Chapter 3 : BJTs Bipolar Junction Transistors

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Chapter 3 BJTs(Bipolar Junction Transistors)
UCET
FACULTY OF ELECTRICAL AND ELECTRONIC ENG.
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Objectives

• At the end of this topics, you will understand
  the
• Transistor structures
• Transistor operation
• Transistor characteristics
• Transistor as a switch and as an amplifier
• Transistor packages and terminal identification

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• 4.1 Introduction
• The basic of electronic system nowdays is
  semiconductor
• device. The famous and commonly use of this
  device is
• BJTs (Bipolar Junction Transistors).
• It can be use as amplifier and logic switches.
• BJT consists of three terminal
• ? collector C
• ? base B
• ?emitter E
• Two types of BJT pnp and npn

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• A single pn junction has two different types of
  bias
• forward bias
• reverse bias
• Thus, a two-pn-junction device has four types of
  bias.

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• Mode of operation
• Cut-off region
• region where the collector current, IC0 A.
• b) Saturation-region
• region where the characteristic to the left of
  VCB0 V.
• c) Active-region
• region where normally employed for linear
  (undistorted) amplifiers.

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• Fig. 4.1 shows the position of the terminals and
  symbol of
• BJT.

• Base is located at the middle and more thin from
  the level of collector and emitter
• The emitter and collector terminals are made of
  the same type of semiconductor material, while
  the base of the other type of material

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• Transistor currents

• Notes
• The arrow is always drawn
• on the emitter
• The arrow always point
• toward the n-type
• The arrow indicates the
• direction of the emitter current
• pnpE? B
• npn B? E

ICthe collector current IB the base current IE
the emitter current
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• By imaging the analogy of diode, transistor can
  be
• construct like two diodes that connetecd together
  as in
• Fig 4.2.
• It can be conclude that the work of transistor
  is base on
• work of diode.

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• 4.2 Transistor operation
• There are 3 types of connection transistor or
  configuration
• in electric circuit
• a) CB (common base)
• b) CE (common emitter)
• c) CC (common collector)
• This configuration is base on which the terminal
  is
• connected to the input signal and output signal.
• Table 4.1 shows the relationship between input
  signal and
• output signal with the transistor configuration.

Table 4.1
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Transistor Operation
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Energy band of a NPN transistor
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• Common-base configuration (CB)
• Fig 4.3 shows the common-base configuration for
  pnp
• and npn transistor.
• CB is derived from the fact that the
• - base is common to both i/p and o/p of the
  configuration.
• - base is usually the terminal closest to or at
  ground potential

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• Common-base configuration for pnp
• For transistor to operate, base-emitter junction
  forward
• bias and base-collector junction reverse bias.
• Fig. 4.4 shows the CB connection

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• Analysis of Common-base configuration for pnp
• Step 1
• B-E junction? must be forward bias

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• Analysis of Common-base configuration for pnp
• Step 2
• BC junction? must be reverse bias
• ICBOICO0 A is a reverse saturation current and
• normally known as leakage current, when IE0A

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• Analysis of Common-base configuration for pnp
• Step 3Overall analysis
• Current flow from base to emitter due to the ve
  supply
• VEB. It produce collector current, IC.
• Small current is produce for base current, IB.
• Leakage current, ICBO is produce by reverse-bias
• process also flow to collector.

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• Approximation
• Once the transistor is ON base-emitter voltage
  can be approximately 0.7V. The variation of VCB
  can be ignored for approximation process when
  analyzing transistor networks without getting
  involved with parameter variations of less
  importance.

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• Analysis of Common-base configuration for pnp
• Current base, IB (?A) is small compare to current
  
• emitter, IE (mA) and current collector,IC (mA).
• The relationship among these current can be
  analyse
• with KCL IE IB IC
• Current collector is produce from the total sum
  of
• current emitter and leakage current.
• Current emitter that flow through collector known
  as
• ?DC IE . The value is big compare to leakage
  current.
• The analysis can be understand by the following
  expression

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• IC IC(majority) IC(minority)
• IC ?IE ICBO
• It can then be summarize to IC ?IE (ignore
  ICBO due to small value)
• ? is a common base current gain factor that
  shows the efficiency by calculating the current
  percent from current flow from emitter to
  collector.The value of ? is typical from 0.9
  0.998.

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• Analysis of Common-base configuration for npn
• For transistor to operate, base-emitter junction
  in forward
• bias and base-collector junction in reverse bias.

