Chapter 8: FET Amplifiers
Introduction
• • • •
Excellent voltage gain g nput mpe ance Lowpower consumption Good frequency range
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FET Small SmallSi nal Model
Transconductance e re a ons p o a c ange n VGS is called transconductance
D
o
e correspon ng c ange n
ranscon uc ance s eno e gm an g ven y: ΔI D ΔV GS
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Gra hical Determination of
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Mathematical Definitions of ID
gm
VGS
2I DSS
gm
VP
VP
gm
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g m0 1
VGS
ID
VP
I DSS gm
VP
2I DSS
Where VGS =0V g m0
Where 1
VGS
1
g m0 1
5
VGS VP
GS
VP
g m0
D
I DSS
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FET Im edance Input impedance: Zi
Output Impedance: Zo
1
rd
os
where: rd
DS
ID
VGS
constant
yos= admittance parameter listed on FET specification sheets.
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FET AC Equivalent Circuit
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CommonSource CS FixedCommon FixedBias Circuit The input is on the gate and the output is on the drain There is a 180 phase shift between input and output
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Input impedance:
Zi
RG
Out ut im edance: Zo o
Av Av
R D  rd D
Vo V Vo Vi
rd 10R D
g m (rd  R D ) gmR D
rd 10R D
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CommonSource CS SelfCommon Self Bias Circuit This is a commonsource amplifier configuration, so the input is on the gate and the output is on the drain There is a 180 phase shift between input and output
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Calculations Input impedance: i
G
Out ut im edance:
Zo o
rd  R D D
rd 10R D
Voltage gain:
Av
Av
g m (rd  R D )
gm R D
rd 10R D
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CommonSource (CS) SelfCommon Self Bias Circuit Removing Cs affects the gain of the circuit.
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Calculations Input impedance:
Z
R
Output impedance: Zo
RD
rd 10R D
Voltage gain: v
Vo Vi
1
V v
Vi
gm R D RD gm RS
RS rd
R 1
gmRS
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rd 10(R D
13
RS )
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CommonSource (CS) Voltage Common VoltageDivider Bias This is a commonsource amplifier configuration, so the in ut is on the ate and the output is on the drain.
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Impedances Input impedance: Zi
R 1  R 2
Zo
rd  R D
Zo
RD
rd 10R D
Av
g m (rd  R D )
Av
gm R D
rd 10R D
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configuration, the input is on the gate, but the output is from the . There is no phase shift between npu an ou pu .
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Impedances Input impedance: i
G
Output impedance:
Zo
rd  R S 
Zo
R S 
Av
Av
gm
Vo Vi
gm rd 10R S
g m (rd  R S ) 1
Vo Vi
1
g m (rd  R S )
gmRS 1
gmRS
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rd 10
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CommonGate CG Circuit Common
and the output is on the drain. There is no phase shift between input and output.
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Calculations Input impedance: Zi
R S 
Zi
R S 
rd 1
RD g m rd
gm
rd 10R D
Zo
R D  rd
Zo
R D r 10 d
RD
gm R D Av
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Vi
19
1
r
RD
v
m
D rd 10R D
rd
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DT
e MOSFET AC E uivalent
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gm and rd can be found in the specification sheet for the FET.
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There is a 180 phase shift between input and output.
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Calculations Input impedance: Zi
RF 1
rd  R D
g m (rd  R D )
RF 1
F
gmR D
rd
D , rd
D
Out ut im edance: Zo
R F  rd R D
Zo
RD R
r  R
r
10R
Voltage gain: Av
Av
g m (R F  rd  R D ) g m R D R F rd R D ,rd
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10R D
23
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Calculations Input impedance:
Z i R 1  R 2 u pu mpe ance:
Zo o
Av v
rd  R D D rd 10
g m (rd  R D ) gm
D rd 10R D
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Summar Table
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Summar Table
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Check the DC bias voltages: If not correct check power supply, resistors, FET. Also check to ensure that the coupling capacitor between amplifier stages is OK. .
Check the AC voltages: , stage
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Practical A
lications
ThreeChannel Audio Mixer Three Silent Switching Phase Shift Networks Motion Detection S stem
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