The Bipolar

Junction
Transistor
For Students, Professionals
and Beyond
eBook 13

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 The B ip ol a r Junc tion Tr a nsis tor

TABLE OF
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CONTENTS
This Basic Electronics Tutorials eBook is focused on the bipolar junction transistor,
or BJT, with the information presented within this ebook provided “as-is” for general
information purposes only.
1. Introduction To Transistors . . . . . . . . . . . . . . . . . . . . . . 1
2. Transistor Biasing . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 All the information and material published and presented herein including the text,
graphics and images is the copyright or similar such rights of Aspencore. This represents
3. Transistor Gain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 in part or in whole the supporting website: www.electronics-tutorials.ws, unless
4. Transistor Operating Regions . . . . . . . . . . . . . . . . . . . . . 4 otherwise expressly stated.

5. Bipolar Junction Transistor Characteristics Curves . . . . . . . 4 This free e-book is presented as general information and study reference guide for the
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6. The Bipolar Transistor as a Switch . . . . . . . . . . . . . . . . . . 5 care has been taken with respect to the accuracy of the information given herein, the
7. Transistor Switch Application . . . . . . . . . . . . . . . . . . . . . 6 author makes no representations or warranties of any kind, expressed or implied, about
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9. The Sziklai Darlington Pair . . . . . . . . . . . . . . . . . . . . . . 8
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 The B ip ol a r Junc tion Tr a nsis tor

Figure 1. The NPN and PNP Transistor Types

1. Introduction To The BJT Transistor
The Bipolar Junction Transistor, or BJT for short, is a three-layer device made from two NPN Transistor PNP Transistor
different types of doped semiconductor material. The bipolar transistor is able to control
the flow of electric current through itself by the application of different voltage levels + -
applied to each of its three connecting terminals. IE = IB + IC IC IC = IE - IB IC

The ability of a transistor to control the flow of current allows it to be used as either an C C
analogue amplifier to increase or “amplify” an electronic signal, or as a digital on/off solid + IB - IB
VCE VCE
state switch, depending on its configuration. B B
E E
Bipolar junction transistors are constructed by bonding together doped P-type and N-type VEB VEB
semiconductor materials creating pn-junctions. As there are two pn-junctions within one IE IE
Symbol Symbol
transistor construction, it is therefore known as a bipolar device. Hence the name, Bipolar - - + +
Junction Transistor, or BJT.
The basic construction of a Bipolar Junction Transistors are 3-layer solid + Collector - Collector
Transistor is of two pn-junctions fused together C C
state semiconductors devices
to produce three distinct regions. Each of these used in amplifier and switching
three regions is given a name to identify it from N P
circuits
the other two. The names of these three basic but B Base B Base
very distinct regions as well as their connecting terminals are known and labelled as the + P - N
Emitter ( E ), the Base ( B ) and the Collector ( C ) respectively.
N P
A bipolar junction transistor constructed from a piece of p-type semiconductor material
sandwiched between two n-type regions is called an NPN Transistor. Thus the base- E E
emitter junction forms a single pn-diode, while the base-collector junction forms another - Emitter + Emitter
Construction Construction
pn-diode.
There also exists the complement of the NPN transistor, called the PNP Transistor.
The arrow used in the transistors symbol identifies the base-emitter junction with the
The PNP transistor is constructed again from a piece of n-type semiconductor material
direction of positive current flow always pointing to the n-type material. Also note that the
sandwiched between two p-type regions. In each case, the semiconductor layer in the
polarity of the applied voltages is reversed between the NPN and PNP type.
centre of the doped sandwich is called the Base, (B) with the two outer layers being the
Collector, (C) and the Emitter, (E) as shown in Figure 1.

