Electronic devices-and-circuit-theory-10th-ed-boylestad-chapter-8

Introduction
FETs provide:
• Excellent voltage gain
• High input impedance
• Low-power consumption
• Good frequency range
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
2

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:
D
m
I
g
Δ =
GS
V
Δ
3

Graphical Determination of gm
4

Mathematical Definitions of gm
D
I
Δ
m V
GS
g
Δ
=
⎤
⎥⎦
⎡
DSS
2I
= −
⎢⎣
GS
m V
P
P
V
1
V
g
DSS
m0 V
P
2I
g =
⎡
⎤
Where VGS =0V
⎥⎦
= −
⎢⎣
GS
m m0 V
P
V
g g 1
D
DSS
V
GS
1 − =
P
I
I
V
⎛
V
⎞
I
Where
D
DSS
m0
GS
1 g g = ⎟⎟⎠
m m0 I
P
g
V
⎜ ⎜⎝
= −
5

FET Impedance
Input impedance:
p
Zi = ∞Ω
Output Impedance:
Z = r = 1
o d y
yos
ΔV
where:
Δ
d I GS
V constant
DS
D
V
r = =
Δ
yos= admittance parameter listed on FET specification sheets.
6

FET AC Equivalent Circuit
7

Common-Source (CS) Fixed-Bias Circuit
The input is on the gate and the
output is on the drain
There is a 180° phase shift
between input and output
8

Calculations
Input impedance:
Zi = RG
p Output p
impedance:
Zo = RD || rd
Z R
o D r ≥
10R
d D
≅
Voltage gain:
o
A m d D
g (r || R )
V
v = = −
V
i
v g R
rd 10RD m D
o
V
i
V
A = = −
≥
9

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
There is a 180° phase shift between
input and output
10

Calculations
Input impedance:
Z Zi = R
RG
p Output p
impedance:
Zo = rd || RD
Z R
o D r ≥
10R
d D
≅
Voltage gain:
Av = −gm(rd || RD)
Av = −
gmRD rd ≥
10RD Copyright ©2009 by Pearson Education, Inc.
11

Common-Source (CS) Self-Bias Circuit
Removing Cs affects
the gain of the circuit.
12

Calculations
Input impedance:
Zi i = G
RG
Output impedance:
Z R ≥
o D r 10R
d D
≅
Voltage gain:
Vo gmRD
A = =
−
R R
D S
d
m S
i
v
r
V 1 g R
+
+ +
Vo o
m gmRD
D
A r 10(R R )
v 1 g R d D S
m S
i
g
V
≥ + +
= = −
13

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

Impedances
Input impedance:
Zi = R1 || R2
Output impedance:
Zo = rd || RD
Z R ≥
o D r 10R
d D
≅
Voltage gain:
Av = −gm(rd || RD )
Av = −
gmRD rd ≥
10RD Copyright ©2009 by Pearson Education, Inc.
15

Source Follower (Common-Drain) Circuit
In a common-drain amplifier
configuration, the input is on the
gate, but the output is from the
source.
There is no phase shift between
input and output.
16

Impedances
Input impedance:
Z Zi = R
RG
Output impedance:
Z = r || R || 1
m
o d S g
1
Z R || ≅ ≥
rd 10RS
m
o S g
Voltage g (r || R )
m d S
gain:
= =
v +
1 g (r || R )
V
o
V
A
m d S
i
V
A = =
+
≥ v 1 g R d
r 10
m S
m S
o
i
g R
V
17

Common-Gate (CG) Circuit
The input is on the source
and the output is on the
drain.
There is no phase shift
between input and output.
18

Calculations
Input impedance:
⎤
⎥⎦
⎡
+
⎢⎣
r +
R
=
d D
i S 1 g r
m d
Z R ||
1
Z R || ≅ ≥
rd 10RD
m
i S g
Output impedance:
Zo = RD || rd
Voltage o D rd 10 Z ≅ R ≥
⎤
⎥⎦
⎡
⎢
⎣
+
D
R
= = ⎦
A d
v = m D rd ≥
10RD g R
m D
gain:
o
r
V
A g R
⎤
⎥⎦
⎡
+
⎢⎣
D
R
d
i
v
r
1
V
19

D-Type MOSFET AC Equivalent
20

E-Type MOSFETAC Equivalent
gm and rd can be found in
the specification sheet for
the FET.
21

Common-Source Drain-Feedback
There is a 180° phase shift
between input and output.
22

Calculations
Input impedance:
R r || R
F d D
1 g (r || R )
Z
m d D
i +
+
=
R ||R 10R
F
i
R
Z ≅
≥
RF rd RD ,rd 10RD
>> +
1 g R
m D
Output impedance:
p p
Zo = RF || rd ||RD
Zo ≅ RD RF >>rd || RD , rd ≥10RF d D d D
Voltage gain:
Av = −gm(RF || rd || RD )
Av ≅ −gmRD RF>>rd||RD,rd≥10RD
23

Common-Source Voltage-Divider Bias
24

Calculations
Input impedance:
Zi=R1|| R2
O t Output t i d
impedance:
Zo = rd || RD
Zo RD rd 10 Z ≅ R ≥
Voltage gain:
Av = −gm(rd || RD)
A ≅ g R
Av −gmRD rd ≥10RD
25

Summary Table
more…
26

Summary Table
27

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

Practical Applications
Three-Channel Audio Mixer
Silent Switching
Phase Shift Networks
Motion Detection System
29

Electronic devices-and-circuit-theory-10th-ed-boylestad-chapter-8

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