characteristics, when ID = 0.1 mA and VDS = + 8V.
rd = [VDS /ID ]| VGS= ─ 2V
= + 8V/0.1 mA = 80 k
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2008 EDC Lesson 14- " , Raj Kamal, 19
AC Equivalent CircuitAC Equivalent Circuit
G
Vgs
+ gmVgs
S
D
S
rd
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2008 EDC Lesson 14- " , Raj Kamal, 20
SummarySummary
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2008 EDC Lesson 14- " , Raj Kamal, 21
We learnt• Small signal case of Characteristics• Trans-conductance • Input impedance• Output impedance
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2008 EDC Lesson 14- " , Raj Kamal, 22
End of Lesson 14 End of Lesson 14