FET AmplifiersFET Amplifiers
Chapter 8
Boylestad
Electronic Devices and Circuit TheoryElectronic Devices and Circuit Theory
Electronic Devices and Circuit TheoryBoylestad
© 2013 by Pearson Higher Education, IncUpper Saddle River, New Jersey 07458 • All Rights Reserved
Ch.8 Summary
Introduction
FETs provide:
• Excellent voltage gain
• High input impedance
• Low-power consumption
• Good frequency response
Electronic Devices and Circuit TheoryBoylestad
© 2013 by Pearson Higher Education, IncUpper Saddle River, New Jersey 07458 • All Rights Reserved
Ch.8 Summary
FET Small-Signal Model
Transconductance: The ratio of a change in ID to the corresponding change in VGS
• Transconductance is denoted gm and given by:
GS
Dm ΔV
ΔIg
Electronic Devices and Circuit TheoryBoylestad
© 2013 by Pearson Higher Education, IncUpper Saddle River, New Jersey 07458 • All Rights Reserved
Ch.8 Summary
Geographical Determination of gm
Electronic Devices and Circuit TheoryBoylestad
© 2013 by Pearson Higher Education, IncUpper Saddle River, New Jersey 07458 • All Rights Reserved
Ch.8 Summary
Mathematical Definitions of gm
GS
Dm ΔV
ΔIg
P
GS
P
DSSm V
V
V
Ig 1
2
P
DSSm V
Ig
20
DSS
Dm
P
GSmm I
Ig
V
Vgg 00 1
For VGS = 0 V
Electronic Devices and Circuit TheoryBoylestad
© 2013 by Pearson Higher Education, IncUpper Saddle River, New Jersey 07458 • All Rights Reserved
Ch.8 Summary
FET Impedence
Ω iZ
Input impedance:
Output Impedance:
osdo y
rZ1
constant VGS
D
DSd ΔI
ΔVrwhere
yos= admittance parameter listed on FET spec sheets
Electronic Devices and Circuit TheoryBoylestad
© 2013 by Pearson Higher Education, IncUpper Saddle River, New Jersey 07458 • All Rights Reserved
Ch.8 Summary
FET AC Equivalent Circuit
Electronic Devices and Circuit TheoryBoylestad
© 2013 by Pearson Higher Education, IncUpper Saddle River, New Jersey 07458 • All Rights Reserved
Ch.8 Summary
Common-Source (CS) Fixed-Bias
The input is applied to the gate and the output is taken from the drain
There is a 180 phase shift between the circuit input and output
Electronic Devices and Circuit TheoryBoylestad
© 2013 by Pearson Higher Education, IncUpper Saddle River, New Jersey 07458 • All Rights Reserved
Ch.8 Summary
Calculations
Gi RZ
dDo ||rRZ
10
RrDo
Dd
RZ
Input impedance:
Output impedance:
)( Ddmi
ov ||Rrg
V
VA
Dd RrDmi
ov Rg
V
VA
10
Voltage gain:
Electronic Devices and Circuit TheoryBoylestad
© 2013 by Pearson Higher Education, IncUpper Saddle River, New Jersey 07458 • All Rights Reserved
Ch.8 Summary
Common-Source (CS) Self-Bias
This is a common-source amplifier configuration, so the input is applied to the gate and the output is taken from the drain.
There is a 180 phase shift between input and output.
Electronic Devices and Circuit TheoryBoylestad
© 2013 by Pearson Higher Education, IncUpper Saddle River, New Jersey 07458 • All Rights Reserved
Ch.8 Summary
Calculations
Gi RZ
Ddo ||RrZ
RrDo
Dd
RZ10
Input impedance:
Output impedance:
)( Ddmv ||RrgA
Dd RrDmv RgA10
Voltage gain:
Electronic Devices and Circuit TheoryBoylestad
© 2013 by Pearson Higher Education, IncUpper Saddle River, New Jersey 07458 • All Rights Reserved
Ch.8 Summary
Common-Source (CS) Self-Bias
Removing Cs affects the gain of the circuit.
Electronic Devices and Circuit TheoryBoylestad
© 2013 by Pearson Higher Education, IncUpper Saddle River, New Jersey 07458 • All Rights Reserved
RrDo
Dd
RZ10
Ch.8 Summary
Calculations
Gi RZ
Output impedance:
d
SDSm
Dm
i
ov
rRR
Rg
Rg
V
VA
1
)(101 SDd RRrSm
Dm
i
ov Rg
Rg
V
VA
Voltage gain:
Input impedance:
Electronic Devices and Circuit TheoryBoylestad
© 2013 by Pearson Higher Education, IncUpper Saddle River, New Jersey 07458 • All Rights Reserved
Ch.8 Summary
Common-Source (CS) Voltage-Divider Bias
This is a common-source amplifier configuration, so the input is applied to the gate and the output is taken from the drain.
