Topic 1 : AC Coupled Amplifiers

1. Part 2.2
Multi-Stage and
Differential Amplifiers
Assoc Prof Zheng Yuanjin email: yjzheng@ntu.edu.sg
EE2002 Analog Electronics

2. AC Coupled Amplifier:
An Example of a 3-Stage Amplifier
C-C
amplifier
V
CC
C-S amplifier C-E amplifier
R
D1 R1 RC2R
G1 R3
RI C   C3   C5
 
1 Q2
Q3
M1
C  
6
R
in
RG2 +C  
4
C  
2
+
-
R2vi
R4RE2 RE3
R
S1 v
oR
L
R
out
-
Provides high Rin
and moderate Av
Provides high Av Gives low
high gain stage from low
R and buffersout
RL
• Input and output of overall amplifier is AC-coupled through capacitors C and C .1 6
• Bypass capacitors C and C are used to get maximum voltage gain from the2 4
two inverting amplifiers.
EE2002 Analog Electronics Multi-Stage and Differential Amplifiers 6
AC Coupled Amplifier:
An Example of a 3-Stage Amplifier

3. AC-Coupled Amplifier:
DC Equivalent Circuit
C-C
amplifier
V
CC
C-S amplifier C-E amplifier
R
C2R1
R
D1
R3
RI C   C3   C  
5
1 Q2
Q3
M1
C  
6
RG1 +
C  
2
C  
4
+
-
R2vi
R4
R
S1 RE2 RE3
RL vo
-
Inter-stage coupling capacitors C and C transfer AC signals between amplifiers3 5
but provide isolation at DC, and prevent Q-points of Xtrs from being affected.
EE2002 Analog Electronics Multi-Stage and Differential Amplifiers 7
AC Coupled Amplifier:
DC Equivalent Circuit

4. AC-Coupled Amplifier:
Small-signal Equivalent Circuit
  V
CC
4
R R R R R RB2 1 2 B3 3
R
1 RC2
R
D1
R3
RI C  
3
C  
5
C  
1 Q2 Q3M1
C  
6
RinRG1
+
+
-
C  
2
C4
 
v
i R2 RE2
R
4
R
E3R
S1
vo
R
out
R
L
-
+ v -R
I
B3 3 E3G1 D1 B2 C2
++ + r
p3
g v g v
m2 2
m1 1
r
o1
R
out+
-
vi v2 rp2
g v
m3 3
vo
-
Rin
R v
1
R
B2 RE3 R
L
G1 RD1
ro2 RC2 RB3
ro3
- - C3S1 E2
R  R RR  R R I 2 C2 B3I1 D1 B2
EE2002 Analog Electronics Multi-Stage and Differential Amplifiers 8
AC Coupled Amplifier:
Small-signal Equivalent Circuit

7. AC-Coupled Amplifier:
Output Resistance
+ v -R
I
B3 3G1 D1 B2 C2 E3
++ + r
p3
g v g vm1 1 m2 2
+
-
R
out
R
L
vi v2 rp2 vo
-
RG1 v1
R
E3ro1 RI1
r
o2 RI2 g v
m3 3
ro3
- - C3S1 E2
R  R R R  R R
I 2 C2 B3
I1 D1 B2
r  r Rp 3 o2 I 2
 r  r R  r  r R
 p 3 o2
I 2 r R p
3
o2 I 2
Rout   1 o3 E3  1  33
EE2002 Analog Electronics Multi-Stage and Differential Amplifiers 11
AC Coupled Amplifier:
Output Resistance

10. AC-Coupled Amplifier in General
v
N
R
I
v1 v2 v3
+
Stage
1
Stage
2
Stage v
o+
-
RLvi
R
in2Rin
N -R
outRout(N-1)
To find RoutTo find Rin To find Avt(i)
v
i+1
i
xix vi
Stage StageStage
1
+
-
+
-
v
x
v
x
R
in(i+1)
iRin2 Rout(N-1)
N
EE2002 Analog Electronics Multi-Stage and Differential Amplifiers 14

11. Multi-Stage and
Differential Amplifiers
-End of Lecture-