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• Analysis of Common-base configuration for npn
• Step 1
• B-E junction is forward bias due to the ve VEB
  is
• connected to n-material of emitter.
• Electron is inject from VEB will flow to emitter
  and
• become current emitter, IE.
• This electron flow through emitter and enter base
  area.
• Since base is made from p-type only small
  electron from
• emitter will combined with hole at base. This
  will generate
• base current, IB.
• At collector, all the current emitter, IE become
  IC due
• to the electron from emitter is collected by C
  cause by
• VCB. Only small current is produce flow to base
  for
• Producing current base, IB.

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• Analysis of Common-base configuration for npn
• Step 2
• B-C junction is reverse bias due to ve VCB is
  connected
• to n-type of collector.
• Thus no current flow occur and the minority
  carrier of
• the current take place. Lastly the current that
  been generated
• is leakage current, ICBO.

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• Common-base characteristics
• Require two set of characteristics
• ? input parameters
• ? output parameters
• Input parameters
• Procedures
• Set VCB at one value (fixed value of VCC)
• Measure the current emitter, IE for a few
  different value of VBE(different value of VEE)
• The complete circuit is shows in Fig .4.5

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• Fixed value VCB, the increase of VBE will affect
  of increasing IE in
• exponential after one value of voltage
  (0.7-silicon/0.3-germanium).
• Before this IE was very small or
• no IE . This condition similar with the diode in
  forward bias.

Input characteristics for a common-base npn
transistor
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Output characteristics for a common-base npn
transistor
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Input characteristics for a common-base pnp
transistor
Output characteristics for a common-base pnp
transistor
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• Biasing
• Proper biasing CB configuration in active region
  by
• approximation IC ? IE (IB ? 0 u A)

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Transistor as an amplifier
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Simulation of transistor as an amplifier
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Quiz

• Draw a theoritical circuit for NPN and PNP type
  common-base transistor
• What is a?
• Using a PNP type transistor, design a circuit
  that will be using a 10V input voltage to draw an
  output of 200V.

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Common Emitter configuration
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• Common-emitter configuration (CE)
• Fig. 4.7 shows the configuration of CE for the
  pnp
• and npn transistors.
• It is called common-emitter configuration since
  
• emitter is common or reference to both i/p and
  o/p
• terminals.
• emitter is usually the terminal closest to or at
  ground
• potential.
• Almost amplifier design is using connection of CE
  
• due to the high gain for current and voltage.
• Two set of characteristics are necessary to
  describe the
• behavior for CE input (base terminal) and output
  (collector
• terminal) parameters.

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Proper Biasing common-emitter configuration in
active region
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• IB is microamperes compared to
• miliamperes of IC.
• IB will flow when VBE gt 0.7 V
• for silicon and 0.3 V for Germanium
• Before this value IB is very small
• and no IB.
• BE junction is forward bias
• Increasing VCE will reduce IB
• for different values.

• Input characteristics for a common-emitter NPN
  transistor

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• For small VCE (VCE lt VCESAT, IC increase
  linearly with increasing of VCE
• VCE gt VCESAT IC not totally depands on VCE ?
  constant IC
• IB(uA) is very small compare to IC (mA). Small
  increase in IB cause big increase in IC
• IB0 A ? ICEO occur.
• Noticing the value when IC0A. There is still
  some value of current flows.

Output characteristics for a common-emitter npn
transistor
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Determination of collector current from graph
output of a common-emitter configuration
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Determination of IC using input characteristic
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• Beta (?) or amplification factor
• The ratio of dc collector current (IC) to the dc
  base current (IB) is dc beta (?dc ) which is dc
  current gain where IC and IB are determined at a
  particular operating point, Q-point (quiescent
  point).
• Its define by the following equation
• 30 lt ?dc lt 300 ? 2N3904

• On data sheet, ?dchFE with h is derived from ac
  hybrid equivalent cct. FE are derived from
  forward-current amplification and common-emitter
  configuration respectivley.

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• For ac conditions an ac beta has been defined as
  the changes of collector current (IC) compared to
  the changes of base current (IB) where IC and IB
  are determined at operating point.
• On data sheet, ?achfe
• It can defined by the following equation

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Example From o/p characteristics of CE
configuration find ?ac and ?dc with an operating
point at IB25 ?A and VCE 7.5V.
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Solution
IB2
IC2
Q-point
?IC
IB1
IC1
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Relationship analysis between a and ß
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Common-Collector configuration
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• Common-collector configuration (CC)
• Also called emitter-follower (EF).
• Fig. 4.8 shows the configuration of CC for the
  pnp
• and npn transistors.
• It is called common-emitter configuration since
  both the
• signal source and the load share the collector
  terminal as a
• common connection point.
• The o/p voltage is obtained at emitter terminal.
• The input characteristic of CC configuration is
  similar
• with CE configuration.
• Fig 4.9 shows the o/p characteristic of CC for
• npn transistor. All the current relationship for
  CE
• configuration are true for CC configuration.