Electronic Devices and Circuit TheoryBoylestad
© 2013 by Pearson Higher Education, IncUpper Saddle River, New Jersey 07458 • All Rights Reserved
Ch.8 Summary
Impedances
2 1||RRZi
Ddo ||RrZ
RrDo
Dd
RZ10
Input impedance:
Output impedance:
)( Ddmv ||RrgA
Dd RrDmv RgA10
Voltage gain:
Electronic Devices and Circuit TheoryBoylestad
© 2013 by Pearson Higher Education, IncUpper Saddle River, New Jersey 07458 • All Rights Reserved
Ch.8 Summary
Source Follower (Common-Drain)
In a common-drain amplifier configuration, the input is applied to the gate, but the output is taken from the source.
There is no phase shift between input and output.
Electronic Devices and Circuit TheoryBoylestad
© 2013 by Pearson Higher Education, IncUpper Saddle River, New Jersey 07458 • All Rights Reserved
Ch.8 Summary
Impedances
mSdo g
||||RrZ1
Sd Rrm
So g
||RZ 10
1
Gi RZ
Input impedance:
Output impedance:
)(1
)(
Sdm
Sdm
i
ov ||Rrg
||Rrg
V
VA
101
dr
Sm
Sm
i
ov Rg
Rg
V
VA
Voltage gain:
Electronic Devices and Circuit TheoryBoylestad
© 2013 by Pearson Higher Education, IncUpper Saddle River, New Jersey 07458 • All Rights Reserved
Ch.8 Summary
Common-Gate (CG) Circuit
The input is applied to the source and the output is taken from the drain.
There is no phase shift between input and output.
Electronic Devices and Circuit TheoryBoylestad
© 2013 by Pearson Higher Education, IncUpper Saddle River, New Jersey 07458 • All Rights Reserved
Ch.8 Summary
CalculationsInput impedance:
Output impedance:
dm
DdSi rg
Rr||RZ
1
Dd Rrm
Si g||RZ 10
1
dDo ||rRZ
10drDo RZ
d
D
d
DDm
i
ov
rR
rR
Rg
V
VA
1
RrDmv DdRgA 10Voltage gain:
Electronic Devices and Circuit TheoryBoylestad
© 2013 by Pearson Higher Education, IncUpper Saddle River, New Jersey 07458 • All Rights Reserved
Ch.8 Summary
D-Type MOSFET AC Equivalent
Electronic Devices and Circuit TheoryBoylestad
© 2013 by Pearson Higher Education, IncUpper Saddle River, New Jersey 07458 • All Rights Reserved
Ch.8 Summary
E-Type MOSFET AC Equivalent
gm and rd can be found in the specification sheet for the FET.
Electronic Devices and Circuit TheoryBoylestad
© 2013 by Pearson Higher Education, IncUpper Saddle River, New Jersey 07458 • All Rights Reserved
Ch.8 Summary
Common-Source Drain-Feedback
There is a 180 phase shift between input and output.
Electronic Devices and Circuit TheoryBoylestad
© 2013 by Pearson Higher Education, IncUpper Saddle River, New Jersey 07458 • All Rights Reserved
Ch.8 Summary
Calculations
Input impedance:
Output impedance:
)(1
Ddm
DdFi ||Rrg
||RrRZ
DdDdF R,r||RrRDm
Fi Rg
RZ 10 1
DdFo ||R||rRZ
DdDdF R,r||RrRDo RZ 10
)( DdFmv ||R||rRgA Dmv DRdr,D||RdrFRRgA 10 Voltage gain:
Electronic Devices and Circuit TheoryBoylestad
© 2013 by Pearson Higher Education, IncUpper Saddle River, New Jersey 07458 • All Rights Reserved
Ch.8 Summary
Common-Source Voltage-Divider Bias
The input is applied to the gate and the output is taken from the drain.
There is a 180º voltage phase shift between input and output.
Electronic Devices and Circuit TheoryBoylestad
© 2013 by Pearson Higher Education, IncUpper Saddle River, New Jersey 07458 • All Rights Reserved
Ch.8 Summary
Calculations
21||RRZ i
Input impedance:
Output impedance:
Ddo ||RrZ
10drDo RZ
)( Ddmv ||RrgA Dd RrDmv RgA 10Voltage gain:
Electronic Devices and Circuit TheoryBoylestad
© 2013 by Pearson Higher Education, IncUpper Saddle River, New Jersey 07458 • All Rights Reserved
Ch.8 Summary
Summary Table
Electronic Devices and Circuit TheoryBoylestad
© 2013 by Pearson Higher Education, IncUpper Saddle River, New Jersey 07458 • All Rights Reserved
Ch.8 Summary
Summary Table
Electronic Devices and Circuit TheoryBoylestad
© 2013 by Pearson Higher Education, IncUpper Saddle River, New Jersey 07458 • All Rights Reserved
Ch.8 Summary
Summary Table
Electronic Devices and Circuit TheoryBoylestad
© 2013 by Pearson Higher Education, IncUpper Saddle River, New Jersey 07458 • All Rights Reserved
Ch.8 Summary
Troubleshooting
.
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:
If not correct check FET, capacitors and the loading effect of the next stage
Electronic Devices and Circuit TheoryBoylestad
© 2013 by Pearson Higher Education, IncUpper Saddle River, New Jersey 07458 • All Rights Reserved
Ch.8 Summary
Practical Applications
• Three-Channel Audio Mixer
• Silent Switching
• Phase Shift Networks
• Motion Detection System