1
ECE 255: L20
Basic Amplifier Configurations: II
(Sedra and Smith, 7th Ed., Sec. 7.3)
Mark Lundstrom School of ECE
Purdue University West Lafayette, IN USA
Spring 2019 Purdue University
Lundstrom: 2019
Announcements
2
1) There is no HW due Monday, but HW7 (Practice
Problems for Exam 2) will be discussed in class on Monday.
2) Exam 2 is Tuesday, March 5, 6:30-7:30 PM PHYS 112
3) Professor Janes will hold a help session on Tuesday in ME 1061 at 1:30 PM.
4) No class on Friday, March 8
Lundstrom: 2019
Announcements
For Exam 2, you should be prepared on:
Lundstrom: 2019
1) DC analysis / design of 4-resisitor bias circuit 2) Small signal models 3) Reducing a circuit diagram to a s.s. equiv. circuit 3) Amplifier configurations 4) CE and CS in detail
(HW5- HW7)
3
Three basic BJT amplifiers
4
+υi−
+
υ0
−
C E
B
CE
+υi−
+
υ0
−
C E
B
CB
B
+υi−
+υ0−
C E
CC
Three basic MOS amplifiers
5
+υi−
+
υ0
−
D S
G
CS
+υi−
+
υ0
−
D S
G
CG
G
+υi−
+υ0−
D S
CD
Outline
6
1) Basic amplifier considerations
2) CE / CS
3) CB / CG
4) Common base example 5) CC (emitter follower) / CD (source follower)
Lundstrom: 2019
Amplifier with source and load
7
+υi−
+υ0−
Rin+−
Ro
Avoυi+−
Rseries
υs RL
Gv =Rin
Rin + Rseries× Avo ×
RLRL + Ro
Lundstrom: 2019
Gi = AvoRinRL
Gp = GvGi
Avo Rin Ro
Common emitter
8
+
υi
−
+
υ0
−
C E
ib = ii
ic
RC+−
Rseries
υsB
Lundstrom: 2019
Small signal analysis directly on the circuit diagram.
Common emitter results
9
Aυo =υoυi
= −gmRC || ro
Rin = rπ
Ro = RC || ro
open circuit voltage gain
Aυ = −gm RC || ro || RL( ) with load attached
Lundstrom: 2019
Common source
10 Lundstrom: 2019
+
υi
−
+
υ0
−
D S
ig = ii
id
RD+−
Rseries
υsG
Small signal analysis directly on the circuit diagram.
Common source results
11
Aυo =υoυi
= −gmRD || ro
Rin = ∞
Ro = RD || ro
open circuit voltage gain
Aυ = −gm RD || ro || RL( ) with load attached
Lundstrom: 2019
Common emitter with “emitter degeneration”
12
+
υi
−
+
υ0
−
C E
ib = ii
ic
RC+−
Rseries
υs
RE
Lundstrom: 2019
Small signal analysis directly on the circuit diagram.
Common emitter with emitter resistor
13
Aυo = −rπ
rπ + β +1( )REgmRC( )
Rin = rπ + β +1( )RE
Ro = RC
Lundstrom: 2019
open circuit voltage gain
Aυo = −rπ
rπ + β +1( )REgmRC || RL( ) with load attached
Common source with source degeneration
14 Lundstrom: 2019
+
υi
−
+
υ0
−
D S
ig = ii
ic
RD+−
Rseries
υs
RE
G
Common source with source resistor
15
Aυo = −1
1+ gmRSgmRD( )
Rin = ∞
Ro = RD
Lundstrom: 2019
open circuit voltage gain
Aυo = −1
1+ gmRSgmRD || RL( ) with load attached
rπ → ∞ β →∞
Aside: taking limits
16 Lundstrom: 2019
rπ → ∞ β →∞
Aυo = −rπ
rπ + β +1( )REgmRC( )
Rin = rπ + β +1( )RE
CS analysis with equivalent circuits
17 Lundstrom: 2019
+
υi
−
+
υ0
−
D S
ig = ii
ic
RD+−
Rseries
υs
RE
G
1) CS with hybrid-pi model
18
+υgs−
+
υ0
−
D
ig = ii id
RD+−
Rseries
υs
gmυgs
G
S RS
+υi
−
Hybrid pi vs. T-model
19
S
D G
ig id
gmυgs
+υgs−
is
S
D
G
ig
id
gmυgs
+υgs−
is1gm
Lundstrom: 2019
Draw the s.s. model using T-model
20 Lundstrom: 2019
+
υi
−
+
υ0
−
D S
ig = ii
ic
RD+−
Rseries
υs
RE
G
Common source with source resistor
21
+υgs−
+
υ0
−
D id
RD
+−
Rseries
υs
gmυgsG
S RS
+υi
−
1 gm
Common source with source resistor
22
Aυo = −1
1+ gmRSgmRD( )
Rin = ∞
Ro = RD
Lundstrom: 2019
open circuit voltage gain
Aυo = −1
1+ gmRSgmRD || RL( ) with load attached
Outline
23 Lundstrom: 2019
1) Basic amplifier considerations
2) CE / CS
3) CB / CG
4) Common base example 5) CC (emitter follower) / CD (source follower)
Common base
24
+υi
−
+
υ0
−
C E
ic
RC
+−
Rseries
υs RE
B
Lundstrom: 2019
Draw the small signal equivalent circuit.
