Department of Electrical and ECSE-330B Electronic Circuits ...
Department of Electrical and ECSE-330B Electronic Circuits ... Electronics... · Outline of Chapter...
Transcript of Department of Electrical and ECSE-330B Electronic Circuits ... Electronics... · Outline of Chapter...
![Page 1: Department of Electrical and ECSE-330B Electronic Circuits ... Electronics... · Outline of Chapter 5 • 1- Introduction to The Bipolar Junction Transistor • 2- Active Mode Operation](https://reader034.fdocuments.in/reader034/viewer/2022051919/600b6c1d625fb748711d803d/html5/thumbnails/1.jpg)
Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 1
Outline of Chapter 5• 1- Introduction to The Bipolar Junction Transistor• 2- Active Mode Operation of BJT• 3- DC Analysis of Active Mode BJT Circuits• 4- BJT as an Amplifier• 5- BJT Small Signal Models• 6- CEA, CEA with RE, CBA, & CCA • 7- Integrated Circuit Amplifiers
![Page 2: Department of Electrical and ECSE-330B Electronic Circuits ... Electronics... · Outline of Chapter 5 • 1- Introduction to The Bipolar Junction Transistor • 2- Active Mode Operation](https://reader034.fdocuments.in/reader034/viewer/2022051919/600b6c1d625fb748711d803d/html5/thumbnails/2.jpg)
Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 2
Small Signal Model• Apply small signal
at base: vs(t)=vbe(t)• Results in signal
current, ic(t), at collector
• Can we generate a small signal model based on what we know so far
)()( tvgti bemc =
Cbemc
Ccc
RtvgtvRitv
)()()(0−==−
![Page 3: Department of Electrical and ECSE-330B Electronic Circuits ... Electronics... · Outline of Chapter 5 • 1- Introduction to The Bipolar Junction Transistor • 2- Active Mode Operation](https://reader034.fdocuments.in/reader034/viewer/2022051919/600b6c1d625fb748711d803d/html5/thumbnails/3.jpg)
Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 3
Small Signal Model: vbe controls ic
• In response to signal input between base and emitter (vbe), signal current flows in collector (ic)
• Can describe using hybrid-π small signal model
gmvbe+-
vbe
B C
E
C
B
Evbe
ic
)()( tvgti bemc = Hybrid-π small signal model
![Page 4: Department of Electrical and ECSE-330B Electronic Circuits ... Electronics... · Outline of Chapter 5 • 1- Introduction to The Bipolar Junction Transistor • 2- Active Mode Operation](https://reader034.fdocuments.in/reader034/viewer/2022051919/600b6c1d625fb748711d803d/html5/thumbnails/4.jpg)
Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 4
Output Resistance and Small Signal Model
• For constant VBE and small VCE, BJT in saturation:
ICC
IB
B
IEE
VCE
VBE
+VCE
_
Linear dependence on IC vs. VCE for constant VBEdefined as the Early Effect
Active Mode; Early Effect
![Page 5: Department of Electrical and ECSE-330B Electronic Circuits ... Electronics... · Outline of Chapter 5 • 1- Introduction to The Bipolar Junction Transistor • 2- Active Mode Operation](https://reader034.fdocuments.in/reader034/viewer/2022051919/600b6c1d625fb748711d803d/html5/thumbnails/5.jpg)
Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 5
Modeling the Early Effect• Extrapolated curves
intersect at common point (-VA, 0)
• VA is the Early voltage– typically 50 to 100V
⎟⎟⎠
⎞⎜⎜⎝
⎛+⎟⎟
⎠
⎞⎜⎜⎝
⎛=
A
CE
T
BESC V
VVVII 1expIC
VCE
![Page 6: Department of Electrical and ECSE-330B Electronic Circuits ... Electronics... · Outline of Chapter 5 • 1- Introduction to The Bipolar Junction Transistor • 2- Active Mode Operation](https://reader034.fdocuments.in/reader034/viewer/2022051919/600b6c1d625fb748711d803d/html5/thumbnails/6.jpg)
Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 6
BJT Small Signal Output Resistance
C
A
constvCE
Co I
VVIr
BE
=⎥⎥⎦
⎤
⎢⎢⎣
⎡
∂∂
≡
−
=
1
.
