Bipolar Junction Transistor Study and plan

Faculty of Electronics and Telecommunication
University of Engineering and Technology - VNU
Linh kiện Điện tử
Electronic Devices
1) ELECTRONIC DEVICES, 9th edition, Thomas L. Floyd, Prentice Hall
2) Semiconductor Physics And Devices, 3rd ed. - J. Neamen
3) Robert L. Boylestad, Louis Nashelsky, Electronic Devices and Circuit Theory, 8th Edition,
Prentice Hall
4) Linh kiện bán dẫn và vi mạch, Hồ Văn Sung, NXB Giáo Dục, 2007
5) Giáo trình linh kiện điện tử, Trương Văn Tám, ĐH Cần Thơ, 2003.
References

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 The bipolar junction transistor (BJT) is constructed with three doped
semiconductor regions separated by two pn junctions
 Regions are called emitter, base and collector
The invention of the
bipolar transistor in
1948 ushered in a
revolution in electronics
TS. Phạm Ngọc Thảo

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TRANSFER＋ RESISTOR
→ TRANSISTOR
Vi = 200 mV  VL = 50V
 Increase x250 times
AMPLIFIER (KHUẾCH ĐẠI)

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General-purpose/
small-signal
Power transistors
and packages.
Multiple-
transistor
packages

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pnp npn
Common-Base
CB Configuration
Common-Collector
CC Configuration
Common-Emitter
CE Configuration
Input
signal
Output
signal
Input
signal
Input
signal
Input
signal
Input
signal
Input
signal
Output
signal
Output
signal
Output
signal
Output
signal
Output
signal

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The base–emitter and collector–base junctions of a transistor are
both reverse-biased.
The base–emitter and collector–base junctions are forward-biased.
The base–emitter junction is forward-biased, whereas the collector–
base junction is reverse-biased.
E

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Linh kiện bán dẫn và vi mạch 8
Cutoff mode
The base–emitter and collector–base
junctions of a transistor are both
reverse-biased.
VCE ≈ VCC
Saturation mode
The base–emitter and collector–base
junctions of a transistor are both
forward-biased.
VCE ≈ 0

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The base–emitter junction is forward-biased, whereas the
collector– base junction is reverse-biased.
Linh kiện bán dẫn và vi mạch 9
Active mode
(pnp) UC<UB<UE
(npn) UC>UB>UE

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Active mode
The base–emitter junction is forward-biased, whereas the
collector– base junction is reverse-biased.

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Chuyển tiếp P-N giữa cực Base và cực
Emitter được phân cực thuận bởi nguồn
VEE. Chuyển tiếp P-N giữa cực Base và
cực Collector được phân cực nghịch bởi
nguồn VCC.
Điện tử từ cực âm của nguồn VEE di
chuyển vào vùng Emitter qua vùng
Base, đáng lẽ trở về cực dương của
nguồn VEE nhưng vì:
+ Vì vùng Base rất hẹp với 2 vùng kia
+ Nguồn VCC >> VEE cho nên đa số điện
tử bị hấp dẫn về nó
Do đó, số lượng điện tử từ vùng Base
vào vùng thu tới cực dương của nguồn
VCC rất nhiều so với số lượng điện tử từ
vùng Base tới cực dương của nguồn VEE.
Sự dịch chuyển của điện tử tạo thành
dòng điện.
Dòng đi vào cực Base dgl dòng IB;
Dòng đi vào cực Collector dgl dòng IC;
Dòng đi vào cực Emitter dgl dòng IE.
n n
p

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 Quy ước về chiều dòng điện là chiều từ cực dương đi qua
dây dẫn và các linh kiện tiêu thụ điện tới cực âm của
nguồn điện.
 Trong trường hợp npn, dòng dịch chuyển của các electron
tích điện âm dịch chuyển ngược chiều với chiều của dòng
điện
 npn còn được gọi là đèn (bóng) bán dẫn, BJT, ngược.
 Trong trường hợp pnp, dòng dịch chuyển của các lỗ trống
tích điện dương dịch chuyển cùng chiều chiều với chiều của
dòng điện
 pnp còn được gọi là đèn (bóng) bán dẫn, BJT, thuận.
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We have
(1) IE = IB + IC
(2) IC =  IE where   0.95~0.99
Replace (2) into (1)
Take
The collector current is comprised of
two currents:
, β: Current gains (Current
amplification factors)