Common base amplifier
25
RC RE
+υi−
υo
rπ
gmυbe
−υbe+
E C
B
υo = −gmυbeRC
υbe = −υi
υo = +gmυiRC
Av =υoυi
= +gmRC
Ro = RC
Rin = ?
Effect of r0?
Looking into current sourse
26
RC RE
+υi−
υo
rπ
gmυbe
−υbe+
E C
B
Rth =1gm
Rth = ∞
Lundstrom: 2019
CB input resistance
27
RC RE
+υx−
υo
rπ
gmυbe
−υbe+
E C
B
Rin ⇒
ix
Rin = RE || ?
Lundstrom: 2019
CB input resistance (ii)
28
RC
+υx−
υo
rπ
gmυbe
−υbe+
E C
B
′Rin ⇒
ix
Rin = RE || rπ || ?
Lundstrom: 2019
CB input resistance (iii)
29
RC
+υx−
υo
gmυbe
−υbe+
E C
B
′′Rin ⇒
ix
Rin = RE || rπ || ? ′′Rin =υxix
= υxgmυx
= 1gm
Lundstrom: 2019
Common base
30
+υi
−
+
υ0
−
C E
ic
RC
+−
Rsυs RE
B
Av =υoυi
= +gmRC
Ro = RC
Rin = RE || rπ ||1gm
Effect of RL?
Lundstrom: 2019
CB analysis on the circuit diagram
31 Lundstrom: 2019
+υi
−
+
υ0
−
C E
RC
+−
Rsυs RE
B
ic
Common base: results
32
Av =υoυi
= +gmRC
Ro = RC
Rin = RE || rπ ||1gm
rπ ||1gm
= rπβ +1
Lundstrom: 2019
Av =υoυi
= +gmRC
Ro = RC
Rin = RE ||rπ
β +1
When can we not do problems by inspection?
33 Lundstrom: 2019
E
C B
ib ic
rπ gmυπ+υπ−
ro
+υce−
rµ
Common emitter amplifier
34 Lundstrom: 2019
+
υi
−
+
υ0
−
C E
ib = ii
ic
RC+−
Rseries
υsB
Small signal equivalent circuit
35 Lundstrom: 2019
+υi =υπ
−
+
υ0
−
C E
ib = ii ic
RC+−
Rseries
υs gmυπrπ
B
rµ
Result
36 Lundstrom: 2019
Sedra and Smith, problem 7.63
Av = −gmRC
Ro = RC
Rin = rπ
Av = −gmRC1−1 gmrµ( )1+ RC rµ
⎡
⎣⎢⎢
⎤
⎦⎥⎥
Ro = RC || rµ
Rin = rπ ||rµ + RC1+ gmRC
⎡
⎣⎢
⎤
⎦⎥
Common base numbers
37
Ro = RC = 10 kΩ
1gm
= 139kΩ = 26Ω
Rin = 25Ω
Lundstrom: 2019
IC = 1.0 mA
gmrπ = β
gm = IC VT = 39 mS
rπ = β gm = 2.6 kΩ
Av = +gmRC = +390RC = 10 kΩ
Rin = RE || rπ ||1gm
RE = 1kΩ
β = 100
Common base to common gate
38 Lundstrom: 2019
+υi
−
+
υ0
−
C E
RC
+−
Rseries
υs RE
B
ic
Common gate
39 Lundstrom: 2019
+υi
−
+
υ0
−
D S
RD
+−
Rseries
υs RS
G
ic
Let’s try to do this directly on the circuit diagram
Input resistance
40 Lundstrom: 2019
+υi
−
+
υ0
−
D S
RD
RS
G
ic
Rin ⇒
Input resistance (ii)
41 Lundstrom: 2019
+υi
−
+
υ0
−
D S
RD
RS
G
Rin = RS || ?