• Derivation of ro in active mode is a small signal approximation that provides expression for BJT output resistance in the active mode:
IC
VCE
VBE
C
Ao I
Vr =
![Page 7: Department of Electrical and ECSE-330B Electronic Circuits ... Electronics... · Outline of Chapter 5 • 1- Introduction to The Bipolar Junction Transistor • 2- Active Mode Operation](https://reader034.fdocuments.in/reader034/viewer/2022051919/600b6c1d625fb748711d803d/html5/thumbnails/7.jpg)
Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 7
BJT Small Signal Model With Output Resistance ro
• The model is updated to include the output resistance, ro , between C & E terminals.
• The resistance is in parallel with the current source
gmvbe+-
vbe
B C
E
ro
C
Ao I
Vr =T
Cm V
Ig =
![Page 8: Department of Electrical and ECSE-330B Electronic Circuits ... Electronics... · Outline of Chapter 5 • 1- Introduction to The Bipolar Junction Transistor • 2- Active Mode Operation](https://reader034.fdocuments.in/reader034/viewer/2022051919/600b6c1d625fb748711d803d/html5/thumbnails/8.jpg)
Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 8
Small Signal: vbe proportional to ib
• Since there is both a voltage and a current at the base node, there must be an equivalent resistance (V=IR)
• Small signal base current: ib
• Small signal base emitter voltage: vbe
• Define small signal input resistance at base as vbe/ib= rπ
bbe irv ⋅= π
![Page 9: Department of Electrical and ECSE-330B Electronic Circuits ... Electronics... · Outline of Chapter 5 • 1- Introduction to The Bipolar Junction Transistor • 2- Active Mode Operation](https://reader034.fdocuments.in/reader034/viewer/2022051919/600b6c1d625fb748711d803d/html5/thumbnails/9.jpg)
Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 9
Derive an Expression for BJT Small Signal Base Resistance – rπ
• Define expression for rπ:
• Make use of linearity:
• Take derivative and simplify:
⎟⎟⎠
⎞⎜⎜⎝
⎛==
T
BESCB V
VIII expββ
1−
⎥⎥⎦
⎤
⎢⎢⎣
⎡
∂∂
≡=OPBE
B
b
be
VIr
iv
π
mC
T
OPBE
C
OPBE
B
gIV
VI
VIr ββ
βπ =⋅
=⎥⎥⎦
⎤
⎢⎢⎣
⎡
∂∂
=⎥⎥⎦
⎤
⎢⎢⎣
⎡
∂∂
=−− 11
1
B
T
m IV
gr ==
βπ
![Page 10: Department of Electrical and ECSE-330B Electronic Circuits ... Electronics... · Outline of Chapter 5 • 1- Introduction to The Bipolar Junction Transistor • 2- Active Mode Operation](https://reader034.fdocuments.in/reader034/viewer/2022051919/600b6c1d625fb748711d803d/html5/thumbnails/10.jpg)
Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 10
Hybrid-π Small Signal Models
πrvi be
b =
βib
B C
E
rorπibgmvbe+
-vbe
B C
E
rorπ
( ) bbmbem iirgvg βπ ==
B
T
m IV
gr ==
βπ
VCCS based model CCCS based model
![Page 11: Department of Electrical and ECSE-330B Electronic Circuits ... Electronics... · Outline of Chapter 5 • 1- Introduction to The Bipolar Junction Transistor • 2- Active Mode Operation](https://reader034.fdocuments.in/reader034/viewer/2022051919/600b6c1d625fb748711d803d/html5/thumbnails/11.jpg)
Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 11
Transistor View: Inclusion of ro and rπ
![Page 12: Department of Electrical and ECSE-330B Electronic Circuits ... Electronics... · Outline of Chapter 5 • 1- Introduction to The Bipolar Junction Transistor • 2- Active Mode Operation](https://reader034.fdocuments.in/reader034/viewer/2022051919/600b6c1d625fb748711d803d/html5/thumbnails/12.jpg)
Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 12
Small-Signal: vbe proportional to ie
eebe irv ⋅=
• Since there is both a voltage and a current at the emitter node, there must be an equivalent resistance (V=IR)
• Small signal emitter current: ie• Small signal base emitter
voltage: vbe
• Define small signal input resistance at emitter as vbe/ie= re
