This document discusses multi-stage amplifiers. It begins by explaining the limitations of single-stage amplifiers and how multi-stage amplifiers can increase gain by connecting multiple amplifiers together. While the overall gain is not simply the product of individual stage gains, input and output impedances must be accounted for. Practical multi-stage designs then use coupling capacitors between stages so that biasing networks can be designed independently and allow for DC operation. Negative feedback can also improve the predictability and linearity of multi-stage amplifiers.

1. Top school in India
By:
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2. Multi-stage Amplifiers
 Single stage limitations
 Multi-stage amplifiers
 Feedback
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3. Single Stage Amplifier Limitations
COUT
m
IN
Cm
IN
OUT
Rr
g
r
Rg
v
v
=
=
−=
β
High gain ⇒ high gm and RC
High rIN ⇒ low gm
Low rOUT ⇒ low RC
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4. Simple 2 Stage Amplifier
 To increase the
gain, use two
amplifiers instead
of one.
 Gain is not just
the product of the
individual gains
though
 Input and output
impedances must
be accounted for
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5. Equivalent Circuit
12
2
12
OUTIN
IN
ININ
rr
r
vAv
+
= 22 INOUT vAv =
12
2
12
OUTIN
IN
IN
OUT
rr
r
AA
v
v
+
=
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6. Coupling
 In the simple 2 stage design, the stages are
linked by coupling capacitors
 This means that the biasing networks for
each stage can be designed independently
 It also means that d.c. operation is impossible
 To do this:
 The capacitors must go
 Each stage supplies the bias current for the
following stage
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7. Differential Input 2 Stage Amplifier
 Good points
 Q1 and Q2 form a
standard differential
amplifier
 RC1 forms the collector
resistor for Q2 and
provides base bias
current for Q3
 Q3 forms a common-
emitter amplifier
 Bad point
 vOUT can’t go below 0V
BAD DESIGN!
IB3
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8. Using PNP Transistors
 Analysis of PNP based circuit is almost identical to
NPN
 Only real difference is that the currents in a PNP
transistor all flow in the opposite direction
0.5 V
0.5 V
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9. Practical Two Stage Amplifier
 Q3 and RC2 form a
common emitter
amplifier again
 Base bias current for Q3
is provided by Q2
collector
 Output voltage can vary
almost right up to ±VS
 Multi-stage amplifiers
are usually made up
from alternating NPN
and PNP stagesschool.edhole.com

10. Example – 1. Quiescent Conditions
With vIN = 0, we know
that:
A50
2
100
21 µ=== CC II
mA8
50000
5.0
1050200
5.0
6
1
233
=



−×=






−=∴
−
C
CC
R
II β
3
3
1
312
5.0
β
C
C
BRCC
I
R
III +=+=
0.5 V
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11. Example – 2. Differential Stage
5050000
1025
1050
2
1
2
1
2
1
3
6
1
2
1
2
=×
×
×
×=
==
−
−
C
T
C
Cm
IN
C
R
V
I
Rg
v
v
Ω=×=
==
k200
25/05.0
200
2
2
1 mB
IN
IN
gi
v
r
β
Ω= k50OUTrschool.edhole.com

12. Example – 3. Common Emitter
3201000
25
8
2
3
=×−=
−= Cm
B
OUT
Rg
v
v
Ω=== 625
25/8
200
m
IN
g
r
β
Ω= k1OUTr
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13. Example – 4. Overall Response
200
50000625
625
32050
12
2
12
=
+
××=
+
=
OUTIN
IN
IN
OUT
rr
r
AA
v
v
Ω== k2001ININ rr
Ω== k12OUTOUT rr
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14. Predictability and Distortion
Problems
 IC3
 IC3 depends on current
gain of Q3
 This is unpredictable
 Therefore, gain and
quiescent vOUT are also
unpredictable
 Linearity
 To get near the
maximum possible
output, amplifier goes
into the non-linear regionschool.edhole.com

15. Using Feedback
 In exactly the same way as with the operational
amplifier, non-ideal properties of the multi-stage
amplifier can be compensated for using negative
feedback.
 E.g.
 Gain can be made predictable
 Quiescent output level can be corrected (offset voltage
removed)
 Frequency response extended (constant gain-bandwidth
product)
 Explore these ideas during lab session 3.
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16. Transistors vs. Op-Amps
 Operational Amplifiers
 Very simple to use – easy calculations
 Near ideal properties
 Transistors
 Better frequency response
 Lower noise
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17. Today’s Summary
 Multi-stage amplifiers can be considered as
many separate amplifiers connected together
 Use of the equivalent circuits for the
amplifiers eases analysis
 Direct coupling enables low-frequency use
 Negative feedback can improve linearity and
bandwidth
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18. 2B1 Analogue Electronics Summary
 Practical Op-Amp Design
 How negative feedback works
 GB Product, Slew Rate, Saturation etc.
 Non-Linear Op-Amp Applications
 Comparators, Schmitt Triggers, Precision Rectifiers
 Transistor Amplifiers
 Common Emitter Amplifier
 Differential Amplifier
 Multi-stage amplifiers
 Design and analysis of all the above
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