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 Using empirical methods, measure the parameters of the
circuit to draw BJT characteristic curves.
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Đặc tuyến ngõ vào IB (VBE)
Đặc tuyến truyền dẫn IC (VBE)

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Common-Emitter Configuration
IB = f (VBE)|V =const
CE
IC = f (VCE)|I =const
B

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Common-Collector Configuration

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IB = f (VCB)|V =const
CE
IE = f (VCE)|I =const
B

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 A BJT can be used as a switching device in logic circuits to turn on
or off current to a load. As a switch, the transistor is normally in
either cutoff (load is OFF) or saturation (load is ON)
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BJT as a switch
Switching action of an ideal transistor

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 Signal amplification can be understood as linearly increasing the
signal amplitude electrical signal. BJT can be used for signal
amplification.
 Let BJT amplify the signal the signal requires the BJT bias so that
Base-Emitter is forward biased and Collector-Base is reverse
biased.
 In the amplifier circuit exists both direct (dc) and alternating
components (ac). One-dimensional quantities denoted according
to the primary index rule are capital letters Indexes are also
uppercase (example: IB). Alternating quantities are denoted
according the main index rule is the lower case sub-index is the
lower case (eg Ib)
 The BJT is able to amplify signals due to the Collector current
approximately times the Base current. (IC = βIB)
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BJT as an amplifier

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 The transistor can be employed as an amplifying device
 There is an “exchange” of dc power to the ac domain that permits
establishing a higher output ac power.
 A conversion efficiencyis defined by =Po(ac)/Pi(dc) , where Po(ac) is
the ac power to the load and Pi(dc) is the dc power supplied.
 The factor missing from the discussion above that permits an ac
power output greater than the input ac power is the applied dc
power. It is the principal contributor to the total output power
even though part of it is dissipated by the device and resistive
elements.
 In other words, there is an “exchange” of dc power to the ac
domain that permits establishing a higher output ac power.
 In fact, a conversion efficiencyis defined by h=Po(ac)/Pi(dc), where
Po(ac) is the ac power to the load and Pi(dc) is the dc power supplied
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 There are 2 analysis; dc analysis and ac analysis. The
purpose of dc analysis is to determine the initial operating
values of IC, IB and VCE (Q-point). The goal is to set the Q-point
such that it does not go into saturation or cutoff when an ac
signal is applied. If the Q-point is in active region, the transistor
can operate as an amplifier.
 The purpose of ac analysis is to obtain the gain.
 An amplifier is a system that has a gaining ability to amplify
where a small electrical signal will be converted into a strong
one. Amplifiers are classified as small signal amplifiers
(preamplifiers) and strong signal amplifiers (power amplifiers).
 Amplifiers are able to amplify current, voltage and/or power.
In other words, only amplifiers are able to produce power gain
where as other devices such as transformer are only able to
produce voltage and current gain
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Transistors as a Small Signal Amplifier

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 For saturation mode and cutoff mode, we just provide a sufficiently
large (small) bias voltage so that the junction Base-Emitter , the
junction Collector-Base are both forward (reverse) biasses.
 For active mode, in order to obtain the amplified signal without
distorting, a steady voltage dc must be supplied to the terminals of
the transistor (so that when adding an AC signal , the transistor does
not work into saturation or cutoff modes)
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 β: increases with increase in
temperature
 |VBE|: decreases about 2.5 mV
per degree Celsius (°C) increase
in temperature
 ICO(reverse saturation current):
doubles in value for every 10°C
increase in temperature
 When the temperature changes,
the trasistor parameters will
change because IC = αIE + ICB0,
so when the temperature
changes, the Q-point will change
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A stability factor (S) is defined for
each of the parameters affecting
bias stability as follows:

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Fixed-bias
Configuration
(Mạch định
thiên cố định)
Emitter-bias
configuration
(Mạch định
thiên
Emitter)
Voltage-
divider bias
configuration
(Mạch định
thiên phân áp)
Collector
feedback
configura
tion
(Mạch
định
thiên hồi
tiếp)

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With dc supply, f = 0 Hz,
dung kháng của tụ điện
XC = 1/ (2fC) = 1/(20C) =  

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 Writing Kirchhoff’s voltage equation
for the loop
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Forward Bias of Base–Emitter

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Collector–Emitter Loop
The magnitude of the collector
current is related directly to IB
through
Applying Kirchhoff’s voltage law
around the indicated closed loop
In the other words, we have
Because VE = 0V, so that

