Transcript 2-1 Introduction

Chapter
3
Bipolar Junction
Transistor (BJT)
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Outline
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Introduction
Operation in the Active Mode
Analysis of Transistor Circuits at DC
The transistor as an Amplifier
Graphical Analysis
Biasing the BJT for Discrete-Circuit Design
Configuration for Basic Single Stage BJT Amplifier
High frequency Model
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Introduction
•
•
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Physical Structure
Circuit Symbols for BJTs
Modes of Operation
Basic Characteristic
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Physical Structure
A simplified structure of the npn transistor.
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Physical Structure
A dual of the npn is called pnp type. This is the
simplified structure of the pnp transistor.
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Circuit Symbols for BJTs
The emitter is distinguished by the arrowhead.
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Modes of Operation
Modes
EBJ
CBJ
Cutoff
Reverse
Reverse
Saturation
Forward
Forward
Active
Forward
Reverse
Amplifier
Reverse
active
Reverse
Forward
Performance
degradation
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Application
Switching application
in digital circuits
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Basic Characteristics
• Far more useful than two terminal devices
(such as diodes)
• The voltage between two terminals can
control the current flowing in the third
terminal. We can say that the collector
current can be controlled by the voltage
across EB junction.
• Much popular application is to be an
amplifier
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Operation in the Active Mode
• Current flow
• Current equation
• Graphical representation of transistor’s
characteristics
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Current Flow
Current flow in an npn transistor biased to operate in the
active mode.
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Collector Current
• Collector current is the drift current.
• Carriers are successful excess minority
carriers.
• The magnitude of collector current is almost
independent of voltage across CB junction.
• This current can be calculated by the
gradient of the profile of electron
concentration in base region.
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Base Current
• Base current consists of two components.
Diffusion current
Recombination current
• Recombination current is dominant.
• The value of base current is very small.
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Emitter Current
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•
•
•
Emitter current consists of two components.
Both of them are diffusion currents.
Heavily doped in emitter region.
Diffusion current produced by the majority
in emitter region is dominant.
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Profiles of Minority-Carrier
Concentrations
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Current Equation
• Collector current
iC  I n  I s e
• Base current
 Is
iB 
 
 
iC
• Emitter current
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VT
 vBE VT
e

 Is
iE 

 
iC
vBE
 vBE VT
e

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Explanation for Saturation Current
• Saturation current is also called current scale.
• Expression for saturation current:
Is 
AE qDn n p 0
AE qDn ni

W
N AW
2
• Has strong function with temperature due to
intrinsic carrier concentration.
• Its value is usually in the range of 10-12A to
10-18A.
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Explanation for Common-Emitter
Current Gain
• Expression for common –emitter current
gain:
 Dp N A W 1 W 2 

＝1 

 Dn N D LP 2 Dn b 
• Its value is highly influenced by two factors.
• Its value is in the range 50 to 200 for general
transistor.
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Explanation for Common-Base
Current Gain
• Expression for common –base current gain:
＝

