ECAD & EDC

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The BJT Bipolar Junction Transistor
Note Normally Emitter layer is heavily doped,
Base layer is lightly doped and Collector layer
has Moderate doping.
The Two Types of BJT Transistors
npn
pnp
n
p
n
p
n
p
E
C
E
C
C
C
Cross Section
Cross Section
B
B
B
B
Schematic Symbol
Schematic Symbol
E
E

• Collector doping is usually 109
• Base doping is slightly higher 1010 1011
• Emitter doping is much higher 1017

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BJT Current Voltage - Equations
IE
IC
IE
IC

-
VCE

-
VEC
E
C
E
C
-
-


VBE
VBC
IB
VEB
VCB
IB
-
-


B
B
n p n IE IB IC VCE -VBC VBE
p n p IE IB IC VEC VEB - VCB
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n
I co
-
Inc

VCB
-
p-
- Electrons Holes

Ine
Ipe
n
VBE
-
Bulk-recombination Current
Figure Current flow (components) for an n-p-n
BJT in the active region. NOTE Most of the
current is due to electrons moving from the
emitter through base to the collector. Base
current consists of holes crossing from the base
into the emitter and of holes that recombine with
electrons in the base.
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Physical Structure

• Consists of 3 alternate layers of n- and p-type
  semiconductor called emitter (E), base (B) and
  collector (C).
• Majority of current enters collector, crosses
  base region and exits through emitter. A small
  current also enters base terminal, crosses
  base-emitter junction and exits through emitter.
• Carrier transport in the active base region
  directly beneath the heavily doped (n) emitter
  dominates i-v characteristics of BJT.

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Ic
C
- - - - - - - - - - - - -
- - - -
n
Recombination

VCB
_

• Electrons
• Holes

B
- - - - - - -
- - -
p


IB

-
-
VBE
_

• - - - - - -
• - -
• - - - - - -
• - - -
• -
• - - - - - - - -

n
E
IE
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For CB Transistor IE Ine Ipe Ic Inc- Ico And
Ic - aIE ICo CB Current Gain, a - (Ic- Ico)
. (IE- 0)
For CE Trans., IC ßIb (1ß) Ico where ß -
a , 1- a
is CE Gain
Bulk-recombination current
Inc
ICO
Ipe
Ine
Figure An npn transistor with variable biasing
sources (common-emitter configuration).
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Common-Emitter
Circuit Diagram
Collector-Current Curves
VCE
IC
IC
_
Active Region
VCC
IB
IB
Region of Operation Description
Active Small base current controls a large collector current
Saturation VCE(sat) 0.2V, VCE increases with IC
Cutoff Achieved by reducing IB to 0, Ideally, IC will also be equal to 0.
VCE
Saturation Region
Cutoff Region IB 0
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When analyzing a DC BJT circuit, the BJT is
replaced by one of the DC circuit models shown
below.
DC Models for a BJT
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DC ? and DC ?
? Common-emitter current gain ?
Common-base current gain ? IC ?
IC IB IE The relationships
between the two parameters are ? ? ?
? ? 1 1 -
? Note ? and ? are sometimes referred to as
?dc and ?dc because the relationships being
dealt with in the BJT are DC.
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Output characteristics npn BJT (typical)
Note The PE review text sometimes uses ?dc
instead of ?dc. They are related as follows

• Find the approximate values of bdc and adc
  from the graph.

Input characteristics npn BJT (typical)
The input characteristics look like the
characteristics of a forward-biased diode. Note
that VBE varies only slightly, so we often ignore
these characteristics and assume Common
approximation VBE Vo 0.65 to 0.7V
Note Two key specifications for the BJT are Bdc
and Vo (or assume Vo is about 0.7 V)
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Figure Common-emitter characteristics displaying
exaggerated secondary effects.
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Figure Common-emitter characteristics displaying
exaggerated secondary effects.
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Various Regions (Modes) of Operation of BJT

• Most important mode of operation
• Central to amplifier operation
• The region where current curves are practically
  flat

Active
Saturation

• Barrier potential of the junctions cancel each
  other out causing a virtual short (behaves as on
  state Switch)