Rin ⇒
gmυgs+υgs−
Input resistance (iii)
42 Lundstrom: 2019
+υx−
+
υ0
−
D S
RD
G
Rth ⇒
gmυgs+υgs−
ix
ix = −gmυgs
ix = +gmυx
Rth =υxix
= 1gm
Common base vs. common gate
43
Av =υoυi
= +gmRC
Ro = RC
Rin = RE || rπ ||1gm
Lundstrom: 2019
Av =υoυi
= +gmRD
Ro = RD
Rin = RS ||1gm
CB CG
rπ ||1gm
= rπβ +1
Outline
44
1) Basic amplifier considerations
2) CE / CS
3) CB / CG
4) Common base example 5) CC (emitter follower) / CD (source follower)
Lundstrom: 2019
Realistic Common base amplifier
45
V + = +10 V
RC
RE
V − = −10 V
υi
υo
CC1
CC 2
RoAvo Rin
Compute:
Size of Capacitors?
46
Audio frequencies: 20 – 20,000 Hz
Lundstrom: 2019
Realistic Common base amplifier
47
V + = +10 V
RC
RE
V − = −10 V
υi
υo
CC1
CC 2
RoAvo Rin
Compute:
DC analysis first
DC analysis
48
VBE≈ 0.7 V
β = 100
V + = +10 V
V − = −10 V
RC = 5 kΩ
RE = 10 kΩIE
IC
IB
Lundstrom: 2019
DC analysis
49
VBE≈ 0.7 V
β = 100
V + = +10 V
IE = 0.93mA
V − = −10 V
IC = 0.92 mA
VC = −5.4 V
VE = +0.7 V RC = 5 kΩ
RE = 10 kΩIE
IC
IB
Lundstrom: 2019
gm =ICVT
= 35.4 mS
rπ =βgm
= 2.8 kΩ
Realistic Common base amplifier
50
V + = +10 V
V − = −10 V
υi
υo
CC1
CC 2
RoAvo Rin
Compute:
RC = 5 kΩ
RE = 10 kΩ
Outline
51 Lundstrom: 2019
1) Basic amplifier considerations
2) CE / CS
3) CB / CG
4) Common base example 5) CC (emitter follower) / CD (source follower)
Common collector (emitter follower)
52
+
υi
−
+
υ0
−
C E
ib = ii
ic
+−
Rseries
υs
RE = 1kΩ
Lundstrom: 2019
Let’s do the analysis directly on the circuit diagram
CC by inspection
Aυo =υoυi
= +β +1( )RE
rπ + β +1( )RE
Ro = RE ||rπ + Rseriesβ +1
⎛⎝⎜
⎞⎠⎟
Rin = rπ + β +1( )RE
Lundstrom: 2019 53
Aυo ≈1
Emitter follower
CC is not a unilateral amplifier
CC by inspection
Aυo =υoυi
= +β +1( )RE
rπ + β +1( )RE
Ro = RE ||rπ + Rseriesβ +1
⎛⎝⎜
⎞⎠⎟
Rin = rπ + β +1( )RE
Hybrid pi model
Aυo =υoυi
= + RErπβ +1
+ RE= REre + RE
Rin = rπ + β +1( )RE = β +1( ) re + RE( )
Ro = RE || re +Rseriesβ +1
⎛⎝⎜
⎞⎠⎟
T model Lundstrom: 2019 54
Common collector analysis with hybrid-pi model
55
+
υi
−
+
υ0
−
C E
ib = ii
ic
+−
Rseries
υs
RE
Lundstrom: 2019
Re-draw the circuit using the hybrid-pi model.