1＋
• Its value is less than but very close to unity.
• Small changes in α correspond to very large
changes in β.
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Recapitulation
• Collector current has the exponential
relationship with forward-biased voltage vBE
as long as the CB junction remains reversebiased.
• To behave as an ideal constant current
source.
• Emitter current is approximately equal to
collector current.
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Graphical Representation of
Transistor’s Characteristics
• Characteristic curve relates to a certain
configuration.
• Input curve is much similar to that of the diode,
only output curves are shown here.
• Three regions are shown in output curves.
• Early Effect is shown in output curve of CE
configuration.
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Output Curves for CB
Configuration
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Output Curves for CB
Configuration
• Active region
 EBJ is forward-biased, CBJ is reverse-biased;
 Equal distance between neighbouring output curves;
 Almost horizontal, but slightly positive slope.
• Saturation region
 EBJ and CBJ are not only forward-biased but also turned on;
 Collector current is diffusion current not drift current.
 Turn on voltage for CBJ is smaller than that of EBJ.
• Breakdown region
 EBJ forward-biased, CBJ reverse-biased;
 Great voltage value give rise to CBJ breakdown;
 Collector current increases dramatically.
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Output Curves for CE
Configuration
(a) Conceptual circuit for measuring the iC –vCE characteristics of the BJT.
(b) The iC –vCE characteristics of a practical BJT.
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The Early Effect
• Curves in active region are more sloped
than those in CB configuration.
• Early voltage.
• Effective base width and base width
modulation.
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The Early Effect(cont’d)
• Assuming current scale remains constant,
collector current is modified by this term:
vBE
vCE
iC  I s e
(1 
)
VA
• Narrow base width, small value of Early voltage,
strong effect of base width modulation, strong
linear dependence of Ci on vCE .
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Analysis of Transistor Circuit at
DC
• Equivalent Circuit Models
• Analysis Steps
• Examples
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Equivalent Circuit Models
Large-signal equivalent-circuit models of the npn BJT operating in the
forward active mode. In practical DC analysis, constant voltage drop
model is popular used.
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DC Analysis Steps
a.
Using simple constant-voltage drop model, assuming vBE  0.7V
irrespective of the exact value of currents.
b.
Assuming the device operates at the active region, we can apply the
relationship between IB, IC, and IE, to determine the voltage VCE or
VCB.
c.
Check the value of VCE or VCB, if
i.
VC>VB (or VCE>0.2V), the assumption is correct.
ii.
VC<VB (or VCE<0.2V), the assumption is incorrect. It means the
BJT is operating in saturation region. Thus we shall assume
VCE=VCE(sat) to obtain IC. Here the common emitter current gain
is defined as forced=IC/IB, we will find forced< .
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Examples
• Example 5.4 shows the order of the analysis steps
indicated by the circled numbers.
• Example 5.5 shows the analysis of BJT operating
saturation mode.
• Example 5.6 shows the transistor operating in
cutoff mode.
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Examples(cont’d)
• Example 5.7 shows the analysis for pnp type
circuit. It indicates the the current is affected by illspecified parameter β. As a rule, one should strive
to design the circuit such that its performance is as
insensitive to the value of β as possible.
• Example 5.8 is the bad design due to the currents
critically depending on the value of β.
• Example 5.9 is similar to the example 5.5 except
the transistor is pnp type.
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Examples(cont’d)
• Example 5.10 shows the application of Thévenin’s
theorem in calculating emitter current and so on.
This circuit is the good design for the emitter is
almost independent of β and temperature.
• Example 5.11 shows the DC analysis for two stage
amplifier.
• Example 5.12 shows the analysis of the power
amplifier composed of the complimentary
transistors.
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The Transistor as an Amplifier
• Conceptual Circuits
• Small-signal equivalent circuit models
• Application of the small-signal equivalent circuit
models
• Augmenting the hybrid π model.
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Conceptual Circuit
(a) Conceptual circuit to illustrate the operation of the transistor as an amplifier.
(b) The circuit of (a) with the signal source vbe eliminated for dc (bias) analysis.
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Conceptual Circuit(cont’d)
With the dc sources (VBE and VCC) eliminated (short circuited), thus only
the signal components are present.
Note that this is a representation of the signal operation of the BJT and
not an actual amplifier circuit.
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Small-Signal Circuit Models
• Transconductance
• Input resistance at base
• Input resistance at emitter
• Hybrid π and T model
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Transconductance
• Expression
gm 
I CQ
VT
• Physical meaning
gm is the slope of the
iC–vBE curve at the bias point Q.
• At room temperature,
g m  40ms
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Input Resistance at Base and
Emitter
• Input resistance at base
r 
vbe
V

 T 
ib
I BQ
gm
• Input resistance at emitter
vbe
VT

re 


ie
I EQ
gm
• Relationship between these two resistances
r  (1   )re
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The Hybrid- Model
•
The equivalent circuit in (a) represents the BJT as a voltage-controlled
current source (a transconductance amplifier),
•
The equivalent circuit in (b) represents the BJT as a current-controlled
current source (a current amplifier).
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The T Model
•
These models explicitly show the emitter resistance re rather than the base
resistance r featured in the hybrid- model.
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Augmenting the Hybrid- Model
Expression for the output resistance.
 i
ro   C
v

 CE


v BE  const. 