Cutoff

• Current reduced to zero
• Ideal transistor behaves like an open switch

Note There is also a mode of operation called
inverse active mode, but it is rarely used.
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BJT Trans-conductance Curve
For Typical NPN Transistor 1
Collector Current IC ? IES eVBE/?VT
Transconductance (slope of the curve) gm
IC / VBE IES The reverse saturation current
of the B-E Junction. VT kT/q 26 mV
(_at_ T300oK) ? the emission coefficient and is
usually 1
IC
8 mA
6 mA
4 mA
2 mA
VBE
0.7 V
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Three Possible Configurations of BJT
Biasing the transistor refers to applying
voltages to the transistor to achieve certain
operating conditions. 1. Common-Base
Configuration (CB) input VEB
IE output VCB IC 2. Common-Emitter
Configuration (CE) input VBE IB
output VCE IC 3. Common-Collector
Configuration (CC) input VBC IB (Also
known as Emitter follower) output VEC IE
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Common-Base BJT Configuration
Circuit Diagram NPN Transistor
The Table Below lists assumptions that can be
made for the attributes of the common-base BJT
circuit in the different regions of operation.
Given for a Silicon NPN transistor.
Region of Operation IC VCE VBE VCB C-B Bias E-B Bias
Active ?IB VBEVCE 0.7V ? 0V Rev. Fwd.
Saturation Max 0V 0.7V -0.7VltVCElt0 Fwd. Fwd.
Cutoff 0 VBEVCE ? 0V ? 0V Rev. None/Rev.
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Common-Base (CB) Characteristics
Although the Common-Base configuration is not the
most common configuration, it is often helpful in
the understanding operation of BJT
Vc- Ic (output) Characteristic Curves
IC
mA
Breakdown Reg.
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Active Region
IE
4
IE2mA
Saturation Region
2
Cutoff IE 0
IE1mA
VCB
0.8V
2V
4V
6V
8V
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Common-Collector BJT Characteristics
Emitter-Current Curves
IE
The Common-Collector biasing circuit is basically
equivalent to the common-emitter biased circuit
except instead of looking at IC as a function of
VCE and IB we are looking at IE. Also, since ?
1, and ? IC/IE that means ICIE
Active Region
IB
VCE
Saturation Region
Cutoff Region IB 0
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n p n Transistor Forward Active Mode Currents
Base current is given by
IC
IB
is forward common-emitter current gain
Emitter current is given by
VBE
IE
Forward Collector current is Ico is reverse
saturation current
is forward common- base current gain
In this forward active operation region,
VT kT/q 25 mV at room temperature
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Various Biasing Circuits used for BJT

• Fixed Bias Circuit
• Collector to Base Bias Circuit
• Potential Divider Bias Circuit

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The Thermal Stability of Operating Point SIco
The Thermal Stability Factor SIco SIco
?Ic ?Ico This equation signifies
that Ic Changes SIco times as fast as
Ico Differentiating the equation of Collector
Current IC rearranging the terms we can
write SIco - 1ß 1- ß
(?Ib/?IC) It may be noted that Lower is the
value of SIco better is the stability
Vbe, ß
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The Fixed Bias Circuit
The Thermal Stability Factor SIco SIco
?Ic ?Ico General Equation of SIco
Comes out to be SIco - 1 ß
1- ß (?Ib/?IC)
Vbe, ß
RC
Rb
RC
Applying KVL through Base Circuit we can write,
Ib Rb Vbe Vcc Diff w. r. t. IC, we get (?Ib
/ ?Ic) 0 SIco (1ß) is very large Indicating
high un-stability
Ib
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The Collector to Base Bias Circuit
The General Equation for Thermal Stability
Factor, SIco ?Ic ?Ico Comes
out to be SIco - 1 ß
1- ß (?Ib/?IC)
Vbe, ß
Ic
Applying KVL through base circuit we can write
(Ib IC) RC Ib Rb Vbe Vcc Diff. w. r. t. IC
we get (?Ib / ?Ic) - RC / (Rb RC) Therefore,
SIco - (1 ß) 1
ßRC/(RC Rb) Which is less than (1ß),
signifying better thermal stability
Ib

