The document provides a comprehensive overview of bipolar junction transistors (BJTs), including their construction, operation, and different configurations (common base, common emitter, common collector). It explains DC biasing methods, the role of input and output characteristics, and the operation of BJTs in both amplification and switching modes. Additionally, it covers the characteristics of field effect transistors (FETs), such as JFETs and MOSFETs, within the context of amplifiers.

1. Bipolar Junction Transistor (BJT)
Characteristics
Unit II
Bipolar Junction Transistor: Transistor Construction, Operation, Amplification action.
Common Base, Common Emitter, Common Collector Configuration DC Biasing BJTs:
Operating Point, Fixed-Bias, Emitter Bias, Voltage-Divider Bias Configuration. Collector
Feedback, Emitter-Follower Configuration. Bias Stabilization. CE, CB, CC amplifiers and AC
analysis of single stage CE amplifier (re Model ). Field Effect Transistor: Construction and
Characteristic of JFETs. AC analysis of CS amplifier, MOSFET (Depletion and
Enhancement)Type, Transfer Characteristic
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REC 101 Unit II by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad

2. Bipolar Junction Transistor (BJT): Construction
• Transistor is 3-layer semiconductor device consisting of either n-p-n
layers (npn transistor) or p-n-p layers (pnp transistor).
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REC 101 Unit II by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
Q
E
Emitter
Collector
C
Base B
Q
E
Emitter
Collector
C
Base B
n np
E C
B
p pn
E C
B
npn transistor pnp transistor
Substrate
Collector
base
Emitter
Metal Contact
Oxide layer
Basic epitaxial planar structure

3. Bipolar Junction Transistor (BJT): Construction
• The arrow in the symbol defines the direction of emitter current.
• The emitter layer is heavily doped in comparison to collector.
• The outer layers have widths much greater than the sandwiched p-
or n -type material. (typically 150:1)
• Doping of sandwiched layer is considerably less than outer layers
(typically, 1:10 or less) resulting in low conductance (high resistance)
• The transistor is called bipolar junction transistor (BJT).Term bipolar
reflects the fact that both polarity of charges (holes and electrons)
participate in current flow.
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REC 101 Unit II by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad

4. BJT: Operation
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REC 101 Unit II by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
• EB junction is forward biased and CB
junction is reverse biased
• Large number of majority carriers
diffuse across forward biased p-n
junction into n material.
• In thin base with low conductivity, few carrier go to base terminal
while most carriers diffuse across reverse biased junction. IB is
typically in microamperes, while IC & IE are in milliamperes.
• Using KCL IE= IC + IB
• IC is mainly due to majority carriers. minority carriers (leakage
current: IC with emitter terminal open) also contribute in IC
IC = IC majority + ICO minority
E C
B
p n p
Majority current
component
Minority current
component
VEE VCC
Depletion layer
IE IC
IB

5. BJT: Common Base configuration
• Behaviour of BJT is described by two sets of characteristics; driving
point or input parameters and output parameters.
• The input set for the CB amplifier relates an input current (IE) to an
input voltage (VBE) for various levels of output voltage (VCB).
• The output set for the CB amplifier relates an output current (IC) to
an output voltage (VCB) for various levels of input current (IE).
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REC 101 Unit II by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
E C
B
VEE
VCC
IE IC
IB
Q
VBE VCB
E C
B
VEE VCC
IE IC
IB
Q
VBE VCB
Common base configuration pnp transistor Common base configuration npn transistor

6. BJT: Common Base configuration
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REC 101 Unit II by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
VCB (V)
IE =0 mA
IE =1 mA
2 mA
3 mA
4 mA
5 mA
IC (mA)
6 mA
Cutoff region
Saturationregion
Active region
ICO = ICBO BVCBO
0 10 20 30 40
6
5
4
3
2
1
0
Output characteristics of CB configuration
VBE (V)
VCB =1 V
IE (mA)
0 0.2 0.4 0.6 0.8
7
6
5
4
3
2
1
VCB =10 V
VCB =20 V
Input characteristics of CB configuration

7. BJT: Common Base configuration
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REC 101 Unit II by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
Input characteristics:
• For a VCB, input characteristic is like diode. If transistor ON, VBE0.7 V
output characteristics: has 3 regions : active, cut-off, and saturation
Active region: normally employed for amplification
• EB junction is forward-biased, & CB junction is reverse-biased.
• For IE=0, collector current IC =ICO=ICBO while for higher IE, IC IE
Cut-off region
• EB junction & CB junction both reverse-biased & no Collector current
Saturation region
• EB junction & CB junction both is forward biased
• IC increases exponentially as the voltage VCB increases toward 0 V

