Download Common Source Amplifier

Common Source Amplifier
VDD
RD
C3
C1
C2
vs
RL
RG
I
-VSS
• A discrete common source amplifier can be constructed that is very similar in
form to a common emitter
• Biasing with a current source is quite common, particularly for ICs
• But IC amplifiers do not include decoupling/bypass capacitors and biasing
resistors
Lecture 22-1
Common Source Amplifier
• Design the amplifier for maximum possible output voltage swing
VDD
RD
RS
C3
C1
C2
vs
RL=10kΩ
K=0.25mA/V2
Vt=2V
VDD=10V
RG=500k ( >> RS)
λ=0.02
RG
I=1mA
-VSS
Lecture 22-2
Common Source Amplifier
VDD
RD
RS
C3
C1
C2
vs
RL=10kΩ
K=0.25mA/V2
Vt=2volts
VDD=10volts
RG=500k ( >> RS)
RG
I=1mA
-VSS
Lecture 22-3
Common Source Amplifier
• Analyze the circuit --- first solve for the dc bias point:
IC
+
VCC
10V
1.000 mA
RD
6KΩ
C12
1UF
+
+
M1-NMOS
L=10U
W=10U
D
VSSIN
SIN
C1
1UF
+
S
RG
5E5Ω
C2
1UF
+
VIN
+
0.000 pV
-
VOUTAC
+
0.000 pV
-
-
RL
10KΩ
VD
+
4.000 V
-
IS
1mA
VS
+
VSS
-10V
+
-3.837 V
-
Lecture 22-4
Small Signal Model
VDD
RD
C3
C1
C2
vs
RG
D
G
vs
RL
RG
gm vs
ro
RD
RL
S
I
-VSS
Lecture 22-5
Small Signal Model
D
G
vs
RG
gm vs
ro
RD
RL
S
Lecture 22-6
ac Response
• Frequency response with lamda=0.02
e2
e3
e4
e5
e6
frequency
e7
12
11
10
9
8
7
6
5
DB(VOUTAC/VIN)
Lecture 22-7
Transient Response
• Time-domain response for a 10kHz, 0.2v peak ac input signal with
lamda=0.02
0.0
0.1
0.2
0.3
0.4
0.5
time
0.6 ms
1
V
0
-1
VOUTAC
VIN
Lecture 22-8
Transient Response Distortion?
• Vin multiplied by the midband gain, and shifted in phase by 180°
demonstrates that there is very little distortion
0.0
0.1
0.2
0.3
0.4
0.5
time
0.6 ms
1
V
0
-1
VOUTAC
VIN
VIN*(-3.75)
Lecture 22-9
ac Response
• Response for a 10kHz, 0.2v peak ac input signal with lamda=0
• How can we change the design so that lamda is practically zero?
e2
e3
e4
e5
e6
frequency
e7
12
11
10
9
8
7
6
5
DB(VOUTAC/VIN)
Lecture 22-10
Improving the Gain
• Assuming that the peak ac output is less than 1.0 volt, we can use a larger
value of RD to increase the gain but still keep the transistor out of the triode
region
IC
+
VCC
10V
1.000 mA
RD
10KΩ
C12
1UF
+
+
M1-NMOS
L=10U
W=10U
D
VSSIN
SIN
C1
1UF
+
S
RG
5E5Ω
C2
1UF
+
VIN
+
0.000 pV
-
VOUTAC
+
0.000 pV
-
-
RL
10KΩ
VD
+
0.000 pV
-
IS
1mA
VS
+
VSS
-10V
+
-3.920 V
-
Lecture 22-11
Improving the Gain
e2
e3
e4
e5
e6
frequency
e7
14
13
12
11
10
9
8
DB(VOUTAC/VIN)
Lecture 22-12
Improving the Gain
• But further increase in RD is not possible
0.0
0.1
0.2
0.3
0.4
0.5
time
0.6 ms
1
V
0
-1
VOUTAC
VIN
VIN*(-4.7)
Lecture 22-13
Current Mirrors
3.3V
R
M1
M2
Lecture 22-14
Current Mirrors - Output Resistance
3.3V
R
IREF
Io
ID
M1
M2
Lecture 22-15
Current Mirrors-Accuracy
3.3V
R
IREF
Io
ID
M1
M2
Lecture 22-16
Current Steering
M4
3.3V
R
+
V SG5
-
M5
I4
IREF
I2
I3
I5
ID
M1
M2
M3
Lecture 22-17