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Common Collector Configuration
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Characteristics of Common Collector
The Characteristics are similar to those of the
Common-Emitter. Except the vertical axis is
IE. IE IB1 IB2
IB3 VCE
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• Limits of operation for transistor
• Many BJT transistor used as an amplifier. Thus it
  is
• important to notice the limits of operations.
• At least 3 maximum values is mentioned in data
  sheet.
• There are
• a) Maximum power dissipation at collector PCmax
  or PD
• b) Maximum collector-emitter voltage VCEmax
  sometimes named as VBR(CEO) or VCEO.
• c) Maximum collector current ICmax
• There are few rules that need to be followed for
  BJT
• transistor used as an amplifier. The rules are
• i) transistor need to be operate in active
  region!
• ii) IC lt ICmax
• ii) PC lt PCmax

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Transistor limits of operation
Note VCE is at maximum and IC is at minimum
(ICmaxICEO) in the cutoff region. IC
is at maximum and VCE is at minimum (VCE max
VCEsat VCEO) in the saturation region. The
transistor operates in the active region between
saturation and cutoff.
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Example 1
Refer to the fig. Step1 The maximum collector
power dissipation, PDICmax x VCEmax (1)
18m x 20 360 mW Step 2 At any point on the
characteristics the product of and must be equal
to 360 mW. Ex. 1. If choose ICmax 5 mA,
subtitute into the (1), we get VCEmaxICmax 360
mW VCEmax(5 m)360/57.2 V Ex.2. If choose
VCEmax18 V, subtitute into (1), we
get VCEmaxICmax 360 mW (10) ICmax360m/1820 mA
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Derating PDmax

• PDmax is usually specified at 25C.
• The higher temperature goes, the less is PDmax
• Example
• A derating factor of 2mW/C indicates the power
  dissipation is reduced 2mW each degree centigrade
  increase of temperature.

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Example 2 Transistor 2N3904 used in the
circuit with VCE 20 V.This circuit used at
temperature 1250C. Calculate the new maximum IC.
Transistor 2N3904 have maximum power dissipation
is 625 mW. Derating factor is 5 mW/0C.
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Solution Step 1 Temperature increase 1250C
250C 1000C Step 2 Derate transistor 5 mW/0C
x 1000C 500 mW Step 3 Maximum power
dissipation at 1250C 625 mW500 mW125 mW. Step
4 Thus ICmax PCmax / VCE125m/20 6.25
mA. Step 5 Draw the new line of power
dissipation at 1250C .
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• Example 3
• The parameters of transistor 2N3055 as follows
• - Maximum power dissipation _at_ 250C115 W
• - Derate factor0.66 mW/0C.
• This transistor used at temperature 780C.
• Find the new maximum value of power dissipation.
• Find the set of new maximum of IC if VCE10V, 20
  V and 40 V.

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Solution Step 1 Temperature increase 780C
250C 530C Step 2 Derate transistor 0.66mW/0C
x 530C 35 mW Step 3 Maximum power dissipation
at 780C 115W 35W80 mW. Step 4 ICmax PCmax
/ VCE80m/10 8 mA (point C) ICmax PCmax /
VCE80m/20 4 mA. (point B) ICmax PCmax /
VCE80m/40 2 mA (point A)
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Step 5 Draw the new line of power dissipation at
780C .
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Transistor Specification Sheet
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Transistor Terminal Identification
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Transistor Testing
1. Curve Tracer Provides a graph of the
characteristic curves. 2. DMM Some DMMs will
measure ?DC or HFE. 3. Ohmmeter
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• References
• Thomas L. Floyd, Electronic Devices, Sixth
  edition, Prentice Hall, 2002.
• Robert Boylestad, Electronic Devices and
  Circuit Theory, Eighth edition, Prentice Hall,
  2002.
• 3. Puspa Inayat Khalid, Rubita Sudirman, Siti
  Hawa Ruslan,
• ModulPengajaran Elektronik 1, UTM, 2002.
• 4. Website http//www2.eng.tu.ac.th