This document discusses a 3-stage AC-coupled amplifier. It consists of a common-collector (C-C) amplifier, followed by common-source (C-S) and common-emitter (C-E) amplifiers. Capacitors are used for AC coupling between stages to isolate the DC levels while allowing the transfer of AC signals. Bypass capacitors are also used to maximize voltage gain from the inverting amplifiers. The small-signal equivalent circuit is derived and used to determine the overall output resistance of the multi-stage amplifier configuration. Finally, general formulas are presented for calculating input resistance, output resistance, and voltage gain for N-stage 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-