![Page 13: Department of Electrical and ECSE-330B Electronic Circuits ... Electronics... · Outline of Chapter 5 • 1- Introduction to The Bipolar Junction Transistor • 2- Active Mode Operation](https://reader034.fdocuments.in/reader034/viewer/2022051919/600b6c1d625fb748711d803d/html5/thumbnails/13.jpg)
Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 13
Derive an Expression for BJT Small Signal Emitter Resistance – re
• Define expression for re:
• Make use of linearity:
• Take derivative and simplify:
⎟⎟⎠
⎞⎜⎜⎝
⎛==
T
BESCE V
VIII expαα
1−
⎥⎥⎦
⎤
⎢⎢⎣
⎡
∂∂
≡=OPBE
Ee
e
be
VIr
iv
mC
T
OPBE
C
OPBE
Ee gI
VVI
VIr αα
α=
⋅=
⎥⎥⎦
⎤
⎢⎢⎣
⎡
∂∂
=⎥⎥⎦
⎤
⎢⎢⎣
⎡
∂∂
=−− 11
1
E
T
me I
Vg
r ==α
![Page 14: Department of Electrical and ECSE-330B Electronic Circuits ... Electronics... · Outline of Chapter 5 • 1- Introduction to The Bipolar Junction Transistor • 2- Active Mode Operation](https://reader034.fdocuments.in/reader034/viewer/2022051919/600b6c1d625fb748711d803d/html5/thumbnails/14.jpg)
Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 14
T-Models
gmvbe
+
-vbe
B
C
E
ro
re
αie
B
C
E
ro
reie
VCCS based model CCCS based model
e
bee r
vi = ( ) eeembem iirgvg α==E
T
me I
Vg
r ==α
![Page 15: Department of Electrical and ECSE-330B Electronic Circuits ... Electronics... · Outline of Chapter 5 • 1- Introduction to The Bipolar Junction Transistor • 2- Active Mode Operation](https://reader034.fdocuments.in/reader034/viewer/2022051919/600b6c1d625fb748711d803d/html5/thumbnails/15.jpg)
Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 15
Hybrid-π Small Signal Model – Summary
T
Cm V
Ig =C
Ao I
Vr =B
T
IVr =π
• Useful to use model when:– emitter terminal at signal ground– input applied at base
gmvbevbe
![Page 16: Department of Electrical and ECSE-330B Electronic Circuits ... Electronics... · Outline of Chapter 5 • 1- Introduction to The Bipolar Junction Transistor • 2- Active Mode Operation](https://reader034.fdocuments.in/reader034/viewer/2022051919/600b6c1d625fb748711d803d/html5/thumbnails/16.jpg)
Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 16
– input applied at base or emitter
T Small Signal Model – Summary
• Useful to use model when:
– emitter terminal not at signal ground
T
Cm V
Ig =C
Ao I
Vr =E
Te I
Vr =
e
beebe
eebbe
rriiriir
ririv
)1()1(;)/(
+=+==
==
ββ
π
π
π
• Note:
![Page 17: Department of Electrical and ECSE-330B Electronic Circuits ... Electronics... · Outline of Chapter 5 • 1- Introduction to The Bipolar Junction Transistor • 2- Active Mode Operation](https://reader034.fdocuments.in/reader034/viewer/2022051919/600b6c1d625fb748711d803d/html5/thumbnails/17.jpg)
Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 17
PNP Small Signal Model• Small-signal model for
pnp’s identical to npn• Re-orient the model to
reflect direction of current flow and voltage polarity
• Common mistakes:– Get location of B, E,
and C terminals wrong– DC current flows one way,
signal current flows other way
Note: Vbe = -Veb
![Page 18: Department of Electrical and ECSE-330B Electronic Circuits ... Electronics... · Outline of Chapter 5 • 1- Introduction to The Bipolar Junction Transistor • 2- Active Mode Operation](https://reader034.fdocuments.in/reader034/viewer/2022051919/600b6c1d625fb748711d803d/html5/thumbnails/18.jpg)
Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 18
Example: Calculate Common Emitter Voltage Gain
VA
m mg 4.42= Ω= kro 8.70Ω= kr 36.2π
Use Hybrid-π model, and assume DC operating point is such that the following small signal parameters apply:
Ω= kRC 2
![Page 19: Department of Electrical and ECSE-330B Electronic Circuits ... Electronics... · Outline of Chapter 5 • 1- Introduction to The Bipolar Junction Transistor • 2- Active Mode Operation](https://reader034.fdocuments.in/reader034/viewer/2022051919/600b6c1d625fb748711d803d/html5/thumbnails/19.jpg)
Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 19
Small Signal Example
X
![Page 20: Department of Electrical and ECSE-330B Electronic Circuits ... Electronics... · Outline of Chapter 5 • 1- Introduction to The Bipolar Junction Transistor • 2- Active Mode Operation](https://reader034.fdocuments.in/reader034/viewer/2022051919/600b6c1d625fb748711d803d/html5/thumbnails/20.jpg)
Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 20
Small Signal Example
Cobemout Rrvgv ⋅−=
VVRrgvv
vvA Com
be
out
s
outV /5.82−=⋅−===
bes vv =
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Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 21
Small Signal Analysis – Comments• Analysis procedure:
– Solve for DC operating point
– Calculate small-signal parameters, gm, rπ, re, ro
– Construct small-signal equivalent circuit
– Compute voltage and current gains, input and output resistances.
• What to do about ro?– Often, including ro
complicates circuit analysis
– Omitting ro from small signal model results in acceptable accuracy
– Sometimes, ro must be included
• The often used rule of thumb: if neither end of ro is at a signal ground, you can ignore it
![Page 22: Department of Electrical and ECSE-330B Electronic Circuits ... Electronics... · Outline of Chapter 5 • 1- Introduction to The Bipolar Junction Transistor • 2- Active Mode Operation](https://reader034.fdocuments.in/reader034/viewer/2022051919/600b6c1d625fb748711d803d/html5/thumbnails/22.jpg)
Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 22
Small Signal Approximation
• Small signal analysis requires the use of “small”signals to be valid; how small is “small enough”?
• BJT based on pn junction, thus vbe constraint follows that for diode
• For linear approximation to be valid:
• Works for vbe less than approximately 10mV in amplitude
1<<⋅ T
d
Vnv
1<<T
be
Vv
![Page 23: Department of Electrical and ECSE-330B Electronic Circuits ... Electronics... · Outline of Chapter 5 • 1- Introduction to The Bipolar Junction Transistor • 2- Active Mode Operation](https://reader034.fdocuments.in/reader034/viewer/2022051919/600b6c1d625fb748711d803d/html5/thumbnails/23.jpg)
Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 23
What is the Limit on vs in Order to Remain in Small Signal Approximation?
bes vv =• For this circuit, since vs = vbe, vs must remain less than 10 mV in order for small signal model to be valid• What modifications can be made to increase allowable magnitude of vs?
![Page 24: Department of Electrical and ECSE-330B Electronic Circuits ... Electronics... · Outline of Chapter 5 • 1- Introduction to The Bipolar Junction Transistor • 2- Active Mode Operation](https://reader034.fdocuments.in/reader034/viewer/2022051919/600b6c1d625fb748711d803d/html5/thumbnails/24.jpg)
Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 24
Outline of Chapter 5
• 1- Introduction to The Bipolar Junction Transistor• 2- Active Mode Operation of BJT• 3- DC Analysis of Active Mode BJT Circuits• 4- BJT as an Amplifier• 5- BJT Small Signal Models• 6- CEA, CEA with RE, CBA, & CCA• 7- Integrated Circuit Amplifiers
![Page 25: Department of Electrical and ECSE-330B Electronic Circuits ... Electronics... · Outline of Chapter 5 • 1- Introduction to The Bipolar Junction Transistor • 2- Active Mode Operation](https://reader034.fdocuments.in/reader034/viewer/2022051919/600b6c1d625fb748711d803d/html5/thumbnails/25.jpg)
Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 25
The Common Emitter Amplifier (CEA)• Input at base; output at collector• The emitter has a small-signal
ground and is “common” to both input and output.
