Lecture 6 - University of California, Berkeleyee105/fa07/lectures/Lecture 6.pdf · EE105 Fall 2007...
Transcript of Lecture 6 - University of California, Berkeleyee105/fa07/lectures/Lecture 6.pdf · EE105 Fall 2007...
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Lecture 6
OUTLINE
ANNOUNCEMENTS• HW#3, Prob. 2: Re-draw I-V plots for WB reduced by a factor of 2.
• In case of a major earthquake: – Try to duck/crouch on the floor in front of the seats for cover.
– Once the earthquake stops, evacuate the building in an orderly manner.
EE105 Fall 2007 Lecture 6, Slide 1 Prof. Liu, UC Berkeley
OUTLINE• BJT (cont’d)
– PNP transistor (structure, operation, models)
• BJT Amplifiers– General considerations
Reading: Chapter 4.6-5.1
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Current Flow in a “Long-Base” PN Junction
• The quasi-neutral N-type and P-type regions have low
resistivity, whereas the depletion region has high resistivity.– When an external voltage VD is applied across the diode, almost
all of this voltage is dropped across the depletion region.
Jtot
EE105 Fall 2007 Lecture 6, Slide 2 Prof. Liu, UC Berkeley
x
0 a-b
p
n
n
p
A
D
pD
p
nA
n
xp
n
D
D
L
L
N
N
LN
DLN
D
J
J⋅⋅==
=
0
E
x
• A relatively small E-field exists in the
quasi-neutral regions ���� drift current
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Review of BJT Operation (Active Mode)
• The emitter junction is forward biased.� Carriers diffuse across the emitter junction; thus, minority-carrier concentrations
are enhanced (by ) at the edges of the emitter-junction depletion region.
More minority carriers are “injected” into the base vs. emitter, because the emitter is more heavily doped than the base.
• The collector junction is reverse biased (or not strongly forward biased).� Minority-carrier concentrations are ~0 (since ) at the edges of the
TD VVe
/
0/ ≅VV
e
EE105 Fall 2007 Lecture 6, Slide 3 Prof. Liu, UC Berkeley
� Minority-carrier concentrations are ~0 (since ) at the edges of the emitter-junction depletion region.
• The minority-carrier concentration gradient in the quasi-neutral base region (of width WB) results in minority-carrier diffusion toward the collector junction.• If WB is much shorter than the minority-carrier diffusion length, then most of
the minority carriers injected from the emitter will reach the collector-junction depletion region, and then drift into the quasi-neutral collector.
• The collector current is primarily due to carriers “collected” from the base.
0/ ≅TD VV
e
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Common-Emitter Current Gain, ββββ• Assuming that no minority-carrier recombination occurs
within the quasi-neutral base region:
– The collector current is equal to the current due to minority-carrier
injection from the emitter into the base:
( )1/
2
−= TBE VV
BB
iBEC e
WN
nqDAI
EE105 Fall 2007 Lecture 6, Slide 4 Prof. Liu, UC Berkeley
– The base current is equal to the current due to minority-carrier
injection from the base into the emitter:
• The current gain β can thus be expressed as a function of the
BJT physical parameters:
BBWN
( )β
CVV
EE
iEEB
Ie
WN
nqDAI TBE ≡−= 1
/2
BBE
EEB
WND
WND=β
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Impact of Early Effect on BJT Currents
• For a fixed value of VBE, WB decreases with increasing VCE
(because the width of the collector-junction depletion region increases with increasing reverse bias), so that the minority-carrier concentration gradient in the quasi-neutral base region increases. Thus, IC increases (slightly) with increasing VCE.
+≅ CEVViBEV
enqDA
I TBE 1/
2
EE105 Fall 2007 Lecture 6, Slide 5 Prof. Liu, UC Berkeley
• The base current is not impacted:
• Thus, the current gain β increases with increasing VCE.
βCVV
EE
iEEB
Ie
WN
nqDAI TBE ≡=
/2
+≅A
CE
BB
iBEC
Ve
WNI TBE 1
TBE VV
S
A
CE
E eIV
VI
/
0
0 1
+
+=
β
β
A
CE
A
CE
BBE
EEB
V
V
V
V
WND
WND+≡+= 0ββ
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• The voltage across an independent voltage source does not vary with time.
� Its small-signal voltage is always zero.
Thus, it is regarded as a short circuit for the purpose of small-signal analysis.
Small-Signal Models for Independent Sources
EE105 Fall 2007 Lecture 6, Slide 6 Prof. Liu, UC Berkeley
• The current through an independent current source does not vary with time
� Its small-signal current is always zero.
Thus, it is regarded as an open circuit for the purpose of small-signal analysis.