8. BJT: Common Base configuration
Alpha ()
• Where IC and IE are the currents at the point of operation
• for practical devices alpha typically extends from 0.90 to 0.998
• Since alpha is defined solely for the majority carriers
IC =  IE + ICBO
• Where ICBO is very small and can be ignored in some approximations
Breakdown voltage
• In active region VCB increases beyond a point, due to avalanche
effect, IC rises suddenly.
• The largest permissible VCB before breakdown is labelled as VCBO
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REC 101 Unit II by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
E
C
I
I
gaincurrentbasecommon

9. BJT: Common Emitter Configuration
• The input set for the CE amplifier relates an input current (IB) to an
input voltage (VBE) for various levels of output voltage (VCE).
• The output set for the CE amplifier relates an output current (IC) to
an output voltage (VCE) for various levels of input current (IB).
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REC 101 Unit II by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
E
C
B
VBB
VCC
IE
IC
IB
Q
VBE
VCE
E
C
B
VBB
VCC
IE
IC
IB
Q
VBE
VCE
Common emitter configuration pnp transistor Common emitter configuration npn transistor

10. BJT: Common Emitter Configuration
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REC 101 Unit II by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
Output characteristics of CE configuration
VBE (V)
VCE =20 V
IB (mA)
0 0.2 0.4 0.6 0.8
70
60
50
40
30
20
10
VCE =10 V
VCE =1 V
Input characteristics of CE configuration
VCE (V)
IB =0 A
10 A
20 A
40 A
50 A
IC (mA)
60 A
Saturationregion
Active region
ICEO = ICBOVCE Saturation
0 5 10 15 20
6
5
4
3
2
1
30 A
Cutoff region

11. BJT: Common Emitter Configuration
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REC 101 Unit II by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
Input characteristics:
• For a VCE, input characteristic is like diode. If transistor ON, VBE0.7 V
output characteristics: has 3 regions : active, cut-off, and saturation
Active region: normally employed for amplification
• BE junction is forward-biased, & CB junction is reverse-biased.
• exists to the right of the vertical line at VCE saturation and above the
curve for IB =0. For IB=0, collector current IC =ICEO=ICBO /(1-)
• IC curves are not as horizontal as in CB configuration, VCE affects IC .
Cut-off region: Region below the curve for IB=0,
Saturation region: region to the left of VCE saturation
• IC increases exponentially as VCB increases toward VCE saturation

12. BJT: Common Emitter Configuration
Beta ()
• Where IC and IB are the currents at the point of operation
• for practical devices Beta typically ranges in between 50 to 400
Relation between Alpha and Beta
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REC 101 Unit II by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
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13. BJT: Common Collector Configuration
• CC configuration is also known as emitter follower configuration as
the emitter voltage follows the base voltage.
• This configuration is mostly used as a buffer.
• CC configuration is used mainly for impedance-matching, as it has
high-input & low-output impedance, opposite to CB/CE Configuration
10/31/2017 13
REC 101 Unit II by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
C
E
B
VBB
VEE
IC
IE
IB
Q
VBE
VCE
C
E
B
VBB
VEE
IC
IE
IB
Q
VBE
VCE
Common emitter configuration pnp transistor Common emitter configuration npn transistor

14. BJT: Common collector Configuration
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REC 101 Unit II by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
Input characteristics of CC configuration
VCB (V)
VEC =2 V
IB (mA)
0 1 2 3 4 5 6
70
60
50
40
30
20
10
VEC =4 V
Output characteristics of CC configuration
VEC (V)
IB =0 A
10 A
20 A
40 A
50 A
IE (mA)
Saturationregion
Active region
0 5 10 15 20
5
4
3
2
1
30 A
Cutoff region
• Input characteristics of the CC configuration is different as current IB
decreases with increase in base collector voltage
• Output characteristics of the CC configuration are the same as of CE
configuration.

15. BJT: Amplification
• BJT can be used in two modes
– As a switch (operates in cut-off / saturation region of characteristics)
– As an amplifier (operates in active region of characteristics)
• BJT can be employed as an amplifying device in active region (BE
Junction forward biased and CB junction reverse biased).
• In amplification output ac signal power is greater than the input ac
signal power
• Increased ac power is contributed by dc power of transistor biasing
• Amplification factor is ratio of output to input parameter
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REC 101 Unit II by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
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V
V
V
A gainVoltage
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