• By-pass (red) capacitors and coupling (green) capacitors are used to DC-bias transistor and couple signals.
• Use Hybrid-π model with the output resistance, ro.
01
01
=∞=
∞==
ωω
ω
whenCj
CwhenCj
![Page 26: Department of Electrical and ECSE-330B Electronic Circuits ... Electronics... · Outline of Chapter 5 • 1- Introduction to The Bipolar Junction Transistor • 2- Active Mode Operation](https://reader034.fdocuments.in/reader034/viewer/2022051919/600b6c1d625fb748711d803d/html5/thumbnails/26.jpg)
Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 26
CEA DC Analysis – Confirm Active• DC Analysis
– Thevenin:
– Analysis:
mAII EC 82.11
=+
=ββ
VkIV COUT 18.3)1(5 =Ω−=
( ) ( )
18755000//3000
25.153
3553
55
==
=⎟⎠⎞
⎜⎝⎛
+−+⎟
⎠⎞
⎜⎝⎛
+=
EQ
EQ
Rkk
kkk
kV
VkIV EE 515.05)3( =−Ω=
VVV EB 215.17.0 =+=
mAk
RV
I
II
kIRIV
EQ
EQE
EB
EEQBEQ
84.1)3(
1
57.0)1/(
05)3(7.0
=Ω+
+
+−=
+=
=+Ω−−−
β
β
![Page 27: Department of Electrical and ECSE-330B Electronic Circuits ... Electronics... · Outline of Chapter 5 • 1- Introduction to The Bipolar Junction Transistor • 2- Active Mode Operation](https://reader034.fdocuments.in/reader034/viewer/2022051919/600b6c1d625fb748711d803d/html5/thumbnails/27.jpg)
Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 27
CEA Voltage Gain
inbe vv = ( )Ω−= krvgv obemout 1
( )Ω−== krgvvA om
in
outVO 1
Ω==
Ω==
==
54945
4.1359
0728.0
C
AO
B
T
T
Cm
IVr
IVr
VA
VIg
π
Voltage gain analysis
![Page 28: Department of Electrical and ECSE-330B Electronic Circuits ... Electronics... · Outline of Chapter 5 • 1- Introduction to The Bipolar Junction Transistor • 2- Active Mode Operation](https://reader034.fdocuments.in/reader034/viewer/2022051919/600b6c1d625fb748711d803d/html5/thumbnails/28.jpg)
Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 28
Rin Rout
CEA Input/Output Resistance
Input Resistance:
Ω==
≡
786|| EQIN
in
inIN
RrRivR
π Ω=Ω= 9821krR oOUT
Output resistance: set vin = 0; thus vbe = 0; leads to gmvbe = 0 and thus open circuited; note one node of rotied to ground
![Page 29: Department of Electrical and ECSE-330B Electronic Circuits ... Electronics... · Outline of Chapter 5 • 1- Introduction to The Bipolar Junction Transistor • 2- Active Mode Operation](https://reader034.fdocuments.in/reader034/viewer/2022051919/600b6c1d625fb748711d803d/html5/thumbnails/29.jpg)
Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 29
CEA Summary
( )Comin
outVO Rrg
vvA −==
πrRIN =
CoOUT RrR =
• Large gain parameters
• RIN & ROUT both moderate
• RIN & ROUT not ideal for any amplifier configuration
• In practice, not usually used by itself for gain
![Page 30: Department of Electrical and ECSE-330B Electronic Circuits ... Electronics... · Outline of Chapter 5 • 1- Introduction to The Bipolar Junction Transistor • 2- Active Mode Operation](https://reader034.fdocuments.in/reader034/viewer/2022051919/600b6c1d625fb748711d803d/html5/thumbnails/30.jpg)
Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 30
CEA with RE
• There is NO small-signal ground in the emitter
• Since there is neither collector nor emitter at signal ground– Use T small-
signal model– Neglect ro
RE=3kΩ
![Page 31: Department of Electrical and ECSE-330B Electronic Circuits ... Electronics... · Outline of Chapter 5 • 1- Introduction to The Bipolar Junction Transistor • 2- Active Mode Operation](https://reader034.fdocuments.in/reader034/viewer/2022051919/600b6c1d625fb748711d803d/html5/thumbnails/31.jpg)
Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 31
CEA with RE - Voltage Gain Analysis
• Draw small-signal T-model equivalent circuit.