1

VA
'
IC
Output resistance represents the Early Effect(or base width modulation)
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Models for pnp Type
• Models derived from npn type transistor apply
equally well to pnp transistor with no changes of
polarities. Because the small signal can not
change the bias conditions, small signal models
are independent of polarities.
• No matter what the configuration is, model is
unique. Which one to be selected is only
determined by the simplest analysis.
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Graphical Analysis
a.
Graphical construction for the determination of the dc base current in
the circuit.
b.
Load line intersects with the input characteristic curve.
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Graphical Analysis(cont’d)
Graphical construction for determining the dc collector current IC and the
collector-to-emitter voltage VCE in the circuit.
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Small Signal Analysis
Graphical determination of the signal components vbe, ib, ic, and vce when a
signal component vi is superimposed on the dc voltage VBB
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Effect of Bias-Point Location on
Allowable Signal Swing
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a.
Load-line A results in
bias point QA with a
corresponding VCE which
is too close to VCC and
thus limits the positive
swing of vCE.
b.
At the other extreme,
load-line B results in an
operating point too close
to the saturation region,
thus limiting the negative
swing of vCE.
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Biasing in BJT Amplifier Circuit
• Biasing with voltage
Classical discrete circuit bias arrangement
 Single power supply
 Two-power-supply
With feedback resistor
• Biasing with current source
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Classical Discrete Circuit Bias
Arrangement
by fixing VBE
by fixing IB.
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Classical Discrete Circuit Bias
Arrangement
•
Both result in wide variations in IC and hence in VCE
and therefore are considered to be “bad.”
•
Neither scheme is recommended.
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Classical Biasing for BJTs Using a
Single Power Supply
Circuit with the voltage divider supplying the base replaced with its Thévenin
equivalent.
Stabilizing the DC emitter current is obtained by considering the negative
feedback action provided by RE
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Classical Biasing for BJTs Using a
Single Power Supply
• Two constraints
VBB  VBE
RE 
RB
1 
• Rules of thumb VBB  13 VCC
I C RC  13 VCC
VCB  13 VCC
I RB!  I RB 2  (0.1I E , I E )
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Two-Power-Supply Version
• Resistor RB can be eliminated in
common base configuration.
• Resistor RB is needed only if the
signal is to be capacitively
coupled to the base.
• Two constraints should apply.
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Biasing with Feedback Resistor
Resistor RB provides negative feedback.
IE is insensitive to β provided that RC  RB (1  ）
The value of RB determines the allowable signal swing at the collector.
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Biasing Using Current Source
(a) Q1 and Q2 are required to be identical and have high β.
(b) Short circuit between Q1’s base and collector terminals.
(c) Current source isn’t ideal due to finite output resistor of Q2
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Application of the Small-Signal
Models
a.
Determine the DC operating point of BJT and in
particular the DC collector current IC(ICQ).
b.
Calculate the values of the small-signal model parameters,
such as gm=IC/VT, r=/gm=VT/IB, re=/gm=VT/IE.
c.
Draw ac circuit path.
d.
Replace the BJT with one of its small-signal models. The
model selected may be more convenient than the others
in circuits analysis.
e.
Determine the required quantities.
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Basic Single-Stage BJT Amplifier
• Characteristic parameters
• Basic structure
• Configuration
 Common-Emitter amplifier
 Emitter directly connects to ground
 Emitter connects to ground by resistor RE
 Common-base amplifier
 Common-collector amplifier(emitter follower)
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Characteristic Parameters of
Amplifier
This is the two-port network of amplifier.
Voltage signal source.
Output signal is obtained from the load resistor.
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Definitions
• Input resistance with no load
vi
Ri 
ii R  
L
• Input resistance
Rin
vi

ii
• Open-circuit voltage gain
Avo
vo

vi
RL  
• Voltage gain A  vo
v
vi
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Definitions(cont’d)
• Short-circuit current gain
Ais 
io
ii
RL  0
• Current gain
Ai 
io
ii
• Short-circuit transconductance
io
Gm 
vi
RL  0
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Definitions(cont’d)
• Open-circuit overall voltage gain
v0
Gvo 
vsig
RL  
• Overall voltage gain
v0
Gv 
vsig
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Definitions(cont’d)
Output resistance of amplifier proper
vx
Ro 
ix
Output resistance
Rout
vi  0
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vx

ix
vsig 0
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Definitions(cont’d)
Voltage amplifier
Voltage amplifier
Transconductance amplifier
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Relationships
• Voltage divided coefficient
vi
Rin

