The document discusses FET amplifiers, including: - FETs provide excellent voltage gain, high input impedance, low power consumption, and a good frequency range. - Transconductance (gm) is the relationship between a change in drain current (ID) to the corresponding change in gate-source voltage (VGS). - There are several common FET amplifier configurations - common-source, common-gate, common-drain, and their input/output relationships and calculations for voltage gain, input and output impedances.

1. Chapter 8:
FET Amplifiers

2. Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Introduction
FETs provide:
• Excellent voltage gain
• High input impedance
• Low-power consumption
• Good frequency range
2

3. Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
FET Small-Signal Model
Transconductance
The relationship of a change in ID to the corresponding change in
VGS is called transconductance
Transconductance is denoted gm and given by:
GS
D
m
V
I
g
Δ
Δ

3

4. Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Graphical Determination of gm
4

5. Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Mathematical Definitions of gm
GS
D
m
V
I
g











P
GS
P
DSS
m
V
V
1
V
2I
g
P
DSS
m0
V
2I
g 








P
GS
m0
m
V
V
1
g
g
DSS
D
P
GS
I
I
V
V
1 

DSS
D
m0
P
GS
m0
m
I
I
g
V
V
1
g
g 










Where VGS =0V
Where
5

6. Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky



i
Z
os
d
o
y
1
r
Z 

constant
V
D
DS
d GS
I
V
r 



Input impedance:
FET Impedance
Output Impedance:
where:
yos= admittance parameter listed on FET specification sheets.
6

7. Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
FET AC Equivalent Circuit
7

8. Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Common-Source (CS) Fixed-Bias Circuit
The input is on the gate and the
output is on the drain
8
There is a 180 phase shift
between input and output

9. Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Calculations
G
i R
Z 
d
D
o r
||
R
Z 
10R
r
D
o
D
d
R
Z


Input impedance:
Output impedance:
9
)
R
||
(r
g
V
V
A D
d
m
i
o
v 


D
d 10R
r
D
m
i
o
v R
g
V
V
A




Voltage gain:

10. Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Common-Source (CS) Self-Bias Circuit
This is a common-source amplifier
configuration, so the input is on the gate
and the output is on the drain
10
There is a 180 phase shift between
input and output

11. Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Calculations
G
i R
Z 
D
d
o R
||
r
Z 
10R
r
D
o
D
d
R
Z


Input impedance:
Output impedance:
11
)
R
||
(r
g
A D
d
m
v 

D
d 10R
r
D
m
v R
g
A



Voltage gain:

12. Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Common-Source (CS) Self-Bias Circuit
Removing Cs affects
the gain of the circuit.
12

13. Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Calculations
G
i R
Z 
10R
r
D
o
D
d
R
Z


Input impedance:
Output impedance:
13
d
S
D
S
m
D
m
i
o
v
r
R
R
R
g
1
R
g
V
V
A






)
R
(R
0
1
r
S
m
D
m
i
o
v S
D
d
R
g
1
R
g
V
V
A 





Voltage gain:

14. Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Common-Source (CS) Voltage-Divider Bias
This is a common-source
amplifier configuration, so the
input is on the gate and the
output is on the drain.
14

15. Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Impedances
2
1
i R
||
R
Z 
D
d
o R
||
r
Z 
10R
r
D
o
D
d
R
Z


Input impedance:
Output impedance:
15
)
R
||
(r
g
A D
d
m
v 

D
d 10R
r
D
m
v R
g
A



Voltage gain:

16. Copyright ©2009 by Pearson Education, Inc.
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Source Follower (Common-Drain) Circuit
In a common-drain amplifier
configuration, the input is on the
gate, but the output is from the
source.
16
There is no phase shift between
input and output.

17. Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Impedances
m
S
d
o
g
1
||
R
||
r
Z 
S
d 10R
r
m
S
o
g
1
||
R
Z 

G
i R
Z 
Input impedance:
Output impedance:
17
)
R
||
(r
g
1
)
R
||
(r
g
V
V
A
S
d
m
S
d
m
i
o
v



10
r
S
m
S
m
i
o
v d
R
g
1
R
g
V
V
A 



Voltage gain:

18. Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Common-Gate (CG) Circuit
The input is on the source
and the output is on the
drain.
18
There is no phase shift
between input and output.

19. Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Calculations
Input impedance:
Output impedance:









d
m
D
d
S
i
r
g
1
R
r
||
R
Z
D
d 10R
r
m
S
i
g
1
||
R
Z 

d
D
o r
||
R
Z 
10
r
D
o d
R
Z 

19
















d
D
d
D
D
m
i
o
v
r
R
1
r
R
R
g
V
V
A 10R
r
D
m
v D
d
R
g
A 

Voltage gain:

20. Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
D-Type MOSFET AC Equivalent
20

21. Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
E-Type MOSFET AC Equivalent
gm and rd can be found in
the specification sheet for
the FET.
21

22. Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Common-Source Drain-Feedback
22
There is a 180 phase shift
between input and output.

23. Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Calculations
Input impedance:
Output impedance:
)
R
||
(r
g
1
R
||
r
R
Z
D
d
m
D
d
F
i



D
d
D
d
F 10R
r
,
R
||
r
R
D
m
F
i
R
g
1
R
Z 



D
d
F
o ||R
r
||
R
Z 
D
d
D
d
F 10R
r
,
R
||
r
R
D
o R
Z 


23
)
R
||
r
||
(R
g
A D
d
F
m
v 

D
m
v D
10R
d
,r
D
||R
d
r
F
R
R
g
A 



Voltage gain:

24. Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Common-Source Voltage-Divider Bias
24

25. Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Calculations
2
1
i R
||
R
Z 
D
d
o R
||
r
Z 
Input impedance:
Output impedance:
10
r
D
o d
R
Z 

25
)
R
||
(r
g
A D
d
m
v 

D
d 10R
r
D
m
v R
g
A 


Voltage gain:

26. Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Summary Table
more…
26

27. Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Summary Table
27

28. Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
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
28

29. Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Practical Applications
Three-Channel Audio Mixer
Silent Switching
Phase Shift Networks
Motion Detection System
29