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Load-Line Analysis
Load-line analysis: (Left) the network;
(Right) the device characteristics
The network of an output
equation that relates the
variables IC and VCE in the
following manner:
(y = ax +b)
The load line established by

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Movement of the Q-point
with increasing level of IB
Effect of an increasing
level of RC on the load
line and the Q-point
Effect of lower values of
VCC on the load line and
the Q-point

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= IC x RC

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BJT bias circuit with emitter resistor dc equivalent

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Base–Emitter Loop Writing Kirchhoff’s voltage law around
the indicated loop
where
Fixed-bias

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Writing Kirchhoff’s voltage law for the
indicated loop
where
VE is the voltage from emitter to
ground and is determined by
VC is the voltage from collector to
ground can be determined from
OR

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The collector–emitter loop equation
that defines the load line is
Load-Line Analysis
The load line established by
Load line for the emitter-bias configuration

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Voltage-divider bias configuration
Defining the Q-point for the voltage-divider
bias configuration
 Exact Analysis
 Approximate Analysis

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Exact Analysis
DC components of the
voltage-divider configuration
Re-drawing the input side
of the network
Determining RTh
https://3ce.vn/dinh-ly-thevenin/  RTH and VTH or ETH
Định lý Thevenin

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Exact Analysis
Determining ETh
Inserting the Thévenin
equivalent circuit
The voltage source VCC is returned to the
network and the open-circuit Thévenin
voltage of determined as follows by
applying the voltage-divider rule:
IB_Q point can be determined by first
applying Kirchhoff’s voltage law in the
clockwise direction for the loop
indicated:

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Approximate Analysis The resistance Ri is the equivalent
resistance between base and ground
The voltage across R2, which is actually
the base voltage determined using the
voltage-divider rule (hence the name for
the configuration)
Because Ri =(β +1)RE  βRE the condition that will define whether the
approximate approach can be applied is
IB << I2
I1 ~ I2  R1 nt R2

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The level of VE can be calculated from
The emitter current can be determined from
The collector-to-emitter voltage is determined by
Approximate Analysis
The Q-point (as determined by IC(Q) and VCE(Q)) is independent of β

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Load-Line Analysis
The similarities with the output circuit of the emitter-biased
configuration result in the same intersections for the load line of the
voltage-divider configuration
The level of IB is of course determined by a different equation for the
voltage-divider bias and the emitter-bias configurations.

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Base–Emitter Loop
DC bias circuit with voltage feedback
Base–emitter loop for
the network

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Base–Emitter Loop
Writing Kirchhoff’s voltage law around
the indicated loop
Substituting
In general, the equation for IB has the
following format
IC’= IC + IB
IC = β IB
IE = IC + IB
IC = β IB

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Collector–emitter loop
for the network
Applying Kirchhoff’s voltage law around the
indicated loop
which is exactly as obtained for the emitter-
bias and voltage-divider bias configurations

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 The key to transistor small-signal analysis is the use of the
equivalent circuits (models)
 The re model became the more desirable approach because an
important parameter of the equivalent circuit was determined by
the actual operating conditions
 The re model is really a reduced version of the hybrid  model used
almost exclusively for high-frequency analysis. This model also
includes a connection between output and sinput to include the
feedback effect of the output voltage and the input quantities.
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Transistor circuit The network of next figure following
removal of the dc supply and
insertion of the short-circuit
equivalent for the capacitors.

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Defining the important
parameters of any system
Demonstrating the reason for the
defined directions and polarities

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Circuit of a small figure redrawn for small-signal ac analysis
Current gain

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The ac equivalent of a transistor network is obtained by:
1. Setting all dc sources to zero and replacing them by a short-
circuit equivalent
2. Replacing all capacitors by a short-circuit equivalent
3. Removing all elements bypassed by the short-circuit equivalents
introduced by steps 1 and 2
4. Redrawing the network in a more convenient and logical form

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Finding the input
equivalent circuit
for a BJT transistor
Equivalent circuit
for the input side
of a BJT transistor
BJT equivalent circuit
Ie (= Ib + Ic)

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Dynamic resistance
Now, for the input side:
Solving for Vbe:
and

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Improved BJT equivalent circuit
BJT equivalent circuit
re model for the common-emitter transistor
configuration including effects of ro

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If the slope of the curves is extended
until they reach the horizontal axis, it
is interesting to note in Figure that
they will all intersect at a voltage
called the Early voltage.