VBE
IE
-
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The Potential Devider Bias Circuit
The General Equation for Thermal Stability
Factor, SIco - 1 ß
1- ß (?Ib/?IC)
IC
Applying KVL through input base circuit we can
write IbRTh IE RE Vbe VTh Therefore, IbRTh
(IC Ib) RE VBE VTh Diff. w. r. t. IC
rearranging we get (?Ib / ?Ic) - RE / (RTh
RE) Therefore, This shows that SIco is
inversely proportional to RE and It is less than
(1ß), signifying better thermal stability
Ib
IC
Thevenin Equivalent Ckt
IC
Ib
Rth R1R2 Vth Vcc R2 R1R2
R1R2
Self-bias Resistor
Thevenins Equivalent Voltage
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A Practical C E Amplifier Circuit
Input Signal Source
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BJT Amplifier (continued)
If changes in operating currents and voltages are
small enough, then IC and VCE waveforms are
undistorted replicas of the input signal. A small
voltage change at the base causes a large voltage
change at the collector. The voltage gain is
given by The minus sign indicates a 1800 phase
shift between input and output signals.
An 8 mV peak change in vBE gives a 5 mA change in
iB and a 0.5 mA change in iC. The 0.5 mA change
in iC gives a 1.65 V change in vCE .
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A Practical BJT Amplifier using Coupling and
Bypass Capacitors
In a practical amplifier design, C1 and C3 are
large coupling capacitors or dc blocking
capacitors, their reactance (XC ZC 1/wC)
at signal frequency is negligible. They are
effective open circuits for the circuit when DC
bias is considered. C2 is a bypass capacitor.
It provides a low impedance path for ac current
from emitter to ground. It effectively removes
RE (required for good Q-point stability) from
the circuit when ac signals are considered.

• AC coupling through capacitors is used to inject
  an ac input signal and extract the ac output
  signal without disturbing the DC Q-point
• Capacitors provide negligible impedance at
  frequencies of interest and provide open circuits
  at dc.

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D C Equivalent for the BJT Amplifier (Step1)
DC Equivalent Circuit

• All capacitors in the original amplifier circuit
  are replaced by open circuits, disconnecting vI,
  RI, and R3 from the circuit and leaving RE
  intact. The the transistor Q will be replaced by
  its DC model.

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A C Equivalent for the BJT Amplifier (Step 2)
Ro
R1IIR2RB
Rin

• Coupling capacitor CC and Emitter bypass
  capacitor CE are replaced by short circuits.
• DC voltage supply is replaced with short
  circuits, which in this case is connected to
  ground.

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A C Equivalent for the BJT Amplifier (continued)
All externally connected capacitors are assumed
as short circuited elements for ac signal

• By combining parallel resistors into equivalent
  RB and R, the equivalent AC
• circuit above is constructed. Here, the
  transistor will be replaced by its
• equivalent small-signal AC model (to be
  developed).

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A C Analysis of CE Amplifier
1) Determine DC operating point and
calculate small signal parameters 2) Draw the
AC equivalent circuit of Amp. DC Voltage
sources are shorted to ground DC Current
sources are open circuited Large capacitors
are short circuits Large inductors are open
circuits 3) Use a Thevenin circuit (sometimes a
Norton) where necessary. Ideally the base
should be a single resistor a single
source. Do not confuse this with the DC
Thevenin you did in step 1. 4) Replace transistor
with small signal model 5) Simplify the circuit
as much as necessary. Steps to Analyze a
Transistor Amplifier 6) Calculate the small
signal parameters and gain etc.
Step 1
Step 2
Step 3
Step 4
Step 5
p-model used
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Hybrid-Pi Model for the BJT
Transconductance
Input resistance Rin

• The hybrid-pi small-signal model is the intrinsic
  low-frequency representation of the BJT.
• The small-signal parameters are controlled by the
  Q-point and are independent of the geometry of
  the BJT.