• Neglect rO
• Voltage gain
inEe
ebe v
Rrrv+
=
Cbemout Rvgv −=
Ee
eCm
in
outVO Rr
rRgvvA
+−==
RC
vin
gmvbe
+
-vbe re
vout
RE
![Page 32: Department of Electrical and ECSE-330B Electronic Circuits ... Electronics... · Outline of Chapter 5 • 1- Introduction to The Bipolar Junction Transistor • 2- Active Mode Operation](https://reader034.fdocuments.in/reader034/viewer/2022051919/600b6c1d625fb748711d803d/html5/thumbnails/32.jpg)
Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 32
RIN: the β+1 Reflection Rule
• The β+1 rule takes advantage of the relationship between iEand iB
b
Eeb
b
xbe
in
inIN i
Ririi
vvivR +
=+
=≡ π
• Rule generally applicable only when ro neglected
ib
VXie
EIN RrR )1( ++= βπ
be ii )1( += β
vin
vout
When looking into the base, the input resistance is the base resistance plus (β+1) times total resistance in the emitter
![Page 33: Department of Electrical and ECSE-330B Electronic Circuits ... Electronics... · Outline of Chapter 5 • 1- Introduction to The Bipolar Junction Transistor • 2- Active Mode Operation](https://reader034.fdocuments.in/reader034/viewer/2022051919/600b6c1d625fb748711d803d/html5/thumbnails/33.jpg)
Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 33
The β+1 Reflection Rule in Reverse: Looking Into the Emitter
ib
VX
ve
e
Bbee
e
xeb
in
inIN i
Ririi
vvivR
−−−
=−+
=≡
)1( ++=
βB
eINRrRbe ii )1( += β
Veb
-
+
+-
• RB looks (β+1) smaller from emitter perspective
• When looking into the emitter, the input resistance is the emitter resistance PLUS whatever is in the base divided by (β+1)
xebinxebin vvvvvv =−+= ;
vin
![Page 34: Department of Electrical and ECSE-330B Electronic Circuits ... Electronics... · Outline of Chapter 5 • 1- Introduction to The Bipolar Junction Transistor • 2- Active Mode Operation](https://reader034.fdocuments.in/reader034/viewer/2022051919/600b6c1d625fb748711d803d/html5/thumbnails/34.jpg)
Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 34
CEA with RE– Input/Output Resistance• Neglect rO
• Input Resistance• Using the β+1 reflection
rule:
• Output resistance: set vin = 0; thus vbe = 0; leads to gmvbe = 0 and thus open circuited; by inspection:
Ω= kROUT 1
Rout( )( )[ ]EeEQIN RrRR ++= 1|| β
Rin
![Page 35: Department of Electrical and ECSE-330B Electronic Circuits ... Electronics... · Outline of Chapter 5 • 1- Introduction to The Bipolar Junction Transistor • 2- Active Mode Operation](https://reader034.fdocuments.in/reader034/viewer/2022051919/600b6c1d625fb748711d803d/html5/thumbnails/35.jpg)
Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 35
The CEA With RE – Summary
COUT RR =
moderate
moderate
• Moderate gain due to REwhich reduces gain
• Large RIN good for voltage and transconductanceamplifier configuration
• Moderate ROUT a problem, but fixable
Ee
eCm
in
outVO Rr
rRgvvA
+−==
( )( )EeIN RrR ++= 1β large
![Page 36: Department of Electrical and ECSE-330B Electronic Circuits ... Electronics... · Outline of Chapter 5 • 1- Introduction to The Bipolar Junction Transistor • 2- Active Mode Operation](https://reader034.fdocuments.in/reader034/viewer/2022051919/600b6c1d625fb748711d803d/html5/thumbnails/36.jpg)
Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 36
The Common Base Amplifier (CBA)• The input is ve(t)• The output is vc(t)• The base has a small-
signal ground and is “common” to both input and output.