Small signal equivalent circuit
56
+
υi
− +
υ0
−
E
+−
Rseries
υs
RE
Lundstrom: 2019
rπ gmυbe
B
+υbe−
C
Small signal analysis
57 Lundstrom: 2019
Aυo = ?
Ro = ?
Rin = ? RE = 1kΩ
CC numbers
Aυo =101( )1
2.6 + 101( )1 = +0.98
Rin = 2.6 kΩ+ 101( )1kΩ = 104 kΩ
Ro = 1kΩ ||2.6 kΩ101
= 25Ω
1gm
= 139kΩ = 26Ω
Aυo =υoυi
= +β +1( )RE
rπ + β +1( )RE
Ro = RE ||rπ
β +1⎛⎝⎜
⎞⎠⎟≈ RE ||
1gm
⎛⎝⎜
⎞⎠⎟
Rin = rπ + β +1( )RE
58
“emitter follower”
IC = 1.0 mA gm = 39 mS rπ = 2.6 kΩβ = 100
CC is a “voltage buffer”
Lundstrom: 2019 59
+υi−
+υ0−
Rin+−
Ro
Avoυi+−
Rseries
υs RL
Gv =Rin
Rin + Rseries× Avo ×
RLRL + Ro
Gi = AvoRinRL
Gp = GvGi
Avo Rin Ro
CC to CD
60
+
υi
−
+
υ0
−
C E
ib = ii
ic
+−
Rseries
υs
RE
Lundstrom: 2019
Common Drain (Source follower)
61
+
υi
−
+
υ0
−
D S
ic
+−
Rseries
υs
RS
Lundstrom: 2019
Let’s do the analysis directly on the circuit diagram
G
+υgs−
CC vs. CD
Aυo =υoυi
= +β +1( )RE
rπ + β +1( )RE
Ro = RE ||rπ
β +1⎛⎝⎜
⎞⎠⎟≈ RE ||
1gm
⎛⎝⎜
⎞⎠⎟
Rin = rπ + β +1( )RE
62
Aυo =υoυi
= + gmRS1+ gmRS
Ro = RS ||1gm
⎛⎝⎜
⎞⎠⎟
Rin = ∞
source follower
Lundstrom: 2019
Outline
63 Lundstrom: 2019
1) Basic amplifier considerations
2) CE / CS
3) CB / CG
4) Common base example 5) CC (emitter follower) / CD (source follower)
BJT Amplifier summary
Common emitter Common base Common collector
Ro = RC
Av =β +1( )RE
rπ + β +1( )RE
Ro = RE ||rπ
β +1⎛⎝⎜
⎞⎠⎟
Rin = rπ + β +1( )RERin = RE ||rπ
β +1
Aυo = +gmRC
Ro = RC
Aυo = −gmRC
Rin = rπ
< 0, | high | >0, | high | ~1
moderate low high
moderate moderate low 64
BJT Amplifier summary
Common emitter with RE
Aυo = −rπ
rπ + β +1( )REgmRC( )
Rin = rπ + β +1( )RE
Ro = RC
< 0 , |moderate|
moderate
high
Lundstrom: 2019 65
MOS Amplifier summary
Common source Common gate Common drain
66 Lundstrom: 2019
Summary
67
The Common Emitter amplifier has a voltage gain that is large in magnitude and with a negative sign. It has moderate input and output resistance.
The Common Base amplifier has a voltage gain that is large in magnitude and with a positive sign. It has low input resistance and moderate output resistance.
The Common Source amplifier has a voltage gain that is moderate in magnitude and with a negative sign. It has a high input resistance and moderate output resistance.
The Common Gate amplifier has a voltage gain that is lmoderate in magnitude and with a positive sign. It has fairly low input resistance and moderate output resistance.
Summary
68
The Common Collector (emitter follower) amplifier has a voltage gain of about one, It has high input resistance and a very low output resistance.
The Common Drain (source follower) amplifier has a voltage gain of a little less than one, It has a very high input resistance and a moderately low output resistance.
Lundstrom: 2019
Basic Amplifier Configurations: II
Lundstrom: 2019 69
1) Basic amplifier considerations
2) CE / CS
3) CB / CG
4) Common base example 5) CC (emitter follower) / CD (source follower)