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PNP Transistor
• The operating principle of a PNP BJT is the same as that of an NPN BJT. Note that the bias-voltage polarities are reversed for the PNP device, compared to an NPN device.– The emitter is biased at a higher potential than the base.– The collector is biased at a lower potential than the base.
EE105 Fall 2007 Lecture 6, Slide 7 Prof. Liu, UC Berkeley
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NPN vs. PNP BJTs
• The directions of current flow and operation modes for NPN and PNP BJTs are shown below:
EE105 Fall 2007 Lecture 6, Slide 8 Prof. Liu, UC Berkeley
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PNP BJT Terminal Currents
T
EBSB
A
EC
T
EBSC
V
VII
V
V
V
VII
=
+
=
exp
1exp
β
EE105 Fall 2007 Lecture 6, Slide 9 Prof. Liu, UC Berkeley
BBE
EEB
T
EBS
A
ECE
T
WND
WND
V
VI
V
VI
V
=
+
+=
0
0
0 exp1
β
β
β
β
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Large-Signal Model for PNP BJT
EE105 Fall 2007 Lecture 6, Slide 10 Prof. Liu, UC Berkeley
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PNP BJT Biasing
• Note that the emitter is biased at a higher potential
than the base and the collector.
EE105 Fall 2007 Lecture 6, Slide 11 Prof. Liu, UC Berkeley
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Small-Signal Analysis
EE105 Fall 2007 Lecture 6, Slide 12 Prof. Liu, UC Berkeley
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PNP BJT Small-Signal Model
• The small-signal model for a PNP transistor is
identical to that of an NPN transistor.– Note that the polarity of the small-signal currents and
voltages are defined to be in the opposite direction with
respect to the large-signal model. This is OK, because the
small-signal model is used only to determine changes in
EE105 Fall 2007 Lecture 6, Slide 13 Prof. Liu, UC Berkeley
small-signal model is used only to determine changes in
currents and voltages.
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Small-Signal Model Example 1
EE105 Fall 2007 Lecture 6, Slide 14 Prof. Liu, UC Berkeley
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Small-Signal Model Example 2
EE105 Fall 2007 Lecture 6, Slide 15 Prof. Liu, UC Berkeley
• Note that the small-signal model is identical to that
in the previous example.
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Small-Signal Model Example 3
EE105 Fall 2007 Lecture 6, Slide 16 Prof. Liu, UC Berkeley
• Note that the small-signal model is identical to that
in the previous examples.
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Small-Signal Model Example 4
EE105 Fall 2007 Lecture 6, Slide 17 Prof. Liu, UC Berkeley
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BJT Amplifiers: Overview
EE105 Fall 2007 Lecture 6, Slide 18 Prof. Liu, UC Berkeley
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Voltage Amplifier
• In an ideal voltage amplifier, the input impedance is infinite and the output impedance is zero.
• In reality, the input and output impedances depart from their ideal values.
EE105 Fall 2007 Lecture 6, Slide 19 Prof. Liu, UC Berkeley
from their ideal values.
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Input/Output Impedances
• The figures below show how input and output
impedances are determined.
– All independent sources are set to zero.
EE105 Fall 2007 Lecture 6, Slide 20 Prof. Liu, UC Berkeley
x
x
i
vimpedance ≡
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Input Impedance Example
• Note that input/output impedances are usually
regarded as small-signal quantities.– The input impedance is obtained by applying a small
change in the input voltage and finding the resultant
change in the input current:
EE105 Fall 2007 Lecture 6, Slide 21 Prof. Liu, UC Berkeley
πri
v
x
x =
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Impedance at a Node
• When calculating I/O impedances at a port, we
usually ground one terminal. We often refer to the
“impedance seen at a node” rather than the
impedance between two nodes (i.e. at a port).
EE105 Fall 2007 Lecture 6, Slide 22 Prof. Liu, UC Berkeley
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Impedance seen at the Collector
• The impedance seen at the collector is equal to the
intrinsic output impedance of the transistor, if the
emitter is grounded.
EE105 Fall 2007 Lecture 6, Slide 23 Prof. Liu, UC Berkeley
oout rR =
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Impedance seen at the Emitter
• The impedance seen at the emitter is approximately equal to the inverse of its transconductance, if the base is grounded.
1
1
+
=x
x
gi
v
EE105 Fall 2007 Lecture 6, Slide 24 Prof. Liu, UC Berkeley
)(
1
∞=
≈
+
A
m
out
mx
V
gR
rg
i
π
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Summary of BJT Impedances
1. Looking into the base, the impedance is rπ if the emitter is (ac) grounded.
2. Looking into the collector, the impedance is ro if emitter is (ac) grounded.
3. Looking into the emitter, the impedance is 1/gm if base is (ac) grounded and Early effect is neglected.
EE105 Fall 2007 Lecture 6, Slide 25 Prof. Liu, UC Berkeley
base is (ac) grounded and Early effect is neglected.