Output resistance
Where, VA is Early Voltage (VA100V for npn)
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Hybrid Parameter Model
Ii
Io
Linear Two port Device
Vo
Vi
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h-Parameters
h11 hi Input Resistanceh12 hr Reverse
Transfer Voltage Ratioh21 hf Forward
Transfer Current Ratioh22 ho Output
Admittance
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Three Small signal Models of CE Transistor
The Mid-frequency small-signal models
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BJT Mid-frequency Analysis using the hybrid-p
model
A common emitter (CE) amplifier

• The mid-frequency circuit is drawn as follows
• the coupling capacitors (Ci and Co) and the
• bypass capacitor (CE) are short circuits
• short the DC supply voltage (superposition)
• replace the BJT with the hybrid-p model
• The resulting mid-frequency circuit is shown
  below.

An a c Equivalent Circuit
ro
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Details of Small-Signal Analysis for Gain Av
(Using ?-model)
Rs
Rs
From input circuit
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C-E Amplifier Input Resistance

• The input resistance, the total resistance
  looking into the amplifier at coupling capacitor
  C1, represents the total resistance presented to
  the AC source.

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C-E Amplifier Output Resistance

• The output resistance is the total equivalent
  resistance looking into the output of the
  amplifier at coupling capacitor C3. The input
  source is set to 0 and a test source is applied
  at the output.

But vbe0.
since ro is usually gtgt RC.
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High-Frequency Response BJT Amplifiers
Capacitances that will affect the high-frequency
response Cbe, Cbc, Cce internal
capacitances Cwi, Cwo wiring capacitances
CS, CC coupling capacitors CE bypass
capacitor
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Frequency Response of Amplifiers
The voltage gain of an amplifier is typically
flat over the mid-frequency range, but drops
drastically for low or high frequencies. A
typical frequency response is shown below.

• For a CE BJT (shown on lower right)
• low-frequency drop-off is due to CE, Ci and Co
• high-frequency drop-off is due to device
  capacitances Cp and Cm (combined to form
  Ctotal)
• Each capacitor forms a break point (simple pole
  or zero) with a break frequency of the form
  f1/(2pREqC), where REq is the resistance seen by
  the capacitor
• CE usually yields the highest low-frequency
  break
• which establishes fLow.

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Amplifier Power Dissipation

• Static power dissipation in amplifiers is
  determined from their DC equivalent circuits.

Total power dissipated in C-B and E-B junctions
is where Total power supplied is
The difference is the power dissipated by the
bias resistors.
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Figure 4.36a Emitter follower.
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An Emitter Follower (CC Amplifier) Amplifier
Figure Emitter follower.
Very high input Resistance Very low out put
Resistance Unity Voltage gain with no phase
shift High current gain Can be used for impedance
matching or a circuit for providing electrical
isolation
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Figure 4.36b Emitter follower.
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Figure 4.36c Emitter follower.
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Capacitor Selection for the CE Amplifier
The key objective in design is to make the
capacitive reactance much smaller at the
operating frequency f than the associated
resistance that must be coupled or bypassed.
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Summary of Two-Port Parameters forCE/CS, CB/CG,
CC/CD
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A Small Signal h-parameter Model of C E -
Transistor
h11
Vceh12
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Small-signal Current Gain and Amplification
Factor of the BJT
The amplification factor is given by For
VCE ltlt VA, mF represents the maximum voltage
gain an individual BJT can provide, independent
of the operating point.
bo gt bF for iC lt IM, and bo lt bF for iC gt IM,
however, bo and bF are usually assumed to be
about equal.
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A Simple MOSFET Amplifier
The MOSFET is biased in the saturation region by
dc voltage sources VGS and VDS 10 V. The DC
Q-point is set at (VDS, IDS) (4.8 V, 1.56 mA)
with VGS 3.5 V. Total gate-source voltage is
A 1 V p-p change in vGS gives a 1.25 mA p-p
change in iDS and a 4 V p-p change in vDS.
Notice the characteristic non-linear I/O
relationship compared to the BJT.
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Eber-Moll BJT Model
The Eber-Moll Model for BJTs is fairly complex,
but it is valid in all regions of BJT operation.
The circuit diagram below shows all the
components of the Eber-Moll Model
IE
IC
E
C
?RIE
?RIC
IF
IR
IB
B
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Eber-Moll BJT Model
?R Common-base current gain (in forward active
mode) ?F Common-base current gain (in inverse
active mode) IES Reverse-Saturation Current of
B-E Junction ICS Reverse-Saturation Current of
B-C Junction IC ?FIF IR IB IE - IC IE
IF - ?RIR IF IES exp(qVBE/kT) 1 IR IC
exp (qVBC/kT) 1 ? If IES ICS are not
given, they can be determined using various
BJT parameters.
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Small Signal BJT Equivalent Circuit
The small-signal model can be used when the BJT
is in the active region. The small-signal
active-region model for a CB circuit is shown
below
iB
iC
B
C
?iB
r?
r? (? 1) ?VT IE
iE
E
_at_ ? 1 and T 25?C r? (? 1) 0.026
IE
Recall ? IC / IB
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The Early Effect (Early Voltage)
IC
Note Common-Emitter Configuration
IB
-VA
VCE
Green Ideal IC Orange Actual IC (IC) IC
IC VCE 1 VA
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Early Effect Example
Given The common-emitter circuit below with IB
25?A, VCC 15V, ? 100 and VA 80. Find
a) The ideal collector current b) The actual
collector current
Circuit Diagram
VCE
IC