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Bipolar junction transistors (BJT’s) are current regulating devices. They control the flow For the NPN transistor shown in Figure 2, when a supply voltage, VCC is connected
of current through them in proportion to the amount of biasing applied to their base between the collector-emitter terminals with zero voltage applied to the base terminal,
terminal, thus acting like a current-controlled device. The principle of operation of the (VBE = 0) the transistor will not conduct as the collector-base pn-junction is effectively
two transistor types, PNP and NPN, is exactly the same the only difference being in their reverse biased. Thus zero current flows into its base so it acts as an open-circuited switch.
biasing and the polarity of the power supply connected to each type.
With Vcc still connected across the collector-emitter Forward-bias is a positive
terminals. If a biasing voltage is now applied to the
2. Transistor Biasing base terminal and whose value is less than 0.7 volts
voltage applied to the p-type
region and a negative voltage
for a silicon transistor, or 0.3 volts for a germanium applied to the n-type region
For a transistor to operate as either an amplifier or an electronic switch, it requires the transistor compared to the voltage on the emitter,
correct external voltages to be applied between all of its junctions. Although the bipolar the transistor will still not conduct. This is because the base biasing voltage is too small to
transistors can be thought of as two diodes back-to-back, it does not however function as overcome the potential barrier voltage of the base-emitter pn-junction.
two individual diodes.
If the base biasing voltage is now increased above 0.7 volts for a silicon device, the diode
This because one diode, the base-emitter junction is forward biased while the other junction conducts with electric current flowing into the forward biased base-emitter
diode, the base-collector junction is reverse biased. Then for transistor action to occur, junction since it acts as a standard pn-junction diode supplying electrons from the
the base-emitter diode must be forward biased and the base-collector diode reverse n-region to the p-region.
biased by the application of the correct biasing voltages and their polarities across these
two junctions as shown in Figure 2. The flow of electrons into the base is now heavily influenced by the positive supply
voltage, VCC applied to the collector due to it being higher than the applied base voltage.
Figure 2. NPN Biasing Voltages Since the collector-base junction is forward biased with regards to the base terminal, this
IC excess of electrons flow from the emitter to the collector circuit attracted by the supply
C voltage, VCC.
C If the base biasing voltage is increased further, the
Collector-Base Reverse-bias is a positive
IC base-emitter junction becomes even more forward
N pn-junction VCB voltage applied to the n-type
+ + biased with the resistance of the conducting
ICE
region and a negative voltage
IB B VCC B IB channel between the collector and the emitter
P VCE
applied to the p-type region
+ decreasing to a very low value allowing even more
+ - current to flow between the collector and the emitter. Thus the transistor now starts to act
IBE -
VBE
N Emitter-Base
VBE
- IE as a closed-circuit switch.
pn-junction
-
E IE = IB + IC For the PNP transistor, the voltage biases and directions of current flow are opposite to
IE those for the NPN transistor as current will only flow from the emitter to the collector
E
when the base terminal is forward biased by a voltage which is 0.7 volts more negative

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than that at the emitter. For current to flow through the emitter to the collector, both the 3.1 Beta (transistor current gain)
base and the collector must be negative with respect to the emitter.
Although the base-collector junction is reverse biased, it acts as a current-controlled
Figure 3. PNP Biasing Voltages current source (CCCS). The amount of collector current, IC flowing through the transistor
depends heavily on the amount of base current, IB. Thus the ratio of collector current to
IC base current ∆IC/∆IB defines a junction transistor’s DC current gain.
C
This DC current gain ratio is given by the Greek letter, β (beta) or simply hFE . That is:
C
Collector-Base
IC Beta, β(dc) = IC/IB
P pn-junction
-
VCB
- Then Beta is defined as being the common emitter DC current gain. Thus the collector
IB B VCC B IB current of a bipolar transistor is found simply by multiplying the base current by the beta
N IEC VCE value and is expressed as: IC = βDC*IB.
-
- + +
IEB Most general purpose transistors, beta ( βDC ) ranges about 25 for high power transistors to
VBE
P Emitter-Base
VEB
+ IE over 400 for small signal transistors, with 100 being a common standard beta value.
pn-junction
+
E IC = IE - IB Unfortunately, for a given bipolar transistor beta is not always a constant value as it is
IE strongly influenced by variations in junction temperature, collector current and can also
E vary between transistors of the same model number. However, there are various biasing
techniques which can be used to reduce the variation of beta.

3.2 Alpha (forward current transfer ratio)

3. Transistor Gain The fraction of: β/(1+β) is called Alpha, and is given the Greek letter, α (alpha). Alpha is
the common base current gain and is defined as the ratio between collector current and
As the power supply, Vcc provides the energy required to move the current through the emitter current: IC/IE. That is:
transistor, most of the electrons injected into the base region from the emitter are attracted
towards the much larger potential present at the collector terminal. Alpha, α(dc) = IC/IB
Since the base and collector currents flow into the In general, the parameter α is a measure of the quality of a transistor and is very close to
The ∆ (delta) symbol simply
emitter region, the flow of current through an NPN unity, 1. Alpha is typically in the range of 0.9 to 0.998 as the emitter current is very similar
means a “change in” the
transistor can be expressed mathematically as being: to that of the collector current. Thus IC ≈ IE.
current value
IE = IB + IC Then a bipolar transistor with a beta value of say 100, ( β(dc) = 100 ) would correspond to
an alpha value of 0.99 ( α = 0.99 ) since : β = α/(1-α).
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Figure 6. Transistor Saturation Region

4. Transistor Operating Regions
C + 3. Saturation Region – maximum current flows into the
As a current controlled device, variations in base current, IB cause proportional variations IC(MAX) base with the base-emitter junction heavily forward biased
in the collector current, IC. Thus the collector current is an increased version of the base NPN +
since VBE >> 0.7V.
IB = MAX VCE =
terminal current showing that the bipolar transistor is a current amplifying device. 0.2V
B
+ - - The base current is so large that the collector-base junction
Bipolar junction transistors have the ability to operate within one of three distinct regions becomes forward biased and the transistor is fully-on.
cut-off, active and saturation. The region of operation depends upon how the voltages on VB>>0.7V
IE(MAX) The transistor appears as a low impedance short circuit
each of the three terminals relate to each other. E - between the collector and emitter terminals.
Figure 4. Transistor Cut-off Region Maximum collector current flows so Ic = I(sat) and VCE = VCE(SAT) ≈ 0.2V. As the transistor is
in saturation, any increase in base current will have little or no affect on the saturated
C 1. Cut-off Region – when VBE < 0.7V, the base-emitter
collector circuit, so the relationship of: IC = βDC*IB is no longer valid.
IC = 0 junction is not forward biased so no current flows into the
NPN base, IB = 0. The transistor is essentially inactive and not
IB = 0 conducting. 5. Bipolar Junction Transistor Characteristics Curves
B VCE = VCC
VB < 0.7V In cut-off mode the transistor appears as a high impedance Then we can see that there is a direct link between the voltage and current at the base
IE = 0 open circuit between the collector and emitter terminals. terminal, and its collector terminal in order to drive the transistor into one of its three
E Thus no collector current flows and the transistor is fully-off. regions. The characteristics of a transistors operation are generally presented in the form
IC = 0 and VCE = VCC = maximum voltage. of a set of graphs commonly referred to as a "transistors characteristics curves".

Figure 5. Transistor Active Region Typical transistor characteristics curves are a set of current-to-voltage lines on a graph
showing the I–V relationship for a given bipolar transistor. I-V characteristics curves can be
C + 2. Active Region – the base-emitter junction is forward biased used to show the input and output behaviour of a device.
IC and VBE ≥ 0.7V. The collector-base junction is reverse biased
IB > 0
NPN
and the transistor operates as a fairly linear amplifier. 5.1 Junction Transistor Input Characteristics
+ - IB Forward Bias
B Collector current is proportional to the base current by the The input characteristics of a typical junction transistor
VB > 0.7V constant beta since IC is βDC*IB. The transistors collector to operating in common-emitter mode represent that of a
+ - forward biased diode for the base-emitter junction.
IB
IE
emitter voltage, VCE is now somewhere between fully-off and P N
fully-on but greater than 0.2 volts. VCE > 0.2V. i
Input characteristic curve of 5.1 shows the transistors base
E - knee
VBE current, IB as a function of its base-emitter voltage, VBE with
the collector open. Thus IB is plotted against VBE.
0.4 0.6 0.7 0.8 1.0 VBE

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When VBE is below the forward bias voltage of 0.7 volts, the barrier potential of the diodes Changing the value of the base biasing current IB, we can create a different set of curves
pn-junction begins to form, but there is insufficient external base voltage to overcome it to plot IC against VCE for different values of IB from zero to saturation. These set or family of
so very little current flows into the base. Thus below 0.7 volts IB is virtually zero. collector current curves will tell us a great deal about the operation of the transistor.
As the base-emitter voltage, VBE starts to increase above 0.7 volts, the height of the 6. The Bipolar Transistor as a Switch
potential barrier is reduced. This allows current to flow across the junction from p-type to
the n-type side. The “knee” point on the characteristics occurs when VBE ≈ 0.7 volts. A bipolar junction transistor can be used as an amplifier or as an electronic switch
depending on its operating region. When used as an ON/OFF electronic switch, the
As the base-emitter voltage increases further, the current flowing through the junction
transistor is biased so that it operates between its cut-off region (switched off) and its
also increases at a much larger rate showing that the pn-junction exhibits an exponential
saturation region (switched on).
current–voltage characteristic for a normal silicon diode.
Figure 7. Open Circuit (Switch) Condition (cut-off)
5.2 Junction Transistor Output Characteristics
• The base terminal is grounded ( 0v )
The output characteristic for a bipolar junction transistor is its collector current, IC as a VCC VCC
• Base-emitter voltage VBE < 0.7 volts
function of collector-emitter voltage, VCE for a given amount of base current, IB. It is these • Base-emitter junction is reverse biased
RL IC = 0 VL = VCC
two quantities of output current, IC and output voltage, VCE that represents the output • Transistor is “fully-OFF” ( cut-off region )
characteristics of the transistor when connected in a common emitter configuration. VOUT VOUT = VCC • No collector current flows ( IC = 0 )
IB = 0 Rin C • VCE = VCC no voltage drop across RL
B Switch
IC (mA) The characteristics curve of 5.2, shows how an NPN • VOUT = VCE = VCC = 1 ( logical one )
open
8 Saturation transistors output responds to a fixed value of base VIN E
• The output is opposite to the input
current. In this example, IB = 10uA. 0v 0v 0v
7 Region
• Operates as an “open switch”
6 Active Region
5 As the collector-emitter voltage VCE is increased for Figure 8. Closed Circuit (Switch) Condition (saturation)
4 IB = 10uA a given base bias current. The transistor starts to
3 • The base terminal is connected to VCC
IC
conduct allowing collector current to flow. Collector VCC VCC
2 VCE
current, IC increases steadily in response to VCE up to • Base-emitter voltage VBE >> 0.7 volts
1
the switching “knee” point of about 2 volts. • Base-emitter junction is forward biased
RL IC = Max. VL = IC*RL
0 1 2 3 4 5 6 7 8 9 10 VCE • Transistor “fully-ON” ( saturation region )
The characteristic curve then flattens out, as above VOUT VOUT = 0v • Max collector current flows: IC = VCC/RL
this value, any large change in the collector-emitter voltage results in very little change of Rin C • VCE < 0.2v ( ideal saturation )
B Switch
collector current for a given base bias current. Thus this curve can also be referred to as closed • VOUT = VCE = 0 ( logical zero )
VIN > 0.7v E
the transistors constant current characteristic since a fixed base current will regulate the 0v • The output is opposite to the input
0v
amount of collector current at a value determined by beta. • Operates as a “closed switch”

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or 232Ω to the nearest preferred value. This LED series resistor, RC will dissipate under
7. Transistor Switch Application 33mW (I2*R) when the LED is illuminated. So a 1/4 watt resistor is more than sufficient.
Transistor switches have many applications from switching digital circuits to high power The DC current gain value of the transistor is 100. Thus IB = IC/βDC = 0.012/100 = 120uA
loads. By driving the transistor hard into its saturation region, nearly all the power supply
voltage VCC is dropped across the connected load allowing relatively large collector load The output from the digital circuit switches between 0 and 5 volts, so applying Kirchhoff’s
currents to flow through the transistor without causing it to overheat as VCE is generally voltage law (KVL) again to the base circuit:
less than 0.5 volts.
VDIGITAL – VRB – VBE = 0, thus: 5 – VRB – 0.7 = 0, therefore VRB = 4.3 volts
Figure 9. Transistor Switching Circuit
VCC = +5v VR B 4.3
Let’s assume we want to use a transistor RB = = = 35834 Ω or 35.8k Ω
as a switch to operate an orange coloured IB 0.00012
RC Light Emitting Diode (LED) indicator lamp
using a Digital or Arduino logic circuit as or 36kΩ to the nearest preferred value. Base resistor, RB will dissipate less than 520uW, so
LED shown in Figure 9. again a standard 1/4 watt resistor is more than sufficient. Reducing RB will increase IB and
drive the transistor harder into its saturation region.
NPN Let’s also assume that the output from
IC
BC107 the logic circuit is 0 (LOW) and +5 volts
VOUT = 5v
ON
OFF
ON
C (HIGH), and the supply voltage, VCC is +5V.
8. Darlington Transistor Configuration
RB B
IB + β = 100 The beta (β) value of the NPN BC107
A Digital We have seen previously that when operated as a switch, the ratio of collector current to
Logic Output
E transistor is 100, VCE when fully-on is 0.2 base current (∆IC/∆IB) is known as the current gain or beta value of the junction transistor.
VBE - volts, and the forward voltage drop, VF of Typical values of β for a standard bipolar transistor are in the range of 50 to 400 and varies
0v
the LED is 2 volts when illuminated. even between transistors of the same part number.
What would be the values of RB and RC if the LED draws a current or 12mA when illuminated. In some cases where the current gain of a single transistor is too low to directly drive a
given load, for example a power transistor. One way to increase the overall current gain is
Since VCC = 5v, applying Kirchhoff’s voltage law (KVL) to the collector circuit:
to use a two interconnected devices acting as one single transistor switch.
VCC – VRC – VLED(ON) – VCE(SAT) = 0, thus: 5 – VRC – 2 – 0.2 = 0, therefore: VRC = 2.8 volts A dual transistor configuration, also known as a “Darlington pair”, consist of two NPN or
PNP transistors connected together back-to-back, so that the emitter current of the first
VR C 2.8 transistor TR1 becomes the base drive current of the second transistor TR2. In other words,
RC = = = 233 Ω the first transistor (TR1) is connected as an emitter follower and the second transistor (TR2)
IC 0.012 is connected as a common emitter amplifier as shown in Figure 10.

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Figure 10. Basic Darlington Transistor Configuration If two identical bipolar transistors are used to make one single Darlington configuration. If
beta β1 is equal to β2, then the overall current gain is given as:
Collector Collector
C C
If β1 = β2
IC IC
IC1
and IC = [β1 + (β1 x β2) + β2] x IB
B
Base IB
TR1 then: IC = (β2 + 2β) x IB
IC2 IB
TR1 Base TR2
B Generally, the value of β2 is much, much greater than that of 2β, in which case the 2β part
TR2
IB2 of the equation can be ignored to simplify the maths. Thus: IC = β2 x IB which shows that a
Darlington pair behaves just like a single transistor but with a very high DC current gain.
NPN IE PNP IE
E E The advantage of using a double transistor arrangement such as this, is that Darlington
Emitter Emitter transistor pairs can have very large current gains allowing collector current loads of
several amperes. For example: the NPN TIP120 or the TIP30xx series and their PNP
Using the NPN Darlington pair as the example, the collectors of two transistors are equivalents TIP125 or TIP29xx series.
connected together, and the emitter of TR1 drives the base of TR2. This configuration
achieves β multiplication since the base current, IB2 is equal to transistor TR1 emitter Thus Darlington transistor pairs are much more sensitive to input signals as only a very
current, IE1 as the emitter of TR1 is connected to the base of TR2. Therefore: small base current is required to switch a much larger load current with the typical gain
of a Darlington configuration being over 1,000. Whereas normally a single transistor stage
IC = IC1 + IC2 produces a DC current gain of only 50 to about 400.
The Darlington configuration
IC = (β1 x IB) + (β2 x IB2) Then we can see that a Darlington transistor pair with
is simply two back-to-back
a DC current gain of 1,000:1 could switch an output
IC = [β1 + (β1 x β2) + β2] x IB load current of 1 ampere in the collector-emitter bipolar transistors
circuit with an input base current of just 1mA.
Where: β1 and β2 are the gains of the individual transistors.
Since the switching characteristics of the Darlington pair are basically the same as
This means that the overall current gain, β is given as the current amplified by the first
those for a single transistor emitter follower circuit, they can both be used in similar
transistor is amplified further by the second transistor as the two current gains multiply.
applications. Making Darlington configurations, either as an NPN pair or PNP pair, ideal
Thus two bipolar junction transistors connected together can make one single Darlington for high power switching applications such as interfacing relays, lamps, solenoids and DC
configuration with a very high beta value. For example, two interconnected transistors motors to low power microcontroller, Arduino’s or TTL & CMOS logic gates.
with a beta of 100 and 50 respectively, would give a total beta value of 5000. That is IC can
be 5000 times greater than IB. Effectively creating a super-beta transistor.

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Figure 11. Darlington Transistor Switch Figure 12. Sziklai Darlington Pair
+VCC Collector Collector
IM C C
Flywheel DC Motor NPN PNP
M Configuration
IC ≈ β2 x IB Configuration
IC ≈ β2 x IB
Diode Control

Digital ON ON TR2 TR2

Output C B IB B
OFF
RB B Base TR1 PNP Base TR1 NPN
β >1000 IB
IB NPN PNP
NPN
E E
TTL or CMOS E
TIP120 Emitter Emitter
0v

The Sziklai Darlington pair configuration of Figure 12. contains two opposite polarity
Here in the circuit of Figure 11. the base of the Darlington transistor is sufficiently sensitive bipolar junction transistors within its configuration. The cascaded combination of an
to respond to any small input current from a switch or directly from a TTL or 5V CMOS NPN and a PNP transistor has the advantage that the Sziklai pair performs the same basic
logic gate. The maximum collector current IC(max) for any Darlington pair is the same as function of a Darlington pair except that it only requires 0.6v for it to turn-ON.
that for the main switching transistor.
Similar to the standard Darlington configuration, the DC current gain is equal to β2 for
One of the main disadvantages of a Darlington transistor pair is the minimum voltage equally matched transistors or as before, is given by the product of the two current gains
drop between the base and emitter when fully saturated. Unlike a single transistor switch for unmatched individual transistors.
which has a saturated voltage drop ( VCE(SAT) ) of between 0.2v and 0.6v when fully-ON. A
Darlington device can have over twice the base-emitter voltage drop as the base-emitter
voltage is the sum of the two individual transistor diode junctions.
10. The Bipolar Transistor as an Amplifier
Thus the collector-emitter saturation voltage, VCE(SAT) of a Darlington’s interconnected As well as being used as a solid state switch to turn connected loads “ON” or “OFF” by
arrangement is much higher than that of a single bipolar transistor. To overcome this controlling their base signal, one of the main applications of bipolar junction transistors
issue the Sziklai Darlington Pair was developed. is in signal amplification. Because as we have seen, a small current applied to the base
terminal can control the flow of a much larger current in the collector terminal from a DC
9. The Sziklai Darlington Pair power source. Bipolar transistor amplifiers
Biasing the transistor in such a way as to operate it in operate entirely in their
The Sziklai Darlington Pair, named after its Hungarian inventor George Sziklai, is a active region due to fixed
complementary or compound Darlington configuration which consists of separate its active region. The transistor will amplify any small
AC signal applied to its base with the emitter grounded. or self-biasing methods
NPN and PNP complementary transistors connected together to overcome some of the
disadvantages of using two NPN or two PNP devices. Thus the basic transistor switch circuit can also be used as an amplifying device.

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One such commonly used configuration of an NPN transistor is that of the "Common- Having created a set of characteristics curves for a particular transistor, we can determine
emitter Amplifier" configuration, producing what is generally called a “Class-A amplifier” its quiescent operating, or Q-point. This defines its operating region to give an undistorted
circuit. But there are many other kinds of BJT amplifier circuit configurations, each with amplified output by setting the required biasing current and voltage conditions.
its own special features, advantages and applications.
The Q-point can be fixed anywhere within the active region but to ensure that both the
10.1 The Common Emitter Amplifier positive and negative half cycles of the ac input waveform are correctly amplified, the
Q-point is generally centred around the middle of the previous characteristic curves to
Biasing a bipolar junction transistor means finding the required value of collector-emitter provide the maximum symmetrical output voltage swing.
voltage VCE and collector current IC required to continually forward bias the base-emitter
junction halfway between its cut-off and saturation regions. 10.2 DC Load Line Construction
Transistor biasing allows the amplifier circuit to accurately reproduce the positive and When operated as a solid state switch, the transistor is biased between its cut-off and
negative halves of any ac input signal, thus producing an undistorted output voltage saturation regions. In its cut-off region, IC = 0 and VCE is at maximum as VCE(OFF) = VCC.
waveform. The values of VCE and IC define the amplifiers quiescent operating point or Likewise when it is in its saturation region, IC is maximum and VCE = 0.2v (zero volts for
ideal condition). Collector current IC is limited only by the collector load resistance, RL.
Q-point which only occurs when the transistor is correctly biased into its active region.
A DC load line can be plotted on the characteristic curves between VCE(OFF) = VCC (point “B”)
We recall previously that input and output characteristics curves can be plotted for any on the x-axis, and IC(SAT) = VCC / RL (point “A”) on the y-axis as shown in Figure 14.
transistor and that the output curve is created using a fixed base current value for varying
values of collector-emitter voltage, VCE. Thus we can create a set or family of output Figure 14. DC Load Line
characteristics curves for different values of base current, IB as shown in Figure 13. IC Saturation DC Load
(mA) Region Active Region
Line
When VCE = 0
Figure 13. Determining the I-V Characteristics of a BJT V A
IC(SAT) = CC
RC
Saturation IB5
IC = IB =
Region Active Region
mA IB6 uA
IC
RC IB4
NPN IB5
Transistor C IB4 Q-point IB3
+ ICQ
RB +
IB IB3
VCE VCC IB2
B + IB2
-
+ - IB1
VBE IB1
V
- BB -
E IB = 0 B
0
0v Cut-off Region 0 IB = 0
0
Cut-off Region
VCE VCE (V)
0 VCEQ When IC = 0
VCE(OFF) = VCC

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 The B ip ol a r Junc tion Tr a nsis tor

Having determined the position of the Q-point on the centre of the load line, the Figure 15. DC Analysis of the Amplifier Circuit
quiescent currents and voltages, ICQ and VCEQ respectively can be found. As the Q-point is VCC = 12v
centred on the load line between points A and B, and with no ac input signal applied, ICQ
will be equal to half IC(SAT) and VCEQ will be equal to half VCC. I1 I2
RB1 RC
20k Ω 1.2k Ω
Thus centring the Q-point on the DC load line allows
for optimum ac operation of the bipolar transistor The position of the Q-point on IC = 5.09mA
amplifier when connected in its common emitter a DC load line determines the = 100
VOUT
configuration allowing the relationship of: IC = βDC*IB maximum undistorted output CIN IB =
signal for a given input signal VIN 50uA
to be valid once again.
VCE (sat) = 0v
However, the Q-point is greatly affected by changes in a transistors beta value. Changes in 2.5uF
operating temperature or the replacement of the transistor for another will cause changes VB = IE =
in beta. Resulting in the movement of the Q-point along the load line affecting its DC 1.83v 5.14 mA
operating point. Biasing the transistors base with a fixed voltage value prevents, or greatly RB2 RE
limits the movement of the Q-point thereby increasing stability of the amplifier circuit. 3.6k Ω 220 Ω VE = 1.13v

0v
10.3 The Base Biasing of a Junction Transistor
There are many different ways to bias the base terminal a bipolar junction transistor, such
as: fixed current bias, emitter feedback biasing, collector feedback biasing, dual supply 2). Find the emitter voltage, VE
biasing, etc. but the most common form of biasing the base of a transistor is by using a
VE = VB - VBE = 1.83 - 0.7 = 1.13 Volts
voltage divider network.
3). Find the emitter current, IE
Consider the common emitter (CE) transistor amplifier of Figure 15. which has a voltage
divider biasing network of RB1 and RB2, plus an emitter resistance, RE. Remember that the IE = VE/RE = 1.13/220 = 5.14mA
common emitter amplifier has a load resistor in its collector circuit. The collector current
through this resistance produces the voltage output of the amplifier. 4). Determine the collector (load) current, IC

1). The open-circuit base biasing voltage divider value:  β   100 

IC =   E
I =   × 0.00514 = 5.09mA
   β +1   100 +1 
R B2  3k6 
VB =   VC C =   ×12 = 1.83V
 R B1 + R B2   20k + 3k6  Note that the difference between IC and IE will be the base current IB as: IB = IE - IC

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 The B ip ol a r Junc tion Tr a nsis tor

5). The quiescent base current, IB Figure 16. Amplifier Characteristics Curves
IB = IE - IC = 0.00514 - 0.00509 = 50uA
6). The voltage drop across load resistor, RC when VCE = 0 (saturation)
VCC
VRC = IC x RC = 0.00509 x 1200 = 6.1 Volts IC = (mA) DC Load
RL + RE Line
9 A IB = 90uA
IC (max) =
7). The collector-emitter voltage, VCE 8.5mA
8 IB = 80uA
IC (max) M
VCE = VCC - VRC - VE = 12 - 6.1 - 1.13 = 4.77 Volts
7 IB = 70uA
8). Biased DC output voltage, VOUT(DC) IB = 60uA
6
IC (Q)
VOUT = VCC - VRC = 12 - 6.1 = 5.9 Volts = 5.09mA Q IB = 50uA

4 IB = 40uA
Thus the Q-point sits halfway along the load line and is inside the Active region of the Q-point
characteristics as shown in Figure 16. 3
IB = 50uA IB = 30uA

Having calculated the DC biasing network, we can turn our attention to the small signal 2
IB = 20uA
analysis of the circuit when an ac input signal is applied. The input signals path to ground
is through the forward-biased base-emitter junction. The dynamic resistance of this
IC (min)
1
N IB = 10uA

junction is given as a resistor, r’e. B

0 Vce
1 2 3 4 5 6 7 8 9 10 11 12 13
As the dynamic resistance of a forward-biased diode junction is equal to 25 mV divided VCE (Q)= 5.9V
by the current flowing through the diode junction, the formula used for calculating when Ic = 0
the dynamic resistance, r’e of the forward-biased base/emitter junction is r’e = 25 mV/ VCE = VCC
VCE (min)
IE. Where IE is the emitter current. Thus the transistors dynamic leg resistance r’e is
calculated as being:
VCE (max)
8). The dynamic emitter leg resistance, r'e

r'e = 25mV / IE = 0.025/0.00514 = 4.9 Ω

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 The B ip ol a r Junc tion Tr a nsis tor

The voltage gain of the transistor amplifier is given as: Av = VOUT/VIN where VOUT = IC x RC For an NPN transistor, the red (positive) multimeter lead is connected to the base (B)
and VIN = IE x (RE + r’e). Since IE is approximately equal to IC (IE≈IC) the two current values terminal, and the black (negative) lead will be connected to either the emitter (E) or the
effectively cancel each other out giving the voltage gain of the circuit as: collector (C) terminal. In both directions, base-to-collector and base-to-emitter should
read between 0.6 and 0.7 volts (for silicon) as they are forward biased. If either direction
V OUT  RC   1200  reads high impedance on the multimeter, then the chances are it’s a PNP transistor in
AV = =   =   =  5.36 reverse bias.
V IN  E
R + r'e   220 + 4.9 
For an PNP transistor, the black (negative) lead will be connected to the base, and the red
Thus for an input signal voltage of 0.5Vpk, the output voltage will be: -5.36*0.5 = -2.68Vpk.
(positive) lead to either the emitter or collector. Again in both directions, base-to-collector
and base-to-emitter should read between 0.6 and 0.7 volts as they are forward biased.
11. Testing a Bipolar Junction Transistor If either direction reads high impedance on the multimeter, then the chances are it’s an
NPN transistor in reverse bias.
Bipolar transistors are three-layer two-junction devices so can be viewed more simply
as two series connected diodes with a common connection. By assuming that a bipolar This type of test can quickly show what type of transistor you have an NPN or a PNP, but
transistor is nothing more than two diodes connected at the base and having separate make sure to note the results and polarise the test leads accordingly. Another very simple
emitter and collector leads, transistor can be identified using just a basic multimeter and effective way to identify whether the device is an NPN or PNP transistor, is to look up
which has a diode test function or a low DC voltage setting. the device part number on the internet for its datasheet and operating parameters.

To determine the type of transistor, the multimeter leads are connected in different End of this Bipolar Junction Transistor eBook
combinations across the three connecting terminals identifying the two diode junctions
of Figure 17. Thus forward biasing a pn-junction will have a low resistance while reverse Last revision: September 2022
biasing the pn-junction will give an infinite resistance. Copyright © 2022 Aspencore
https://www.electronics-tutorials.ws
Figure 17. Bipolar Transistor Two-diode Model for Testing Free for non-commercial educational use and not for resale
C C C C
- + + -
High
0.7V 0.7V Impedance
High With the completion of this Bipolar Junction Transistor eBook you should have gained a
Impedance
basic understanding and knowledge of bipolar transistors. The information provided here
+ - - + should give you a firm foundation for continuing your study of electronics and electrical
B B B B
+ - - + engineering. In ebook 14 you will learn about the Field Effect Transistor, or FET.
High High
0.7V 0.7V
Impedance Impedance For more information about any of the topics covered here please visit our website at:
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NPN PNP NPN PNP

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