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EDC Transistor and FET Characteristics

LessonLesson--14: 14: Small Signal Model of Small Signal Model of JFETJFET

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1. Revision1. Revision

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ModelModel

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2. Small Signal Model2. Small Signal Model

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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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Definition of TransDefinition of Trans--conductance from slopeconductance from slope

VGS

ID

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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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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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Mathematical Formula Mathematical Formula

gm = gm0 [ 1 VGS/ Vp ] gm0 = 2 IDSS/|VP/ Vp |

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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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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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FET INPUT IMPEDENCE FET INPUT IMPEDENCE

Typical Zi = 109 |

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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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rrdd from characteristics slope of line from characteristics slope of line

ID

VGS

Q

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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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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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AC Equivalent CircuitAC Equivalent Circuit

G

Vgs

+ gmVgs

S

D

S

rd

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SummarySummary

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We learnt• Small signal case of Characteristics• Trans-conductance • Input impedance• Output impedance

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End of Lesson 14 End of Lesson 14