• 100 IC/IB
• a)
• IC 100 IB 100 (25x10-6 A)
• IC 2.5 mA

_
VCC
IB

• b) IC IC VCE 1 2.5x10-3 15
  1 2.96 mA
• VA 80
• IC 2.96 mA

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Breakdown Voltage
The maximum voltage that the BJT can
withstand. BVCEO The breakdown voltage for a
common-emitter biased circuit. This breakdown
voltage usually ranges from 20-1000
Volts. BVCBO The breakdown voltage for a
common-base biased circuit. This breakdown
voltage is usually much higher than BVCEO and
has a minimum value of 60 Volts.

• Breakdown Voltage is Determined By
• The Base Width
• Material Being Used
• Doping Levels
• Biasing Voltage

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Potential-Divider Bias Circuit with Emitter
Feedback
Most popular biasing circuit. Problem bdc can
vary over a wide range for BJTs (even with the
same part number) Solution Adding the feedback
resistor RE. How large should RE be? Lets see.
Substituting the active region model into the
circuit to the left and analyzing the circuit
yields the following well known equation
ICEO has little effect and is often neglected
yielding the simpler relationship
Voltage divider biasing circuit with emitter
feedback
Replacing the input circuit by a Thevenin
equivalent circuit yields
Test for stability For a stable Q-point w.r.t.
variations in bdc choose
Why? Because then
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PE-Electrical Review Course - Class 4
(Transistors)
Find the Q-point for the biasing circuit shown
below. The BJT has the following specifications
bdc 100, rsat 100 W (Vo not
specified, so assume Vo 0.7 V)
Example
Example
Repeat Example 3 if RC is changed from 1k to 2.2k.
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PE-Electrical Review Course - Class 4
(Transistors)
Determine the Q-point for the biasing circuit
shown. The BJT has the following specifications
bdc varies from 50 to 400, Vo 0.7 V, ICBO
10 nA Solution Case 1 bdc 50
Example?
Case 2 bdc 400 Similar to Case 1 above.
Results are IC 0.659 mA, VCE 6.14 V
Summary
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PE-Electrical Review Course - Class 4
(Transistors)
BJT Amplifier Configurations and Relationships
Using the hybrid-p model.
Note The biasing circuit is the same for each
amplifier.
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Figure 4.16 The pnp BJT.
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Figure 4.17 Common-emitter characteristics for a
pnp BJT.
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Figure 4.18 Common-emitter amplifier for Exercise
4.8.
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Figure 4.19a BJT large-signal models. (Note
Values shown are appropriate for typical
small-signal silicon devices at a temperature of
300K.
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Figure 4.19b BJT large-signal models. (Note
Values shown are appropriate for typical
small-signal silicon devices at a temperature of
300K.
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Figure 4.19c BJT large-signal models. (Note
Values shown are appropriate for typical
small-signal silicon devices at a temperature of
300K.
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