vsig Rin  Rsig
RL
Av  Avo
RL  Ro
Gv 
Rin
RL
Avo
Rin  Rsig
RL  Ro
Ri
Gvo 
Avo
Ri  Rsig
RL
Gv  Gvo
RL  Rout
Avo  Gm Ro
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Basic Structure
Basic structure of the circuit used to realize single-stage,
discrete-circuit BJT amplifier configurations.
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Common-Emitter Amplifier
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Common-Emitter Amplifier
Equivalent circuit obtained by replacing the transistor with its hybrid- model.
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Characteristics of CE Amplifier
• Input resistance
• Voltage gain
Rin  r
Av   g m (ro // RC // RL )
• Overall voltage gain
Gv  
• Output resistance
 ( RC // RL // ro )
r  Rsig
Rout  RC
• Short-circuit current gain Ais   
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Summary of CE amplifier
•
•
•
•
•
•
Large voltage gain
Inverting amplifier
Large current gain
Input resistance is relatively low.
Output resistance is relatively high.
Frequency response is rather poor.
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The Common-Emitter Amplifier
with a Resistance in the Emitter
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The Common-Emitter Amplifier
with a Resistance in the Emitter
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Characteristics of the CE Amplifier
with a Resistance in the Emitter
• Input resistance Rin  RB //(1   )(re  Re )
• Voltage gain
RC // RL
Av  
re  Re
• Overall voltage gain
Gv  
• Output resistance
 ( RC // RL )
Rsig  (1   )( re  Re )
Rout  RC
• Short-circuit current gain
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Summary of CE Amplifier with RE
• The input resistance Rin is increased by the factor
(1+gmRe)
• The voltage gain from base to collector is reduced
by the factor (1+gmRe).
• For the same nonlinear distortion, the input signal
vi can be increased by the factor (1+gmRe).
• The overall voltage gain is less dependent on the
value of β.
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Summary of CE Amplifier with RE
• The reduction in gain is the price for obtaining the
other performance improvements.
• Resistor RE introduces the negative feedback into
the amplifier.
• The high frequency response is significant
improved.
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Common-Base Amplifier
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Common-Base Amplifier
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Characteristics of CB Amplifier
• Input resistance Rin  re
• Voltage gain
Av  g m ( RC // RL )
• Overall voltage gain
Gv 
• Output resistance
 ( RC // RL )
Rsig  re
Rout  RC
• Short-circuit current gain Ais  
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Summary of the CB Amplifier
•
•
•
•
•
•
•
Very low input resistance
High output resistance
Short-circuit current gain is nearly unity
High voltage gain
Noninverting amplifier
Current buffer
Excellent high-frequency performance
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The Common-Collector
Amplifier or Emitter-Follower
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The Common-Collector
Amplifier or Emitter-Follower
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The Common-Collector
Amplifier or Emitter-Follower
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Characteristics of CC Amplifier
• Input resistance Rib  (1   )(re  ro // RL )
• Voltage gain
(1   )( r // R )
Av 
o
L
(1   )( re  ro // RL )
• Overall voltage gain
RB // Rib
(1   )( ro // RL )
Gv 
RB // Rib  Rsig (1   )( re  ro // RL )
• Output resistance
Rout  re 
RB // Rsig
1 
• Short-circuit current gain Ais  (1   )
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Summary for CC Amplifier or
Emitter-Follower
• High input resistance
• Low output resistance
• Voltage gain is smaller than but very close to unity
• Large current gain
• The last or output stage of cascade amplifier
• Frequency response is excellent well
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Summary and Comparisons
• The CE configuration is the best suited for realizing the
amplifier gain.
• Including RE provides performance improvements at the
expense of gain reduction.
• The CB configuration only has the typical application in
amplifier. Much superior high-frequency response.
• The emitter follower can be used as a voltage buffer and
exists in output stage of a multistage amplifier.
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Internal Capacitances of the BJT
and High Frequency Model
• Internal capacitance
The base-charging or diffusion capacitance
Junction capacitances
 The base-emitter junction capacitance
 The collector-base junction capacitance
• High frequency small signal model
• Cutoff frequency and unity-gain frequency
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The Base-Charging or Diffusion
Capacitance
•
•
Diffusion capacitance almost entirely
exists in forward-biased pn junction
Expression of the small-signal diffusion
capacitance
IC
Cde   F g m   F
VT
•
Proportional to the biased current
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Junction Capacitances
•
The Base-Emitter Junction Capacitance
C je
•
C je0

 2C je0
VBE m
(1 
)
Voe
The collector-base junction capacitance
C 0
C 
V
(1  CB ) m
Voc
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The High-Frequency Hybrid-
Model
•
Two capacitances Cπ and Cμ , where C  Cde  C je
•
One resistance rx . Accurate value is obtained form high frequency
measurement.
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The Cutoff and Unity-Gain
Frequency
•
•
Circuit for deriving an expression for
h fe ( s ) 
IC
IB
vCE  0
According to the definition, output port is short circuit
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The Cutoff and Unity-Gain
Frequency(cont’d)
• Expression of the short-circuit current
transfer function
h fe ( s) 
0
1  s(C  C )r
• Characteristic is similar to the one of firstorder low-pass filter
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The Cutoff and Unity-Gain
Frequency (cont’d)
1
 
(C  C )r
gm
T   0 
C  C
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