Common Source Amplifier VDD
RD C3
C1
C2 RL
vs 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 2 RD K=0.25mA/V C 3 Vt=2V RS C1 VDD=10V RG=500k ( >> RS) C2 R =10kΩ L λ=0.02
vs RG I=1mA
-VSS
Lecture 22-2
Common Source Amplifier
VDD K=0.25mA/V2 RD C3 Vt=2volts VDD=10volts RS C1 RG=500k ( >> RS) Ω C2 RL=10k vs 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
M1-NMOS C1 L=10U 1UF W=10U +- C12 D VOUTAC VD 1UF + + +- S C2 0.000 pV 4.000 V - - 1UF RL VSSIN RG VIN +- 10KΩ + SIN 5E5Ω + 0.000 pV IS - 1mA
VS VSS + + -10V -3.837 V -
Lecture 22-4
Small Signal Model
VDD D RD G C3 C 1 v R R R s G gmvs ro D L C2 RL S vs RG I
-VSS
Lecture 22-5
Small Signal Model
G D v R R R s G gmvs ro D L S
Lecture 22-6
ac Response
• Frequency response with lamda=0.02
frequency e2 e3 e4 e5 e6 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
time 0.0 0.1 0.2 0.3 0.4 0.5 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
time 0.0 0.1 0.2 0.3 0.4 0.5 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?
frequency e2 e3 e4 e5 e6 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
M1-NMOS C1 L=10U 1UF W=10U +- C12 D VOUTAC VD 1UF + + +- S C2 0.000 pV 0.000 pV - - 1UF RL VSSIN RG VIN +- 10KΩ + SIN 5E5Ω + 0.000 pV IS - 1mA
VS VSS + + -10V -3.920 V -
Lecture 22-11 Improving the Gain
frequency e2 e3 e4 e5 e6 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
time 0.0 0.1 0.2 0.3 0.4 0.5 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
I R REF Io
ID M1 M2
Lecture 22-15 Current Mirrors-Accuracy
3.3V
I R REF Io
ID M1 M2
Lecture 22-16 Current Steering + V - SG5 M4 M5 3.3V I4 I R REF I2 I5 I3 ID M1 M2 M3
Lecture 22-17