Mosfet electronics

1. Lecture 19 – FET Amplifiers
RIJIL RAMCHAND

2. 7/10/2023 NITC
A Simple CS Amplifier and Variation in IDS with Vgs

3. CS Amplifier
7/10/2023 NITC
Figure Common-source amplifier.

4. CS Amplifier
7/10/2023 NITC
For drawing an a c equivalent circuit of Amp.
•Assume all Capacitors C1, C2, Cs as short
circuit elements for ac signal
•Short circuit the d c supply
•Replace the FET by its small signal model

5. Analysis of CS Amplifier
7/10/2023 NITC
L
gs
m
L
o
o
gs
o
v
R
v
g
R
i
v
v
v
A





gain,
Voltage
d
D
L
L
m
gs
o
v
r
R
R
R
g
v
v
A 



 ,
D
d
D
d
D
d
o R
r
R
r
R
r
Z



imp.,
put
Out
2
1
imp.,
Input R
R
R
Z
G
in


A C Equivalent Circuit
Simplified A C Equivalent Circuit

6. 7/10/2023 NITC
Analysis of CS Amplifier with Potential Divider Bias
)
R
||
(r
g
Av D
d
m


D
R
10
r
D,
m
d
R
g
Av 

 
)
R
||
(r
g
Av D
d
m


This is a CS amplifier configuration therefore the
input is on the gate and the output is on the drain.
2
1 R
||
R
Zi 
D
d R
||
r
Zo 
D
d
D
10R
r
R
Zo



7. 7/10/2023 NITC
Figure vo(t) and vin(t) versus time for the common-source amplifier

8. 7/10/2023 NITC
FET Amplifier Analysis:
g
s
rd
gmvp
vi = vp
ii
io
vo
d
s
+ +
_ _
mid-frequency CE amplifier circuit
RD RL
RTh
vs
+
_
is
' '
o o i
vi m L L d D L vs vi
i s s i
i
i Th Th 1 2
i
Analysis of the CS mid-frequency circuit above yields:
v v Z
A = = -g R , where R = r R R A = = A
v v R + Z
v
Z = = R , where R = R R
i
 
 
 
L
o i
I vi
i L
o o
o d D P vi I
o i
seen by R
i Z
A = = A
i R
v p
Z = = r R A = = A A
i p
 
 
 
A common source (CS) amplifier is shown
to the right.
Rs
Ci
RL
Co
CSS
vi
vo
+
+
vs
+
_
_
_
io
ii
D
S
G
VDD VDD
R1
RSS
RD
R2
The mid-frequency circuit is drawn as follows:
• the coupling capacitors (Ci and Co) and the
bypass capacitor (CSS) are short circuits
• short the DC supply voltage (superposition)
replace the FET with the hybrid-p model
The resulting eqvt. circuit is shown below.

9. 7/10/2023 NITC
Procedure: Analysis of an FET amplifier
1) Find the DC Q-point. This will insure that the FET is
operating in the saturation region and these values are
needed for the next step.
2) Find gm. If gm is not specified, calculate it using the DC
values of VGS as follows:
3) Calculate the required values (typically Avi, Avs, AI, Zi,
and Zo. Use the formulas for the appropriate amplifier
configuration (CS, CG, CD, etc).
 
 
DSS
D
m GS P
2
GS P
D
m GS T
GS
GS
2I
I
g = = V - V (for JFET's and DM MOSFET's)
V V
I
g = = V - V (for EM MOSFET's)
V
(Note: Uses DC value of V )
K





10. 7/10/2023 NITC
Problem:
Find the values for Avi, Avs, AI, Zi, and Zo for the amplifier shown
below. Assume that Ci, Co, and CSS are large.
Note that the biasing circuit provides VGS = -0.178 V.
The JFET has the following specifications:
IDSS = 4 mA, VP = -1.46 V, rd = 50k Ohm
10 k
Ci
8 k
Co
CSS
vi
vo
+
+
vs
+
_
_
_
io
ii
D
S
G
18 V 18 V
800 k
2 k
500
400 k

11. 7/10/2023 NITC
Figure Common-source amplifier.
An Amplifier Circuit using MOSFET(CS Amp.)

12. 7/10/2023 NITC
Figure Small-signal equivalent circuit for the common-source amplifier.
A small signal equivalent circuit of CS Amp.

13. 7/10/2023 NITC
Figure Source follower.
Common Drain Amplifier (Source Follower)

14. 7/10/2023 NITC
Figure Small-signal ac equivalent circuit for the source follower.
Common Drain Amplifier (Source Follower)

15. 7/10/2023 NITC
Figure Equivalent circuit used to find the output resistance of the source follower.
Common Drain Amplifier (Source Follower)

16. 7/10/2023 NITC
Figure Common-gate amplifier.
Common Gate Amplifier

17. 7/10/2023 NITC
Common Gate Amplifier
Figure Small-signal ac equivalent circuit for the common gate amplifier.

18. 7/10/2023 NITC
'
' ' m L
vi m L m L '
m L
'
L d D L d D L SS L
i Th SS Th
m
o d D d D SS
m
i i i
vs vi vi vi
s i s i s i
i i i
I vi vi vi
L L L
P vi I vi I
CS CG CD
g R
A -g R g R
1 g R
R r R R r R R R R
1
Z R R R
g
1
Z r R r R R
g
Z Z Z
A A A A
R + Z R + Z R + Z
Z Z Z
A A A A
R R R
A A A A A

     
     
     
     
     
     
vi I
Th 1 2
A A
where R = R R
VCC
RD
S
R2
RSS
Rs
Ci
RL
Co
C2
vi
vo
+
+
vs
+
_
_
_
io
ii
Common Gate (CG) Amplifier
R1
D
G
Note: The biasing circuit is the same for each amp.
Rs
Ci
RL
Co
CSS
vi
vo
+
+
vs
+
_
_
_
io
ii
D
S
G
VDD VDD
R1
RSS
RD
R2
Common Source (CS) Amplifier
Rs
Ci
vi
+
vs
+
_
_
ii G
VDD VDD
R1
RSS
R2
Common Drain (CD) Amplifier (also called “source follower”)
RL
Co
vo
+
_
io
D
S
FET Amplifier Configurations and Relationships: