Bipolar Junction Transistor (BJT) ( 08 H ):
PNP and NPN transistors–current components in BJT – BJT static
characteristics (Input and Output) 1H– Early effect- CB, CC,CE
configurations (cut off, active, and saturation regions) CE configuration
as two port network 1 H – Alpha and Beta of a transistor ,Biasing and
load line analysis – Fixed bias and self bias arrangement 1H. Transistor
action, Transistor as an amplifier, Operating point, Load line 1 H,
expressions for current gain, voltage gain, input impedance, output
impedance and power gain 1 H. Power amplifier - power BJT - Thermal
resistance - Maximum power 1 H- Class A, Class B,Class AB and Class
C amplifiers1 H -Basic operational amplifier- Differential amplifier 1H.
1
Prof. Dr. Ali S. Hennache- Department of
Physics - College of Sciences - Al-Imam
Muhammad Ibn Saud Islamic University -
The Bipolar Transistor
OBJECTIVES
After studying the material in this chapter, student will be able to describe and/or
analyze:
transistor architecture,
transistors functioning as switches,
transistor characteristics,
base biasing of amplifier circuits,
signal parameters of the amplifier circuit,
the procedure for measuring input and output impedance,
transistor output characteristic curves
2
Prof. Dr. Ali S. Hennache- Department of
Physics - College of Sciences - Al-Imam
Muhammad Ibn Saud Islamic University -
INTRODUCTION
In 1947. J. Bardeen and walker Brattain were the first to invent the “transistor” by
adding another junction to the p-n junction diode and Schottkty diodes which could
control the flow of majority carriers and to form electronic switching circuits. During the
discovery, the device was nothing more than a special kind of resistor whose value
changes and thus the inventors first called it “the transformative changing variable
resistor “,because of such long name, they shorted the name to “the transforming
resistor” but they were still believing the name was quite long and thus they finally
agreed with what is well known device in technology revolution the “Transistor”
Thus, the name transistor comes from the phrase \transferring an electrical signal
across aresistor".
The “Transforming Resistor”
Look around you, Everything you see is controlled either with computers or electronics
of some sort. Your car today has more computational control systems and even energy,
water, air-conditioning house are electronically controlled and all electronically controlled
these days are solid state and based on the transistor so that means transistor and its
derivatives are all extremely important and an absolutely basic to everyday life. In this
part of the course we will find out how transistors work , how they are made and the
physics behind them and what they do.
3
Prof. Dr. Ali S. Hennache- Department of
Physics - College of Sciences - Al-Imam
Muhammad Ibn Saud Islamic University -
INTRODUCTION
Transistors are thus, solid state devices used for:
1- amplifying, and thus are capable of
• Current gain
• Voltage gain
• Signal-power gain
2- controlling and
3- generating electrical signals.
They are used widely in electronic equipments such as pocket calculators,
radios, communication satellites and in general transistors are used in
information and communication technologies (ICT).
4
Prof. Dr. Ali S. Hennache- Department of
Physics - College of Sciences - Al-Imam
Muhammad Ibn Saud Islamic University -
Bipolar junction transistor fundamentals
A transistor is a three-layer device alternately doped semiconductor regions, whereas a
diode has only two layers. Figure 1(a) shows how two layers of N-type material
sandwich one layer of P-type material to make an NPN transistor. Figure 1(b) shows
how two layers of P-type material sandwich one layer of N-type material to form a PNP
transistor. Most transistors used today are NPN because this is the easiest type to
make from silicon .In both cases, the layer in the center is the base, and the two outer
layers are the emitter and collector. The arrow on the schematic symbol identifies the
emitter and always points to the N-type material.
(b)
(a)
E
Base
Emitter
Collector
(p-type
)
(n-type)
(n-type)
C E
Base
Emitter
Collector
(n-type
)
(p-type)
(p-type)
B
B
Figure 1: A three-layer device (Transistor),(a) shows how two layers of N-type material
sandwich one layer of P-type ,(b) shows how two layers of N-type material sandwich
one layer of P-type.
5
Prof. Dr. Ali S. Hennache- Department of
Physics - College of Sciences - Al-Imam
Muhammad Ibn Saud Islamic University -
C
The Bipolar Transistor
Bipolar transistors, also called BJTs (Bipolar Junction Transistors), are three-terminal
devices that can function as electronic switches or as signal amplifiers. In this chapter,
you will learn how transistors perform both of these important functions. Figure 2. shows
the schematic symbols for the two types of bipolar transistors.
The schematic symbols show the three terminals of the transistor. The leg with the
arrow is called the emitter (E), the collector (C) is the other slanted leg, and the middle
region the base(B) is the leg connected perpendicular to the line connecting the emitter
and the collector and is very thin compared to the diffusion length of minority carriers
These terms refer to the internal operation of a transistor but they are not much help in
understanding how a transistor is used.
Collector
Base
Collector
Base
Figure 2. Transistor symbols
NPN TYPE
6
Emitter
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PNP TYPE
Emitter
npn BJT Structure
The emitter (E) and is heavily doped (n-type).
The collector (C) is also doped n-type.
The base (B) is lightly doped with opposite type to the emitter and
collector (i.e. p-type in the npn transistor).
The base is physically very thin .
7
Prof. Dr. Ali S. Hennache- Department of
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Muhammad Ibn Saud Islamic University -
NPN vs PNP
1) NPN. If the base is at a higher voltage than the emitter, current flows from collector
to emitter.
2) NPN. Small amount of current also flows from base to emitter.
3) NPN. Voltage at base controls amount of current flow through transistor (collector
to emitter).
4) PNP. If the base is at a lower voltage than the emitter, current flows from emitter to
collector.
5) PNP. Small amount of current also flows from emitter to base.
6) PNP. Voltage at base controls amount of current flow through transistor (emitter to
collector).
7) The arrow to shows the direction of current flow.
8
Prof. Dr. Ali S. Hennache- Department of
Physics - College of Sciences - Al-Imam
Muhammad Ibn Saud Islamic University -
TYPES OF TRANSISTORs
Darlington pair
is two transistors connected together to give a very high current gain.
In addition to standard (bipolar junction) transistors, there are (Figure 3):
The Field-Effect Transistor which are usually referred to as FETs
The Metal-Oxide Semiconductor Field-Effect Transistor (MOSFET)
The Uni -Junction Transistor (UJT).
All the three above types are beyond the scope of this chapter
http://www.mikroe.com/old/books/keu/04.htm
Darlington pair
This is two transistors connected together
so that the current amplified by the first is
amplified further by the second transistor.
The overall current gain is equal to the two
individual gains multiplied together:
Prof. Dr.
S. HennacheDepartment of
Figure.
3: Ali
Different
transistors
9
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Current flow convention
- VCE
E
+
Base
Collector
(p-type) (n-type)
Emitter
(n-type)
C
IC
IE
- VBE
+
B
IB
+ VBC
-
We shall treat the transistor as a current node and write according to 1st and 2nd
Law of Kirchhoff the following :
IE = IB + IC
10
and
VEB + VBC + VCE = 0
Prof. Dr. Ali S. Hennache- Department of
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Muhammad Ibn Saud Islamic University -
VCE =  VEC
Current flow convention
+ VEC E
Base
Collector
(n-type) (p-type)
Emitter
(p-type)
IE
C
IC
B
+ VEB
IE = IB + IC
11
-
and
IB
VEB + VBC + VCE = 0
Prof. Dr. Ali S. Hennache- Department of
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Muhammad Ibn Saud Islamic University -
- VCB +
VCE =  VEC
TRANSISTOR CONFIGURATIONS
Most electronic devices take the signal between two input terminals and deliver from it an
output signal between two output terminals.
The BJT has only three terminals so one of these is usually shared (i.e. made common)
between input and output circuits. In this chapter all three methods that a transistor can be
connected will be covered
1-common emitter (CE),
2-common base (CB) and
3-common collector (CC) configurations.
The CE configuration is the one most commonly encountered since it provides both good
current and voltage gain for ac signals.
In the CE configuration the input is between the base and the emitter. The output is
between the collector and the emitter.
12
Prof. Dr. Ali S. Hennache- Department of
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TRANSISTOR CONFIGURATIONS
The p-n junction joining the base and emitter regions is called the baseemitter (B-E) junction. (or emitter-base)
The p-n junction between the base and collector regions is called the
collector-base (C-B) junction.(or base-collector)
In normal operation for analogue (linear amplifier) circuits the emitter-base
junction is forward biased and the collector-base junction is reverse biased.
These ‘bias’ or ‘quiescent’ conditions are set by d.c. bias circuits.
The a.c. (‘analogue’) signal to be amplified is superimposed on top of the d.c.
bias voltages and currents. (Exactly as for dynamic resistance, small variations
about a Q point, in our discussion of diodes.)
The forward bias between the base and emitter injects electrons from the
emitter into the base and holes from the base into the emitter.
13
Prof. Dr. Ali S. Hennache- Department of
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Muhammad Ibn Saud Islamic University -
GENERAL CONNECTION CHARACTERISTICS
Comparison of the characteristic sizes (table below) of the 3 different
transistor connections .
Common
Base
14
Common
Emitter
Common
Collector
Input
Impedance
Low (about 50
Ω)
Medium (1-5
KΩ)
Output
Impedance
High (500KΩ1MΩ)
Medium (about Low (up to 300
50KΩ)
Ω)
Current Gain
Low (<1)
High (50 - 800) High (50-800)
Voltage Gain
Low (about 20) High (about
200)
Low (<1)
Power Gain
Low (about 20) High (up to
10000)
Medium (about
50)
Prof. Dr. Ali S. Hennache- Department of
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High (300-500
KΩ)
SWITCH FUNCTION
If Vin is high, T is ON, switch
is closed and Vout is low.
Digital “0”
If Vin is low, T is OFF, switch
is open and Vout is high.
Digital “1”
Switch function occurs when high base voltage (>0.7 V)saturates the
transistor and it fully conducts current in the C-E path resulting in Vout =0.
or when the the base voltage is negative. Then it cuts off the current in the CE path and Vout =Vcc.
This is the means by which digital or on/off switching can be accomplished
and forms the basis for all digital circuits (including computers)
15
Prof. Dr. Ali S. Hennache- Department of
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TRANSISTOR SWITCHES
A transistor can function as an SPST (single-pole single-throw) switch, but rather than
being mechanically controlled, it is controlled by an electronic signal driving the base
terminal. Figure below shows a comparison between an open SPST switch and an NPN
transistor.
When the switch is open, as shown in Figure 4–
8(a), there is no current flowing in the circuit
and the bulb is off. When the control signal on
the base terminal of the transistor turns the
transistor off, as shown in Figure 4–8(b), the
transistor acts like an open switch. The
resistance between the collector and the emitter
terminals rises infinitely high and prevents
current flow in the circuit. The bulb in series with
the transistor is off.
Figure 4–8 Open switch equals off transistor
16
Prof. Dr. Ali S. Hennache- Department of
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COMPARISON OF SPST SWITCH AND TRANSISTOR
Figure below shows a comparison between the SPST switch and the transistor when
turned on. When the switch is closed, current flows in the circuit and lights the bulb.
Likewise, when the control signal on the base terminal turns the transistor on, the
resistance between the collector and emitter drops to zero, and the current flow lights
the bulb. Actually, the transistor is not a perfect switch. When it is off, the resistance
between the collector and emitter (RCE) does not go to infinity, and when it is on, the
resistance between the collector and emitter (RCE) does not drop to zero. Even though
the transistor is not a perfect switch, it is close enough to function well in most circuits.
“CLOSED” SWITCH = “ON” TRANSISTOR
17
Prof. Dr. Ali S. Hennache- Department of
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Muhammad Ibn Saud Islamic University -
CONFIGURATIONS AS AMPLIFIER
Each configuration, as you will see later,
has particular characteristics that make it
suitable for specific applications. An easy
way to identify a specific transistor
configuration is to follow three simple
steps:
1- Identify the element (emitter, base, or
collector) to which the input signal is
applied.
2- Identify the element (emitter, base, or
collector) from which the output signal is
taken.
3- The remaining element is the common
element, and gives the configuration its
name.
Therefore, by applying these three simple
steps to the circuits in figure on the left ,
we can conclude that this circuit is more
than just a basic transistor amplifier. 1st it
is a common-emitter amplifier,2nd is a
common base amplifier and 3rd is a
common collector amplifier
18
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Transistors applications
•Silicon transistor (bipolar junction transistor)
high gain, bandwidth, analog amplifier
•FET (field effect transistor)
high input impedance, analog amplifier
•MOS FET (Metal Oxide Field Effect Transistor)
digital, fast switching (preferred in computers,
microprocessors)
•CMOS (Complementary Metal Oxide Semiconductor) Transistor
low power, digital switching and analog (preferred in
low power implanted devices)
19
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Current components
1 = hole current lost due to recombination in base, IBR
2 = hole current collected by collector, ~ IC
1 + 2 = hole part of emitter current, IEP
5 = electrons injected across forward biased E-B junction, (– IBE); same as
electron part of emitter current, – IEN
4 = electron supplied by base contact for recombination with
holes lost, – IBR (= 1)
3 = thermally generated e & h making up reverse saturation current of
reverse biased C-B junction. (generally neglected)
20
Prof. Dr. Ali S. Hennache- Department of
Physics - College of Sciences - Al-Imam
Muhammad Ibn Saud Islamic University -
TRANSISTOR CHARACTERISTICS AND DATA SHEETS
There are hundreds of transistors on the market. The differences in these transistors
can be found by examining the electrical characteristics listed on the data sheets. The
three most important characteristics to know are:
123-
the maximum collector current (IC),
maximum power dissipation (PD), and
small signal beta (β).
Maximum collector current (IC)
is the maximum continuous current that can flow in the collector leg of the transistor
without damage to the transistor. Bipolar transistors are available with maximum
collector current ratings from 50 mA to 50 A.
Maximum power dissipation (PD)
is the maximum power the transistor can dissipate without being damaged. An
approximation of the power being dissipated by a transistor can be calculated by
multiplying the voltage across the transistor from collector to emitter (VCE) times the
collector current (IC). Bipolar transistors are available with maximum power
dissipation ratings from 0.2 W to 250 W.
21
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BETA (hfe) OF THE TRANSISTOR
DC beta (β) or (hFE) is the DC current gain of the transistor in the common-emitter
configuration. In most applications, the small signal beta and the DC beta are
interchangeable. In this text, we will assume they are interchangeable unless otherwise
noted. The formula for DC beta is β = IC/IB. Transistors are available with beta ratings
from 10 to 1000.
Maximum base current (IB)
is the maximum current that can flow in the base leg of the transistor without damage
to the transistor.
Collector to base breakdown voltage (VCBO)
is the maximum reverse-biased voltage that can be applied across the collector to base
junction. Bipolar transistors are available with collector to base breakdown voltage
ratings from 20 V to 1500 V.
22
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BETA OF THE TRANSISTOR
The ratio of how the emitter current divides into base and collector current is a
function of the particular transistor being used. Typically only about 1% of the
emitter current will exit the base, and 99% will exit the collector. The ratio of
collector current to base current is a parameter of the individual transistor and is
called beta (β). Beta can be stated mathematically as β = IC/IB.
Beta is sometimes referred to as hFE, which stands for forward current gain in
the common emitter configuration. Beta is the current gain of the transistor
where IC is output current and IB is input current (beta has no units, since it is a
ratio of two currents). The important thing to remember about beta is that it is a
fixed ratio between collector current and base current. Therefore, a small
change in base current will cause a large change in collector current. The base
current is the control current.
23
IE = IB + IC
Transistor current divider
β = IC / IB
Transistor current gain (beta)
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Performance parameters (pnp)
Neglect the reverse leakage (electron) current of C-B junction
Emitter efficiency:
I EP
I EP
 

I EP  I EN
IE
Base transport factor:
αT 
IC
I Ep
Common base dc current gain:
Fraction of emitter current
carried by holes.
 close to 1.
Fraction of holes collected by
the collector.
T close to 1.
I C  T I EP  T I E  dc I E
 dc   T 
Note that  is less than 1.0 but close to 1.0 (e.g.  = 0.99)
24
Prof. Dr. Ali S. Hennache- Department of
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Performance parameters (pnp)
Common emitter dc current gain, dc:
I C   dc I B
I C   dc I E   dc ( I C  I B )
  dc 
 I B
I C  
 1   dc 
 dc 
 dc
 T

1   dc
1   T
Note that  is large (e.g.  = 100)
For NPN transistor, similar analysis can be carried out. However, the emitter
current is mainly carried by electrons.
25
Prof. Dr. Ali S. Hennache- Department of
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Muhammad Ibn Saud Islamic University -
EXAMPLES
EXAMPLE 01
A transistor is connected as shown in Figure 1. It has a base current of 8 μA
and a collector current of 1.2 mA. What is the emitter current and the beta of
the transistor?
Step 1
Calculate emitter current.
Step 2
Calculate beta.
IE = IB + IC
IE = 8 μA + 1.2 mA
IE = 1208 μA
β = IC/IB
β = 2400 μA / 20 μA
β = 120
Figure 1
EXAMPLE 02
A transistor is connected as shown in Figure 1. It has an emitter current of
2.42 mA and a collector current of 2.4 mA. What is the base current and beta
of the transistor?
Step 1
Calculate base current.
IE = IB + IC
IB = IE – IC
IB = 2.42 mA – 2.40 mA = 0.02 mA or 20 μA
26
Step 2
Calculate beta.
Prof. Dr. Ali S. Hennache- Department of
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β = IC/IB
β = 1200 mA/8 mA
β = 150
EXAMPLES
EXAMPLE 03
A transistor is connected as in Figure 1 and has a base current of 16 μA and a
beta of 80. What is the collector current and emitter current of the transistor?
Step 1
Calculate the collector current.
β = IC/IB
IC = β × IB
IC= 80 × 16μA
IC= 1280 μA or 1.28 mA
Step 2
Calculate the emitter current.
IE = IB + IC
IE = 16 μA + 1280 μA = 1296 μA or 1.296 mA
27
Prof. Dr. Ali S. Hennache- Department of
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Muhammad Ibn Saud Islamic University -
examples
EXAMPLE 04
Design a circuit to control the on/off conditions of a 50 Ω load connected to
30 V. The control signal is a voltage switch between 0 V and 4 V. The load
will be on when the control voltage is 4 V and off when the control voltage is
0 V. The transistor used in the circuit will have a beta of 50.
Step 1
Draw a diagram of the switching circuit. (Figure 2 shows a diagram of the
circuit.)
28
Prof. Dr. Ali S. Hennache- Department of
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Examples
Step 2
Calculate the collector current when the load is in the on state.
The supply voltage is divided by the load resistance. The saturation voltage (0.2
V) of the transistor could be subtracted from the supply voltage, but it is not
significant in this case.
IC = IL = 30 V/50 Ω = 600mA
Step 3
Calculate the base current needed.
IB = IC/β = 600 mA/50 = 12 mA
Step 4
Calculate the value of VRB.
VRB = Vcontrol – VBE = 4 V – 0.7 V = 3.3 V
Step 5
Calculate the value of Rb.
Rb = VRB/IB = 3.3 V/12 mA = 275 Ω
29
Prof. Dr. Ali S. Hennache- Department of
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Muhammad Ibn Saud Islamic University -
Electronics “ Phys. 324 ”
Figure 4–10 shows a transistor switching circuit designed to control the on/off condition
of a 12 V bulb.When the control signal equals 5 V, the bulb is on, and when the control
signal equals 0 V, the bulb is off. The current flowing through the bulb is the collector
current. The collector current is zero when the bulb is off and is limited by the bulb
resistance when the bulb is on.
Example 4.4 shows how to design the switching circuitry.
EXAMPLE 4.4
Design the transistor switching
circuit shown in Figure 4–10.
30
Step 1. Calculate the collector current when the
bulb is in the on state. The supply
voltage is divided by the resistance of the load (bulb).
IC = IL = VCC/RL = 12 V/10 Ω = 1.2 A
Step 2. Using beta, calculate the needed base
current.
IB = IC/β = 1.2 A/100 = 12 mA
Step 3. Calculate the value of VRB.
VRB = Vcontrol – VBE = 5 V – 0.7 V = 4.3 V
Step 4. Calculate the value of Rb.
Rb = VRB/IB = 4.3 V/12 mA = 358 Ω (Use the next
lower standard value, 330 Ω.)
Step 5. Draw the switching circuit. (The switching
circuit is shown in Figure 4–10.)
Prof. Dr. Ali S. Hennache- Department of
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Muhammad Ibn Saud Islamic University -
THE TRANSISTOR AMPLIFIER
GAIN AND AMPLIFICATION
Transistor amplifiers are circuits that provide signal gain. Gain is an important
concept in electronics. Let us take a moment to make sure we have a
common understanding of the word.
Gain is the ratio of output to input.
The general formula is Gain = Output/Input. Gain has no units because the
output and the input must be in the same units, and the units cancel.
Figure below shows the symbol used for an amplifier. An amplifier is an
electronic circuit used to obtain gain. In the figure , the amplifier has an input
of 0.6 Vp-p and an output of 6 Vp-p.
By using the gain formula, we can calculate the voltage gain to be:
(6 Vp-p/0.6 Vp-p = 10). The word voltage preceding gain indicates that the
gain ratio is comprised of the output voltage and the input voltage. It is also
possible to have current and power gains.
31
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Expressions for gains
The general formula for the Gain (G) or Amplification (A) is:
GV (AV) = Output / Input
Voltage Gain :
GV (AV) = VOutput / VInput
Current Gain :
GI (AI) = IOutput / IInput
Power Gain :
Gp (Ap) = GV (AV) . GI (AI)
32
Prof. Dr. Ali S. Hennache- Department of
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Electronics “ Phys. 324 ”
Darlington pair
As we already said that this is two transistors connected
together so that the current amplified by the first is
amplified further by the second transistor. The overall
current gain is equal to the two individual gains multiplied
together:
Darlington pair current gain,
hFE = hFE1 × hFE2
(hFE1 and hFE2 are the gains of the individual transistors)
This gives the Darlington pair a very high current gain, such
as 10000, so that only a tiny base current is required to
make the pair switch on.
A Darlington pair behaves like a single transistor with a
very high current gain. It has three leads (B, C and E)
which are equivalent to the leads of a standard individual
transistor. To turn on there must be 0.7V across both the
base-emitter junctions which are connected in series inside
the Darlington pair, therefore it requires 1.4V to turn on.
33
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TRANSISTOR OUTPUT CHARACTERISTIC CURVES
A graph showing collector current (IC) versus collector/emitter voltage
(VCE) with base current (IB) held constant is called an output
characteristic curve. Figure (a) below shows an experimental circuit used to
obtain the needed data to draw an output characteristic curve. Figure (b)
shows the output characteristic curve obtained by holding the base current
constant at 5 μΑ and monitoring the collector current as the collector/emitter
voltage is varied from 0 V to 20 V.
Circuit for developing characteristic curves
Family of output characteristic curves
If the base current is adjusted to a second value and the process is repeated, data can be
obtained for a second curve. This process can be repeated several times, and a family of
output characteristic curves can be obtained as shown in Figure on the right.
34
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Inside transistor
The following drawing shows how the electrons and holes flow within the
transistor
This is generally what happens inside a transistor when voltage is applied.
The purpose of this theory is to explain how can someone use the transistor
to design an amplifier or a switch.
In any transistor scheme the symbol VEE is for the emitter supply, VCC for
the collector supply and VBB for the base supply.
35
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Hybrid parameters
The hybrid parameters [h]
The hybrid parameters are values that characterize the operation of a
transistor, such as the amplification factor, the resistance and others. They
are used to calculate and properly use the transistor in a circuit. Most of the
the hybrid parameter values are given in the datasheet by the manufacturer.
The hybrid parameters for Common Emitter (CE) connection
Here is the first set of hybrid parameters for a transistor connected with
Common Emitter.
hie - input impedance
The first hybrid parameter that we will see is the hie. This parameter is
defined by the result of the division of the VBE with the IB:
hie = VBE / IB
This parameters defines the input resistance of a transistor, when the
output is short-circuited (VCE=0).
36
Prof. Dr. Ali S. Hennache- Department of
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Muhammad Ibn Saud Islamic University -
Hybrid parameters
hfe - Current Gain
This is the most important parameter and is extensively used when calculating a
transistor amplifier. This is actually the only parameter you need to know to begin
designing amplifiers. The equation for this parameter is the following:
hfe = IC / IB
When we have the output of the transistor short-circuited (VCE=0), hfe defines the current
gain of the transistor in common emitter (CE) connection. Using this parameter we can
calculate the output current (IC) from the input current (IB):
IC = IB x hfe
This explains why this parameter is so useful. A typical BJT transistor has typical current
amplification from 30 to 800, while a Darlington pair transistor can have an amplification
factor of 10.000 or more. Another symbol for the hfe is the Greek letter β (Beta).
37
Prof. Dr. Ali S. Hennache- Department of
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Muhammad Ibn Saud Islamic University -
Hybrid parameters
hre - Dynamic transfer ratio reverse voltage
This parameter is calculated with this equation:
hre = VBE / VCE
If the input of the transistor is open (IB=0) then this parameter gives the voltage
gain when the transistor is connected with common emitter (CE).
hoe - Output Conductivity
This parameter is defined with the input open (IB=0) and the transistor
connected in common emitter (CE) connection. The equation is:
hoe = IC / VCE
With the above conditions, this parameter defines the conductivity of the
output. So, the impedance of the output can be defined as follows:
ro = 1 / hoe = VCE / IC
38
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Muhammad Ibn Saud Islamic University -
Output characteristic, Load Lines and Quiescence point
The DC Load Line
One of the most important characteristics is the Common Emitter output
characteristic or IC to VCE characteristic and looks like as below:
This is the IC to VCE characteristic of a
BC547 transistor. The horizontal axis (x)
has the VCE voltage in volts, and the
vertical axis has the IC current, usually in
milliamperes. Between them, there are
several different curves. Each one of these
lines corresponds to a different base
current,
usually
measured
in
microamperes.
The IC to VCE characteristic of a transistor
39
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Muhammad Ibn Saud Islamic University -
Dc load line (operation point)
The DC Load Line is a line (red line ) on these characteristics, which eventually
determines all the points that the transistor will operate at. In other words, the
operation point (usually called Q from the word "Quiescence") will be
somewhere on the DC load line.
To draw this load line, the collector current and the collector-emitter voltage
needed to be known. For example for the characteristics below the load line
drawn with red color is for IC=40mA and VCE=12V.
How to draw the load line
Before one can draw the load line, we must
first discuss and explain the 4 basic
regions of the output characteristic or IC to
VCE
1the saturation area,
2the cut-off area,
3the linear area and
4the breakdown point.
40
Prof. Dr. Ali S. Hennache- Department of
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SATURATION AREA
Region 1:
The Saturation Area
The saturation area is the area where the load line intersects with the saturation
point of the characteristics. In the following curves , the saturation area is
marked with a red transparent filter.
41
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Cut- off area
Region 2:
The Cut-Off area
The cut-off area is the area in which the collector current becomes zero. In
the following curves , the cut-off area is marked with a yellow transparent
filter.
42
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Linear area
Region 3:
The Linear Area
The linear area is the area between the cutoff and the saturation area of the
transistor, as shown bellow with a green mask.
43
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Breakdown point
Region 4:
The Breakdown Point
The breakdown point is the point above which the collector current
increases rapidly and the transistor is destroyed. This area is marked with a
purple mask in the following drawing.
44
Prof. Dr. Ali S. Hennache- Department of
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Muhammad Ibn Saud Islamic University -
Test
Q.1Q.2Q.3Q.4Q.5Q.6Q.7Q.8-
45
What are the three transistor configurations?
Which transistor configuration provides a phase reversal between the
input and output signals?
What is the input current in the common-emitter circuit?
What is the current gain in a common-base circuit called?
Which transistor configuration has a current gain of less than 1?
What is the output current in the common-collector circuit?
Which transistor configuration has the highest input
resistance?
What is the formula for GAMMA
Prof. Dr. Ali S. Hennache- Department of
Physics - College of Sciences - Al-Imam
Muhammad Ibn Saud Islamic University -
AMPLIFIER PARAMETERS
Amplifier Parameters
Any amplifier is said to have certain parameters. These are the particular
properties that make the amplifier perform in a certain way, and so make it
suitable for a given task. Typical amplifier parameters are described below.
Gain
The gain of an amplifier is a measure of the "Amplification" of an amplifier, i.e.
how much it increases the amplitude of a signal. More precisely it is the ratio of
the output signal amplitude to the input signal amplitude, and is given the
symbol "A". It can be calculated for voltage (Av), current (Ai) or power (Ap).
Amplification
The Voltage Amplification (Av) or Gain of a voltage amplifier is given by:
Voltage gain Av = Amplitude of output voltage ÷ Amplitude of input voltage.
46
Prof. Dr. Ali S. Hennache- Department of
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Muhammad Ibn Saud Islamic University -
AMPLIFIER PARAMETERS
Current gain Ai = Amplitude of output current ÷ Amplitude of input
current.
Power gain Ap = Signal power out ÷ Signal power in.
The gain of an amplifier is governed, not only by the components
(transistors etc.) used, but also by the way they are interconnected
within the amplifier circuit.
47
Prof. Dr. Ali S. Hennache- Department of
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Muhammad Ibn Saud Islamic University -
AMPLIFIER PARAMETERS
The dBs can be used to indicate the gain of amplifiers
Converting a power gain ratio to dBs is calculated by multiplying the log of the
ratio by 10:
When voltage gain(Av) or current gain (Ai) is plotted against frequency the
−3dB points are where the gain falls to 0.707 of the maximum (mid band)
gain.
48
Prof. Dr. Ali S. Hennache- Department of
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Muhammad Ibn Saud Islamic University -
AMPLIFIERS CLASSES
There are four classes of operation
for an amplifier(Figure 1). These are:
A, AB, B and C (figure 1 (A), 1(B)
(1C) and (1D) respectively) .Each
class of operation has certain uses
and characteristics.
The class of operation of an amplifier
is determined by the amount of time
(in relation to the input signal) that
current flows in the output circuit
The selection of the "best" class of
operation is determined by the use of
the amplifying circuit.
Figure 1: A comparison of output signals for the different amplifier classes of
operation
49
Prof. Dr. Ali S. Hennache- Department of
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CLASS A AMPLIFIER
CLASS A
The output is a replica of the input. Figure 2 is
an example of a class A amplifier. Although the
output from this amplifier is 180 degrees out of
phase with the input, the output current still
flows for the complete duration of the input and
thus, amplifier current flows for 100% of the
input signal.
The class A operated amplifier is used as an
audio- and radio-frequency amplifier in radio,
radar, and sound systems, just to mention a few
examples.
The class A amplifier has the characteristics of
good FIDELITY( the output signal is just like
the input signal in all respects except
amplitude) and low EFFICIENCY.
Figure 2: A simple class A transistor amplifier.
50
Class A - 100% of the input signal is
used (conduction angle a = 360° or 2π)
Prof. Dr. Ali S. Hennache- Department of
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CLASS B AMPLIFIER
CLASS B
A class B amplifier operates for 50% of the
input signal. A simple class B amplifier is
shown in figure 3.
In the circuit shown in figure 3, the baseemitter bias will not allow the transistor to
conduct whenever the input signal becomes
positive. Therefore, only the negative portion
of the input signal is reproduced in the output
signal.
Figure 3: A simple class B
transistor amplifier.
51
The efficiency of Class B amplifiers is twice the
efficient of class A amplifiers since the
amplifying device only conducts (and uses
power) for half of the input signal. If less than
50% of the input signal is needed, a class C
amplifier is used.
Class B - 50% of the input signal is used
(a = 180° or π)
Prof. Dr. Ali S. Hennache- Department of
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CLASS AB AMPLIFIER
CLASS AB
If the amplifying device is biased in such a way
that current flows in the device for 51% - 99%
of the input signal, the amplifier is operating
class AB.
Class AB amplifiers have better efficiency and
poorer fidelity than class A amplifiers. They are
used when the output signal need not be a
complete reproduction of the input signal, but
both positive and negative portions of the input
signal must be available at the output.
Any amplifier operating between class A
and class B limits is operating class AB.
Figure 4: A simple class AB
transistor amplifier.
52
Class AB - more than 50% but less than
100% is used. (181° to 359°, π < a < 2π)
Prof. Dr. Ali S. Hennache- Department of
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CLASS C AMPLIFIER
CLASS C
Notice that only a small portion of the input
signal is present in the output signal. Since
the transistor does not conduct except during
a small portion of the input signal, this is the
most efficient amplifier. It also has the worst
fidelity. The output signal bears very little
resemblance to the input signal.
Class C amplifiers are used where the
output signal need only be present during
part of one-half of the input signal. Any
amplifier that operates on less than 50% of
the input signal is operated class C.
Figure 5: A simple class C
transistor amplifier.
53
Class C - less than 50% is used (0° to
179°, a < π)
Prof. Dr. Ali S. Hennache- Department of
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Muhammad Ibn Saud Islamic University -
Collector current waveforms operating in for different
classes
Collector current waveforms for transistors operating in (a) class A, (b) class B, (c)
class AB, and (d) class C amplifier stages.
54
Prof. Dr. Ali S. Hennache- Department of
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Muhammad Ibn Saud Islamic University -
PHYS 324 - ELECTRONICS
Field Effect Transistor (FET) ( 07 H ):
Field-Effect Transistors (FET): Construction and
classification 1H, Principle of operation 2H,
Characteristic curves, Characteristic parameters of the
FET 1H, Effect of temperature on FET, Common source
amplifier, Common drain amplifier 1H. Application of
FET as voltage variable resistor and MOSFET as a
switch 1H – Advantages of FET over transistor 1H
55
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FIELD EFFECT TRANSISTOR ( FET)
INTRODUCTION
A field-effect transistor (FET) is a type of transistor commonly used for weaksignal amplification (for example, for amplifying wireless signals).� The device
can amplify analog or digital signals.� It can also switch DC or function as an
oscillator.
In the FET, current flows along a semiconductor path called the channel. �At
one end of the channel, there is an electrode called the source. �At the other
end of the channel, there is an electrode called the drain. �The physical
diameter of the channel is fixed, but its effective electrical diameter can be varied
by the application of a voltage to a control electrode called the gate. The
conductivity of the FET depends, at any given instant in time, on the electrical
diameter of the channel.� A small change in gate voltage can cause a large
variation in the current from the source to the drain.� This is how the FET
amplifies signals.
Field-effect transistors are fabricated onto silicon integrated circuit (IC) chips.�
A single IC can contain many thousands of FETs, along with other components
such as resistors, capacitors, and diodes.
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INTRODUCTION
The junction FET has a channel consisting of N-type semiconductor
(N-channel) or P-type semiconductor (P-channel) material; the gate
is made of the opposite semiconductor type.� In P-type material,
electric charges are carried mainly in the form of electron deficiencies
called holes.� In N-type material, the charge carriers are primarily
electrons. In a JFET, the junction is the boundary between the
channel and the gate. Normally, this P-N junction is reverse-biased
(a DC voltage is applied to it) so that no current flows between the
channel and the gate.� However, under some conditions there is a
small current through the junction during part of the input signal cycle.
57
Prof. Dr. Ali S. Hennache- Department of
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FET CLASSIFICATIONS
Field-Effect Transistors
Junction FET (JFET)
N-CHANNEL
58
P-CHANNEL
Metal-Oxide- Semiconductor
FET (MOSFET).
D-MOSFET Depletion
Mode
MOSFET
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E- MOSFET Enhancemen
t Mode
MOSFET
JFET SYMBOL
–FETs are 3 terminal devices
 Drain
(D)
 Source (S)
 Gate
(G)
–the gate is the control input
–diagram illustrates the notation
used for labelling voltages and
currents
P-CHANNEL
59
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N-CHANNEL
N AND P-CHANNELS
60
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N-CHANNEL
There are three terminals: Drain (D) and Source (S) are connected to n-channel
Gate (G) is connected to the P-type material
61
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P-CHANNEL
P-Channel JFET operates in a similar manner as the N-channel JFET except the
voltage polarities and current directions are reversed Drain (D) and Source (S) are
connected to P-channel and Gate (G) is connected to the N-type material.
62
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D- MOSFET SYMBOLS
63
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BASIC OPERATION OF FET
The field-effect transistor is another type of solid-state device that is becoming
increasingly popular in electronic circuits. These transistors derive their name
from the fact that current flow in them is controlled by variation of an
electric field established by application of a voltage to a control electrode,
referred to as the gate.
In contrast, current flow in bipolar transistors is controlled by variation of the
current injected into the base terminal. Moreover, the performance of bipolar
transistors depends on the interaction of two types of charge carriers (holes
and electrons). Field-effect transistors, however, are unipolar devices; as a
result, their operation is basically a function of only one type of charge carrier,
holes in p-channel devices and electrons in n-channel devices.
A charge-control concept can be used to explain the basic operation of fieldeffect transistors. A charge on the gate (control electrode) induces an equal, but
opposite, charge in a semiconductor layer, referred to as the channel, located
directly beneath the gate. The charge induced in the channel controls the
conduction of current through the channel and, therefore, between the source
and drain terminals which are connected to opposite ends of the channel.
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OPERATION
The JFET is a voltage−controlled device in which current flows from the
SOURCE terminal (equivalent to the emitter in a bipolar transistor) to the DRAIN
(equivalent to the collector). A voltage applied between the source terminal and
a GATE terminal (equivalent to the base) is used to control the source − drain
current. The main difference between a JFET and a bipolar transistor is that in a
JFET no gate current flows, the current through the device is controlled by an
electric field, hence "Field effect transistor". The JFET construction and circuit
symbols are shown in Figures 1, 2 and 3.
Figure on the right shows the simplest
form of construction for a Junction FET
(JFET) . It uses a small slab of N type
semiconductor into which are infused
two P type areas to form the Gate.
Current (electrons) flows through the
device from source to drain along the N
type silicon channel. As only one type of
charge carrier (electrons) carry current in
N channel JFETs, these transistors are
also called "Unipolar" devices.
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65
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JFET OPERATION
In the N channel device, the N channel is sandwiched between two P type
regions (the gate and the substrate) that are connected together electrically to
form the gate. The N type channel is connected to the source and drain
terminals via more heavily doped N+ type regions. The drain is connected to a
positive supply, and the source to zero volts. N+ type silicon has a lower
resistivity than N type. This gives it a lower resistance, increasing conduction
and reducing the effect of placing standard N type silicon next to the aluminium
connector, which because aluminum is a tri−valent material, having three
valence electrons whilst silicon has four, would tend to create an unwanted
junction, similar in effect to a PN junction at this point.
66
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OPERATION
When a voltage is applied between drain and source (VDS) current flows and the
silicon channel acts rather like a conventional resistor. Now if VDS is increased
(with VGS held at zero volts) towards what is called the pinch off value VP, the
drain current ID also at first, increases. The transistor is working in the "ohmic
region" as shown above.
However as drain source voltage VDS increases, the depletion layers at the gate
junctions are also becoming thicker and so narrowing the N type channel
available for conduction.
67
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OPERATION
Figure below is showing the cross section of a N channel planar Junction FET
(JFET) The load current flows through the device from source to drain along a
channel made of N type silicon. In the planar device the second part of the gate is
formed by the P type substrate.
P channel JFETs are also available and the principle of operation is the same as
the N channel type described here, but polarities of the voltages are of course
reversed, and the charge carriers are holes.
68
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N-CHANNEL JFET OPERATION
The nonconductive depletion region becomes thicker with increased reverse bias.
(Note: The two gate regions of each FET are connected to each other.)
69
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OPERATION
gate volt controls the thickness of the channel consider an n-channel
device
- making the gate more positive attracts electrons to the gate
and makes the gate region thicker – reducing the resistance of
the channel. The channel is said to be enhanced
- making the gate more negative repels electrons from the gate
and makes the gate region thinner – increasing the resistance of
the channel. The channel is said to be depleted
70
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BJT VS FET
CURRENT CONTROLLED VS VOLTAGE CONTROLLED DEVICES
Compared to BJTs, FETs have faster switching speeds (time switching On to
Off) and generate less heat per switch, than the BJT.
FETs can be designed to smaller geometries than BJTs, allowing greater
number of individual transistors per square micron of semiconductor space.
However, FETs, particularly the early MOSFETs are more liable to damage
from Electro-Static Discharge (ESD) than were the BJT devices.
71
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TRANSFER CHARACTERISTICS
The input-output transfer characteristic of the JFET is not as straight forward
as it is for the BJT
In a BJT,  (hFE) defined the relationship between IB (input current) and IC
(output current).
In a JFET, the relationship (Shockley’s Equation) between VGS (input voltage)
and ID (output current) is used to define the transfer characteristics, and a
little more complicated (and not linear):
VGS 

ID = IDSS  1 
VP 

2
As a result, FET’s are often referred to a square law devices
72
Prof. Dr. Ali S. Hennache- Department of
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JFET OPERATING CHARACTERISTICS
There are three basic operating conditions for a JFET:
JFET’s operate in the depletion mode only
A. VGS = 0, VDS is a minimum value depending on IDSS and the drain and
source resistance
B. VGS < 0, VDS at some positive value and
C. Device is operating as a Voltage-Controlled Resistor
For an n channel JFET, VGS may never be positive*
For an p channel JFET, VGS may never be negative*
73
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SATURATION
At the pinch-off point:
• any further increase in VGS does not produce any increase in ID.
VGS at
pinch-off is denoted as Vp.
• ID is at saturation or maximum. It is referred to as IDSS.
• The ohmic value of the channel is at maximum.
74
Prof. Dr. Ali S. Hennache- Department of
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FET OUTPUT CHARACTERISTICS
There are 3 main important regions in a field effect transistor:
1Ohmic region
2Pinch-off voltage saturation
3Saturation region
75
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FET as a Voltage-Controlled Resistor
The region to the left of the pinch-off point is called the ohmic region.
The JFET can be used as a variable resistor, where VGS controls the drainsource resistance (rd). As VGS becomes more negative, the resistance (rd)
increases.
rd =
76
ro
VGS 

1 
VP 

2
Prof. Dr. Ali S. Hennache- Department of
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JFET OUTPUT CHARACTERISTICS
As VGS becomes more negative:
• the JFET will pinch-off at a lower voltage (Vp).
• ID decreases (ID < IDSS) even though VDS is increased.
• Eventually ID will reach 0A. VGS at this point is called Vp or VGS(off).
• Also note that at high levels of VDS the JFET reaches a breakdown
situation.
ID will increases uncontrollably if VDS > VDSmax.
77
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In the JFET output characteristics shown in above, the Drain current ID shows
very little change, and the curves are very nearly horizontal at voltages greater
than the pinch off voltage. Almost all of the expected increase in current, due to
the increase in voltage between Source and Drain (VDS), is offset by the
narrowing of the conducting channel due to the growing depletion layers.
78
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P-CHANNEL JFET CHARACTERISTICS
As VGS increases more positively
• the depletion zone increases
• ID decreases (ID < IDSS)
• eventually ID = 0A
Also note that at high levels of VDS the JFET reaches a breakdown situation. ID
increases uncontrollably if VDS > VDSmax.
79
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TRANSFER (TRANSCONDUCTANCE) CURVE
From this graph it is easy to determine the value of ID for a given value of VGS
It is also possible to determine IDSS and VP by looking at the knee where
VGS is 0
80
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TRANSFER CHARACTERISTIC
The transfer characteristic for a JFET, which shows the change in Drain current
(ID) for a given change in Gate−Source voltage (VGS), is shown in Fig 2.4. Because
the JFET input (the Gate) is voltage operated, the gain of the transistor cannot be
called current gain, as with bipolar transistors. The drain current is controlled by
the Gate−Source voltage, so the graph shows milliamperes per volt (mA / V), and
as I / V is CONDUCTANCE (the inverse of resistance V / I) the slope of this
graph (the gain of the device) is called the FORWARD or MUTUAL
TRANSCONDUCTANCE, which has the symbol gm. Therefore the higher the
value of gm the greater the amplification.
Notice that VGS is always shown as being negative;
in reality it may be zero or slightly above zero, but
the gate is always more negative than the N type
channel between source and drain. Note also that
the slope of the curve in the transfer characteristic
is less steep than that of the transfer characteristic
for a typical bipolar transistor. This means that a
JFET will have a lower gain than that of a bipolar
transistor.
81
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TRANSFER CHARACTERISTICS
JFET
D-MOSFET
E-MOSFET
Similar shape for all forms of FET – but with a different offset not a linear
response, but over a small region might be considered to approximate a
linear response
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Differences between Common Source amplifier Common
Drain amplifier Common Gate amplifier
Both the common-gate and common source has voltage gain of greater that
1 compared with the voltage gain of source- follower which is less than or
approximately equal to 1 . The input resistance of both common-source and
source follower is high typically ranges from kilo ohms and above while
common-gate has a low input impedance ranges from hundred ohms or
below.
The output resistance of both common-gate and common-source are
dominated by RD while source follower has low output impedance and is not
dominated by RD
83
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COMPARISON BETWEEN CONFIGURATIONS
Configuration
84
Voltage Gain
Input
Resistance
Current Gain
Output
Resistance
CommonSource
AV >1
-
∞
Moderate to
high
common
Drain
AV ≈ 1
-
∞
Low
CommonGate
AV > 1
Ai ≈ 1
Low
Moderate to
high
Prof. Dr. Ali S. Hennache- Department of
Physics - College of Sciences - Al-Imam
Muhammad Ibn Saud Islamic University -
APPLICATIONS OF FET
In a Field Effect Transistor (FET), as we already said, conduction in a channel
is controlled by the effect of an electric field produced by a voltage applied to
the Gate electrode. There are no forward-biased junctions so the Gate draws
no current. The fact that the Gate in a FET does not draw any current is the
most important characteristic of FETs.
The nonexistent Gate current results in a very high input impedance (ZIN)
which is essential in many applications .
For applications like analog switches and amplifiers of ultra-high input
impedance, FETs have no equivalent. In fact, FETs are the basic building
blocks for op-amps and digital components like switches, microprocessors and
memories. Consider FETs when you want:
Very high ZIN.
A bidirectional analog switch.
A simple current source (2 terminal).
A voltage-controlled resistance.
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SWITCH
The following figure shows the MOSFET equivalent of the BJT switch
discussed in the previous lectures.
This circuit is even simpler because we do not have to concern ourselves with
the inevitable compromise of providing adequate base current with
squandering excessive power. In the MOSFET-equivalent circuit we just apply
a full-swing DC voltage drive to the high-impedance Gate. As long as the
switched-on FET behaves like a resistance that is small compared with the
load, it will bring its Drain close to ground - typical power MOSFET have
RDS(ON) < 0.2W which is fine for this application.
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CURRENT SOURCES
JFETs are used in current sources because JFETs need no Gate bias in
order to get current. This makes the current source circuit much simpler.
JFETs are used as current sources within ICs (especially in op-amps) and
also sometimes in discrete circuits. The simplest JFET current source is
shown below where R1 is the load and ID is the output current of the current
source:
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JFET LIKE A RESISTOR
Given that VGS in the above circuit is zero, the following graph shows the Drain
current curve for various values of VDS. From this graph we can see that ID will be
relatively constant for VDS > 3V. Note that this current source will only work as
long as you keep VDS > 3V, out of the so-called linear region, where the JFET
behaves not like a current source, but like a resistor (due to linear relation
between ID and VDS in that region):
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ADJUSTABLE CURRENT SOURCE
The previous circuit can be modified to create an adjustable current source by
adding a self-biasing resistor, RS as follows:
In the previous circuit, VGS = 0 and ID was close to the maximum current, IDSS.
In this circuit however, VGS = VG - IDRS = - IDRS (since VG is tied to ground).
Because VGS = - IDRS and not zero, the JFET is brought closer to the pinch-off
voltage VP and thereby, reducing ID to a more stable value. The previous circuit
was operating at IDSS, and as shown from the data sheets IDSS can be
unpredictable. Adding RS makes the current source more predictable because it
reduces the Drain current value ID away from the unpredictable IDSS.
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Prof. Dr. Ali S. Hennache- Department of
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ADJUSTABLE CURRENT SOURCE
The following curve illustrates how ID varies with VDS for the above circuit as
the Drain voltage VDD was varied from 1 to 5V. Unlike the previous current
source which was operating at IDSS where it was obvious that ID was not
constant with increasing VDS, there is little variation in ID along the
saturation region in this improved current source
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Prof. Dr. Ali S. Hennache- Department of
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The figure below is already given in the previous lectures and shows how the
Drain current ID varies with the Drain-Source voltage VDS for a few values of
the controlling Gate-Source voltage VGS:
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FET VARIABLE RESISTORS
In the linear region, the curves are approximately straight as long as VDS <
VGS - VT. Actually, these curves extend through both directions through the
zero point, meaning that the device can be used as a voltage controlled
resistor for small signals of both polarities. From the universal equation for ID:
For the linear region we can easily find the ratio ID / VDS by dividing the first
equation by VDS to give:
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Prof. Dr. Ali S. Hennache- Department of
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The previous equation describes the resistance being supplied by the FET
when it is running in the linear region. In fact, RDS in the linear region is also
equal to 1/gm. In other words, the channel resistance in the linear region is the
inverse of the transconductance gm in the saturation region.
Typically the value of resistance we can produce for a FET will range from
10s of Ohms all the way up to an open circuit. A typical application might be
in a circuit to automatically control the gain of an amplifier to keep the output
within the linear region. The range of VDS over which the FET behaves like a
good resistor depends on the FET itself, but is roughly proportional to the
amount by which VG exceeds VT.
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The good-old voltage divider is a very useful circuit:
It is used to produce a fraction of the input voltage VIN at VOUT:
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From the previous equation, we note the following:
Value of R2
VOUT
R2 = R1
VOUT = 0.5VIN
R2 = 2R1
VOUT = (2/3)VIN
R2 very low (short circuit)
VOUT = 0
R2 very high (open
circuit)
VOUT = VIN
Therefore, by varying R2 we can vary VOUT, in other words, by varying R2 we
can determine how much to attenuate the input signal. Now, using FET as a
variable resistor, we can replace R2 in the above voltage divider with a FET
running in the linear region to produce a voltage-controlled attenuator (or
volume control):
Prof. Dr. Ali S. Hennache- Department of
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ADVANTAGES OF FET OVER TRANSISTOR
FET’s (Field – Effect Transistors) are much like BJT’s (Bipolar Junction
Transistors).
Similarities:
• Amplifiers
• Switching devices
• Impedance matching circuits
Differences:
• FET’s are voltage controlled devices whereas BJT’s are current
controlled
devices.
• FET’s also have a higher input impedance, but BJT’s have higher
gains.
• FET’s are less sensitive to temperature variations and because of
there
construction they are more easily integrated on IC’s.
• FET’s are also generally more static sensitive than BJT’s.
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ADVANTAGES OF FET OVER TRANSISTOR
Basic FET Circuits
The following categories represent circuit situations that take advantage of the
unique properties of FETs, and hence work better with FETs rather than with
BJTs or even cannot be built with BJTs:
High-impedance / low current
Buffers or amplifiers for applications where the base current and finite input
impedance of the BJT limit performance. Although you can build such circuits
with discrete FETs, it is always better to use integrated circuits ICs built with
FETs. These ICs often use FETs as a high-impedance front end for a BJT-based
design!
Analog Switches
MOSFETs are excellent voltage-controlled analog-switches. Once again, use
dedicated analog switch ICs rather than building them with discrete FETs.
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Digital Logic
MOSFETs dominate microprocessors, memory and most high-performance
digital logic.
Power Switching
Power MOSFETs are preferable to power BJTs for switching loads. Here, we
can use discrete power FETs.
Variable Resistors (current sources)
FETs behave like a voltage-controlled resistor in the linear region of the Drain
curves (the voltage controls the value of the resistor). FETs also behave like a
voltage-controlled current-source in the saturation region of the Drain curves
(the voltage controls the value of the current).
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Electronics “ Phys. 324 ”
by
Prof. Dr. Ali S. Hennache
Uni Junction Transistor (UJT) ( 04 H ):
Structure and working of UJT 2H- Characteristics. 1H
Application of UJT as a relaxation oscillator 1H.
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UNIJUNCTION TRANSISTOR (UJT)
• UJT consists of a bar of N-type silicon material
(lightly-doped) and a small amount of diffused Ptype material (heavily-doped)
• An emitter terminal E is connected to the P
material to form the PN junction
• Two paths for current flow: B2 to B1; E to B1
• Normally current does not flow in either path until
Emitter voltage is about 10 volts higher than B1
voltage
• Is a special transistor that has two bases and
one emitter
• Has two states: completely on or completely off
• Part of thyristor family which include SCR, triac,
and diac
• UJT is a break over-type switching device
• Useful in industrial circuits: timers, oscillators,
waveform generators, gate control circuits for
SCRs and triacs
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CONSTRUCTION OF A UJT
The basic structure of a uni-junction transistor is shown in figure below. It
essentially consists of a lightly-doped N-type silicon bar with a small piece of
heavily doped P-type material alloyed to its one side to produce single P-N
junction. The single P-N junction accounts for the terminology uni-junction. The
silicon bar, at its ends, has two ohmic contacts designated as base-1 (B1) and
base-2 (B2), as shown and the P-type region is termed the emitter (E). The
emitter junction is usually located closer to base-2 (B2) than base-1 (B1) so that
the device is not symmetrical, because symmetrical unit does not provide
optimum electrical characteristics for most of the applications.
The UJT has one pn junction and is
used mainly as a triggering device in
thyristor circuits and can also be used
in oscillator circuits. The symbol is
similar to a JFET. Note the angle of
the emitter. The other terminals are
called base 1 and base 2. The
characteristics are quite different than
any other transistor.
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Between base B1 and base B2, the resistance of the n-type bar called interbase resistance (RB ) and is in the order of a few kilo ohm.
This inter-base resistance can be broken up into two resistances—the
resistance from B1 to the emitter is RB1 and the resistance from B2 to the
emitter is RB2.
Since the emitter is closer to B2 the value of RB1is greater than RB2.
Total resistance is given by:
RB = RB1 + RB2
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EQUIVALENT CIRCUIT OF A UJT
The resistive equivalent circuit of a UJT shown makes it easier to understand its
operation. The emitter current controls the value of RB1 inversely. The total
resistance or inter-base resistance (RBB) equals the sum of RB1 and RB2. The
standoff ratio () is the ratio RB1/ RBB.
B2
D1
RB2
E
RB1
B1
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EQUIVALENT CIRCUIT
The VBB source is generally fixed and provides a constant voltage from B2 to B1.
The UJT is normally operated with both B2 and E positive biased relative to B1.
B1 is always the UJT reference terminal and all voltages are measured relative
to B1 . VEE is a variable voltage source.
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UJT OPERATION
• When a voltage, called standoff voltage VP, applied to the emitter is about 10
volts higher than the voltage applied to B1 , UJT turns on and current flows
through the B2-B1 path and from the emitter-B1 path
• Current will continue to flow through the UJT until the voltage applied to the
emitter drops to a point that is about 3 volts higher than the voltage applied to B1
• When emitter voltage drops to this point , the UJT will turn off and will
remain off until the voltage applied to the emitter again reaches a level about
10 volts higher than the voltage applied to B1
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UJT OPERATION
Uni-junction transistor can trigger larger thyristors with a pulse at base B1.With
the emitter disconnected, the total resistance RBB, is the sum of RB1 and RB2 .
RBBO ranges from 4-12kΩ for different device types. The intrinsic standoff ratio η
is the ratio of RB1 to RBBO. It varies from 0.4 to 0.8 for different devices.
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OPERATION
As VE increases, current IE increases up IP at the peak point. Beyond the peak
point, current increases as voltage decreases in the negative resistance region.
The voltage reaches a minimum at the valley point. The resistance of RB1, the
saturation resistance is lowest at the valley point.
IP and IV, are datasheet parameters. VP is the voltage drop across RB1 plus a
0.7V diode drop; VV is estimated to be approximately 10% of VBB.
Peak voltage of UJT Vp
Vp=ηVbb +Vd
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PROGRAMMABLE UNI-JUNCTION TRANSISTOR
External PUT (Programmable Uni-junction Transistor ) resistors R1 and R2
replace uni-junction transistor internal resistors RB1 and RB2, respectively.
These resistors allow the calculation of the intrinsic standoff ratio η.
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OPERATION
RBB is known as the interbase resistance, and is the sum of RB1
and RB2:
RBB = RB1 + RB2
(1)
N.B. This is only true when the emitter is open circuit.
VRB1 is the voltage developed across RB1; this is given by the voltage divider
rule:
VRB1 = RB1 / RB1 + RB2
109
(2)
Prof. Dr. Ali S. Hennache- Department of
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OPERATION
Since the denominator of equation 2 is equal to equation 1, the former can
be rewritten as:
VRB1 = (RB1 /RBB ) X VBB
(3)
The ratio RB1 / RBB is referred to as the intrinsic standoff ratio and is denoted
by (the Greek letter eta).
If an external voltage Ve is connected to the emitter, the equivalent circuit can be
redrawn as shown in Fig.5.
If Ve is less than VRB1, the diode is reverse biased and the circuit behaves as
though the emitter was open circuit. If however Ve is increased so that it exceeds
VRB1 by at least 0.7V, the diode becomes forward biased and emitter current Ie
flows into the base 1 region. Because of this, the value of RB1 decreases. It has
been suggested that this is due to the presence of additional charge carriers
(holes) in the bar. Further increase in Ve causes the emitter current to increase
which in turn reduces RB1 and this causes a further increase in current. This
runaway effect is termed regeneration. The value of emitter voltage at which this
occurs is known as the peak voltage VP and is given by:
VP = AVVBB + VD
(4)
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OPERATION
The characteristics of the UJT are illustrated by the graph of emitter voltage against
emitter current (Fig.6).
As the emitter voltage is increased, the current is very small - just a few
microamps. When the peak point is reached, the current rises rapidly, until at
the valley point the device runs into saturation. At this point RB1 is at its
lowest value, which is known as the saturation resistance.
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ON STATE
As VEE increases, the UJT stays in the OFF state until VE approaches the
peak point value V P. As VE approaches VP the p–n junction becomes forwardbiased and begins to conduct in the opposite direction.
As a result IE becomes positive near the peak point P on the VE - IE curve.
When VE exactly equals VP the emitter current equals IP .
At this point holes from the heavily doped emitter are injected into the n-type
bar, especially into the B1 region. The bar, which is lightly doped, offers very
little chance for these holes to recombine.
The lower half of the bar becomes replete with additional current carriers
(holes) and its resistance RB is drastically reduced; the decrease in BB1
causes Vx to drop.
This drop, in turn, causes the diode to become more forward-biased and IE
increases even further.
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OFF STATE
When a voltage VBB is applied across the two base terminals B1 and B2, the
potential of point p with respect to B1 is given by:
VP =[VBB/ (RB1 +RB2)]*RB1=η*RB1
η is called the intrinsic stand off ratio with its typical value lying between 0.5
and 0.8.
The VEE source is applied to the emitter which is the p-side. Thus, the emitter
diode will be reverse-biased as long as VEE is less than Vx. This is OFF state
and is shown on the VE - IE curve as being a very low current region.
In the OFF the UJT has a very high resistance between E and B1, and IE is
usually a negligible reverse leakage current. With no IE, the drop across RE
is zero and the emitter voltage equals the source voltage.
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STATIC EMITTER CHARACTRERISTIC
The static emitter characteristic (a curve showing the relation between emitter
voltage VE and emitter current IE) of a UJT at a given inter base voltage VBB is
shown in figure. From figure it is noted that for emitter potentials to the left of peak
point, emitter current IE never exceeds IEo . The current IEo corresponds very
closely to the reverse leakage current ICo of the conventional BJT. This region, as
shown in the figure, is called the cut-off region. Once conduction is established at
VE = VP the emitter potential VE starts decreasing with the increase in emitter
current IE. This Corresponds exactly with the decrease in resistance RB for
increasing current IE. This device, therefore, has a negative resistance region
which is stable enough to be used with a great deal of reliability in the areas of
applications listed earlier. Eventually, the valley point reaches, and any further
increase in emitter current IE places the device in the saturation region, as shown
in the figure. Three other important parameters for the UJT are IP, VV and IV and
are defined below:
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IMPORTANT PARAMETERS FOR THE UJT
Peak-Point Emitter Current. Ip. It is the emitter current at the peak point. It represents
the rnimrnum current that is required to trigger the device (UJT). It is inversely
proportional to the interbase voltage VBB.
Valley Point Voltage VV The valley point voltage is the emitter voltage at the valley
point. The valley voltage increases with the increase in interbase voltage VBB.
Valley Point Current IV The valley point current is the emitter current at the valley point.
It increases with the increase in inter-base voltage VBB.
Special Features of UJT. The special features of a UJT are :
A stable triggering voltage (VP)— a fixed fraction of applied inter base voltage VBB.
A very low value of triggering current.
A high pulse current capability.
A negative resistance characteristic.
Low cost.
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V-I CHARACTERISTIC
STATIC EMITTER CHARACTERISTIC FOR A UJT
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UJT RATINGS
Maximum peak emitter current :
This represents the maximum allowable value of a pulse of emitter current.
Maximum reverse emitter voltage :
This is the maxi mum reverse-bias that the emitter base junction B2 can
tolerate before breakdown occurs.
Maximum inter base voltage :
This limit is caused by the maxi mum power that the n-type base bar can
safely dissipate.
Emitter leakage current :
This is the emitter current which flows when VE is less than Vp and the
UJT is in the OFF state.
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APPLICATIONS
The UJT is very popular today mainly due to its high switching speed.
A few select applications of the UJT are as follows:
(i) It is used to trigger SCRs and TRIACs
(ii) It is used in non-sinusoidal oscillators
(iii) It is used in phase control and timing circuits
(iv) It is used in saw tooth generators
(v) It is used in oscillator circuit design
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APPLICATIONS
Uni-junction transistor (abbreviated as UJT), also called the double-base
diode is a 2-layer, 3-terminal solid-state (silicon) switching device. The device
has-a unique characteristic that when it is triggered, its emitter current increases
re generatively (due to negative resistance characteristic) until it is restricted by
emitter power supply.
The low cost per unit, combined with its unique characteristic, have warranted its
use in a wide variety of applications.
A few include oscillators, pulse generators, saw-tooth generators, triggering
circuits, phase control, timing circuits, and voltage-or current-regulated supplies.
The device is in general, a low-power-absorbing device under normal operating
conditions and provides tremendous aid in the continual effort to design relatively
efficient systems.
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WORTH POINTS ABOUT UJT
The worth noting points about UJT are given below:
The device has only one junction, so it is called the uni-junction device.
The device, because of one P-N junction, is quite similar to a diode but it differs
from an ordinary diode as it has three terminals.
The structure of a UJT is quite similar to that of an N-channel JFET. The main
difference is that P-type (gate) material surrounds the N-type (channel) material in
case of JFET and the gate surface of the JFET is much larger than emitter
junction of UJT.
In a uni-junction transistor the emitter is heavily doped while the N-region is lightly
doped, so the resistance between the base terminals is relatively high, typically 4
to 10 kilo Ohm when the emitter is open.
The N-type silicon bar has a high resistance and the resistance between emitter
and base-1 is larger than that between emitter and base-2. It is because emitter is
closer to base-2 than base-1.
UJT is operated with emitter junction forward- biased while the JFET is normally
operated with the gate junction reverse-biased.
UJT does not have ability to amplify but it has the ability to control a large ac
power with a small signal. It exhibits a negative resistance characteristic and so it
can be employed as an oscillator.
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FEATURES OF UJT
Special Features of UJT. The special features of a UJT are :
A stable triggering voltage (VP)— a fixed fraction of applied inter base voltage
VBB.
A very low value of triggering current.
A high pulse current capability.
A negative resistance characteristic.
Low cost.
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