• By-pass capacitors and coupling capacitors are used to DC-bias transistor and couple signals.
• Use T model without the output resistance, ro.
by-pass
coupling
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Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 37
CBA – Voltage Gain Analysis
inbe vv −=
• Draw small-signal T-model equivalent circuit.
• Neglect rO
• Circuit analysis - voltage gain
)1( Ω−= kvgv bemout
)1( Ω== kgvvA m
in
outVO
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Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 38
CBA – Short Circuit Current Gain Analysis• Draw small-signal T-model
equivalent circuit.• Neglect rO
• Circuit analysis - current gain
eout ii α−=
⎟⎟⎠
⎞⎜⎜⎝
⎛+
==Ee
E
in
outIS Rr
RiiA α
VX
+
_
_
VX
ineX iii +=
inEe
Ee i
RrRi ⎟⎟
⎠
⎞⎜⎜⎝
⎛+
−=
iX
EXeeX Ririv =−=
![Page 39: Department of Electrical and ECSE-330B Electronic Circuits ... Electronics... · Outline of Chapter 5 • 1- Introduction to The Bipolar Junction Transistor • 2- Active Mode Operation](https://reader034.fdocuments.in/reader034/viewer/2022051919/600b6c1d625fb748711d803d/html5/thumbnails/39.jpg)
Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 39
CBA – Input/Output resistance
Ω= kROUT 1
Rout
• Input Resistance
• Output resistance• With vIN = -vbe = 0
EeIN RrR ||=
Rin
LOW input resistance can be useful !!
![Page 40: Department of Electrical and ECSE-330B Electronic Circuits ... Electronics... · Outline of Chapter 5 • 1- Introduction to The Bipolar Junction Transistor • 2- Active Mode Operation](https://reader034.fdocuments.in/reader034/viewer/2022051919/600b6c1d625fb748711d803d/html5/thumbnails/40.jpg)
Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 40
The Common Collector Amplifier (CCA)• The input is vb(t)• The output is ve(t)• The collector has a small-
signal ground and is “common” to both input and output.
• By-pass capacitors and coupling capacitors are used to DC-bias transistor and couple signals.
• Use T model with the output resistance, ro.
by-pass
coupling
![Page 41: Department of Electrical and ECSE-330B Electronic Circuits ... Electronics... · Outline of Chapter 5 • 1- Introduction to The Bipolar Junction Transistor • 2- Active Mode Operation](https://reader034.fdocuments.in/reader034/viewer/2022051919/600b6c1d625fb748711d803d/html5/thumbnails/41.jpg)
Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 41
CCA – Voltage-Gain Analysis
⎟⎟⎠
⎞⎜⎜⎝
⎛+ΩΩ
=eo
oinout rkr
krvv4//
4//
+
- -+
Since re small, Almost vout=vin
Voltage Division:
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Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 42
The CCA – Current-Gain Analysis
⎟⎟⎠
⎞⎜⎜⎝
⎛++Ω
Ω+==
)1(88)1(
ββ
ein
outIS rk
kiiA
)1/(;
βα
+=−−==+
exin
xinbeeb
iiiiiiiii
eexin rikiv =Ω= )8(
eout ii =
iX
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Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 43
CCA - Input/Output Resistance• Input Resistance; by
inspection using β+1 rule:
RIN
HIGH INPUT RESISTANCE
• Output Resistance, vin= 0
EoeOUT RrrR ////=
ROUTLOW OUTPUT RESISTANCE
( )( ){ }EoeIN RrrkR //1//8 ++Ω= β
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Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I
BJTs 44
CCA Operation – Voltage Buffer• Good voltage buffer
– The VOLTAGE gain is almost unity, and the DC component is only reduced by 0.7V
– Large short circuit current gain
– High input resistance (which reduces loading to the circuits before)
– Low output resistance (which reduces the loading of circuits after)