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Common-Base Configuration
Common-Base BJT transistor
Equivalent circuit for configuration
Of the above figure
No phase shift between the input and
output voltages

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For the common-collector configuration, the model defined for the
common-emitter configuration is normally applied rather than
defining a model for the common-collector configuration
npn versus pnp
The dc analysis of npn and pnp configurations is quite different in the
sense that the currents will have opposite directions and the voltages
opposite polarities.
However, for an ac analysis where the signal will progress between
positive and negative values, the ac equivalent circuit will be the same.

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Common-emitter fixed-bias
configuration
Network of the left figure following the
removal of the effects of VCC, C1, and C2

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The re model

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The re model
Note the explicit absence of β in the equation,
although we recognize that must be utilized to
determine re.

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Phase Relationship
The negative sign in the resulting equation for Av reveals that a
180°phase shift occurs between the input and output signals, as
shown in the figure

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Substituting the re equivalent circuit into
the ac equivalent network of voltage-
divider bias configuration

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From the figure with Vi set to 0V,
resulting in Ib =0 mA and βIb =0 mA,

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The re model has the advantage that the parameters are defined by
the actual operating conditions, whereas the parameters of the
hybrid equivalent circuit are defined in general terms for any
operating conditions.
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The description of the hybrid
equivalent model will begin with
the general two-port system

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Complete hybrid equivalent circuit
h11  input resistance  hi
h12  reverse transfer voltage ratio  hr
h21  forward transfer current ratio  hf
h22  output conductance  ho
The re transistor model

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Common-base configuration
Hybrid equivalent circuit
Common-emitter configuration
Hybrid equivalent circuit

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Effect of removing hre and hoe from
the hybird equivalent circuit
Approximate hybrid
equivalent model
Because hr is normally a relatively small quantity, its removal is
approximated by hr = 0 and hrVo =0, resulting in a short-circuit
equivalent for the feedback element as shown in the left right. The
resistance determined by 1/ho is often large enough to be ignored in
comparison to a parallel load, permitting its replacement by an open-
circuit equivalent for the CE and CB models.
The right figure is quite similar to the general structure of the common-
base and common-emitter equivalent circuits obtained with the re
model.
hrVo= 0
(hr ~ 0) (ho >>)

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Hybrid versus re model: common-emitter configuration
Hybrid versus re model: common-base configuration

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Approximate common-emitter
hybrid equivalent circuit
Approximate common-base
hybrid equivalent circuit
The analysis using the approximate hybrid equivalent circuit of the
common-emitter configuration and of the common-base configuration
is very similar eto that just performed using the re model.

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Fixed-Bias Configuration Substituting the approximate hybrid
equivalent circuit into the ac
equivalent network of the next figure
Using R’ =1/h oe||RC, we obtain

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Fixed-Bias Configuration
Assuming that RB>>h ie and 1/hoe 10RC, we find Ib Ii and Io=Ic=h𝑓𝑒Ib
=hfeIi , and so
Voltage Gain:
Current Gain:

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Two-port system
Substituting the complete hybrid equivalent circuit into the two-port system

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Current Gain, Ai = Io/ Ii
Applying Kirchhoff’s current law to the output circuit yields
Substituting
Rewriting the equation above, we have
Note that the current gain reduces to
the familiar result of Ai =hf if the factor
hoRL is sufficiently small compared to 1

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Voltage Gain, Av = Vo/ Vi
Applying Kirchhoff’s voltage law to the input circuit results in
Substituting and
Solving for the ratio Vo/ V i yields
In this case, the familiar form of
Av =-hfRL /hi returns if the factor
(h iho–hf hr)RL is sufficiently small
compared to hi.

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Input impedance, Zi = Vi/ Ii
For the input circuit
The familiar form of Zi =hi is obtained if the second factor in the
denominator (hoRL) is sufficiently smaller than one.

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Output Impedance, Zo=Vo/Io
The output impedance of an amplifier is defined to be the ratio of the
output voltage to the output current with the signal Vs set to zero.
For the input circuit with Vs = 0,
In this case, the output impedance is
reduced to the familiar form Zo=1/ho
for the transis-tor when the second
factor in the denominator is
sufficiently smaller than the first.

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 Includes parameters that do not appear in the other two models
primarily to provide a more accurate model for high-frequency
effects.
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hybrid  high-frequency transistor
small-signal ac equivalent circuit

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