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Transistor
• A semiconductor device that can be modeled with dependent sources
• Transistor types:– Bipolar Junction Transistor (BJT)
– Field Effect Transistor (FET)
• A transistor has different operating modes with different i-v characteristics
2
INTRODUCTION
TRANSISTORSA bipolar transistor consists of a three-layer "sandwich" of doped
(extrinsic) semiconductor materials, either P-N-P or N-P-N. Each layer
forming the transistor has a specific name (Emitter - E, Base - B and Collector-C), and each layer is provided with a wire contact for
connection to a circuit
4
TERMINOLOGY AND SYMBOLS
•Both, PNP and NPN transistors can be thought of as two very closely spaced PN junctions.
•The base must be small to allow interaction between the two PN junctions.
5
•There are four regions of operation of a BJT transistor
•Since it has three leads, there are three possible amplifiers
6
Qualitative Description of Transistor Operation•Emitter doping is much larger than base doping
•Base doping larger than collector doping
•Current components: IE = IEp + IEn
IC = ICp + ICn
IB = IE - IC = IB1
+ IB2 + IB3
•IB2 = current due to electrons that replace the recombined electrons in the base,
•IB3 = collector current due to thermally-generated electrons in the collector that go in the base
•IB1 = current from electrons being back injected into the forward-biased emitter-base junction
7
Circuit DefinitionsBase Transport Factor aT :
T = ICp /IEp Ideally it would be equal to unity (recombination in the base reduces its value)
Emitter Injection Efficiency : = IEp /(ICp + IEp) = IEp /IE Approaches unity if emitter doping is much larger than base dopingAlpha-dc:
dc= IC /IE = (ICp+ ICn ) /(Iep + IEn) = ICp /(Iep + IEn ) = dc
Beta-dc:
dc = IC /IB = IC /(IE - IC) = dc /(1- dc) Current gain is large when dc approaches unity
Collector-reverse Saturation Current:
IBCo = ICn IC = ICp + ICn = dcIE + IBCo
8
Collector Current in Common-emitter Configuration:
IC =dc(IC + IB) +IBCo
IC ={dc /(1-dc)}* IB +IBCo /(1- dc) IC =dc +IECo
IECo =(1+ dc) IBCoLarge Current Gain Capability:
Small base current IB forces the E-B junction to be forward biased and inject large number of holes which travel through the base to the collector.
9
+ N P NEmitter
Base +
-
FB RB
Bipolar Transistor Biasing (NPN)
Collector
10
P N PEmitter Collector
Base
+
+
-
FB RB
Bipolar Transistor Biasing (PNP)
11
Bipolar Transistor Operation (PNP)•90% of the current carriers pass through the reverse biased base - collector PN junction and enter the collector of the transistor.
•10% of the current carriers exit transistor through the base.
•The opposite is true for a NPN transistor.
12
Transistor Characteristic Curve
IC
VCE
Q-Point
IB
0 uA
10 uA
20 uA
30 uA
40 uA
50 uA
60 uA
70 uA80 uA90 uA
Saturation
Cutoff
13
Bipolar Transistor Amplifiers
•Amplifier Classification–Amplifiers can be classified in three ways:
•Type (Construction / Connection)–Common Emitter–Common Base–Common Collector
•Bias (Amount of time during each half-cycle output is developed).
–Class A, Class B, Class AB, Class C
•Operation–Amplifier–Electronic Switch
14
Common Emitter Schematic
RB
RC
Q1
+
0
+VCC
Input Signal
+
0
Output Signal
Output Signal Flow Path
Input Signal Flow Path
15
• DC Kirchoff Voltage Law Equations and Paths
Kirchoff Voltage Law
RB
RC
Q1
+VCC
Base - Emitter Circuit
ICRC + VCE - VCC = 0
IBRB + VBE - VCC = 0
Collector - Emitter Circuit
16
Common Emitter Operation
Positive Going Signal
Negative Going Signal
Output Signal
Input Signal
+
+
0
0 Base becomes more (+) WRT Emitter FB IC VRC
VC VOUT ( Less + )
Base becomes less (+) WRT Emitter FB IC VRC
VC
VOUT ( More + )
RC
RB
Q1
17
Common Base Schematic
+
0
+
0+VCC
RBRCRE
Q1
CC
Input Signal Flow Path
Output Signal Flow Path
18
• DC Kirchoff Voltage Law Equations and Paths
Kirchoff Voltage Law
+VCC
RBRCRE
Q1
CC
Base - Emitter CircuitIBRB + VBE + IERE - VCC = 0
Collector - Emitter CircuitICRC + VCE + IERE - VCC = 0
19
Common Base Operation
Positive Going Signal
Negative Going Signal
+VCC
RBRCRE
Q1
CC
Base becomes more (+) WRT Emitter FB IC VRC
VC
VOUT ( More + )
Base becomes less (+) WRT Emitter FB IC VRC
VC VOUT ( Less + )Input
Signal
0Output Signal
+
0
20
Common Collector Schematic
RB
RE
Q1
+
0
+VCC
Input Signal +
0
Output Signal
Output Signal Flow Path
Input Signal Flow Path
21
• DC Kirchoff Voltage Law Equations and Paths
Kirchoff Voltage Law
RB
RE
Q1
+VCC
Base - Emitter CircuitIBRB + VBE + IERE - VCC = 0
Collector - Emitter CircuitICRC + VCE + IERE - VCC = 0
22
Common Collector Operation
Positive Going Signal
Negative Going Signal
RB
RE
Q1
+VCC
Base becomes more (+) WRT Emitter FB IE VRE
VE
VOUT ( More + )
Base becomes less (+) WRT Emitter FB IE VRE
VE VOUT ( Less + )Input
Signal
0 0
+ +
Output Signal
23
Transistor Bias Stabilization
Used to compensate for temperature effects which affects semiconductor operation. As temperature increases, free electrons gain energy and leave their lattice structures which causes current to increase.
24
Types of Bias Stabilization•Self Bias: A portion of the output is fed back to the input 180o out of phase. This negative feedback will reduce overall amplifier gain.•Fixed Bias: Uses resistor in parallel with Transistor emitter-base junction.•Combination Bias: This form of bias stabilization uses a combination of the emitter resistor form and a voltage divider. It is designed to compensate for both temperature effects as well as minor fluctuations in supply (bias) voltage.•Emitter Resister Bias: As temperature increases, current flow will increase. This will result in an increased voltage drop across the emitter resistor which opposes the potential on the emitter of the transistor.
25
Self Bias Schematic
RB
RC
Q1
+VCC
+
=
Initial Input
Self Bias Feedback
Resulting Input
+
+
+
+
o o
o
o
VOUT
26
Emitter Bias Schematic
RB
RC
Q1
+VCC
+
o
VOUT
RE
++
+
+
-
-Initial Input
+
o
CE
DC Component
AC Component
27
Combination Bias Schematic
RB1
RC
Q1
+VCC
+
o
VOUT
RE
++
+
+
-
-Initial Input
+
o
CE
DC Component
AC Component
RB2
28
Amplifier Frequency Response
•The range or band of input signal frequencies over which an amplifier operates with a constant gain.•Amplifier types and frequency response ranges.
•Audio Amplifier–15 Hz to 20 KHz
•Radio Frequency (RF) Amplifier–10 KHz to 100,000 MHz
•Video Amplifier (Wide Band Amplifier)–10 Hz to 6 MHz
29
Class ‘A’ Amplifier Curve
IC
VCE
IB
0 uA
10 uA
20 uA
30 uA
40 uA
50 uA
60 uA
70 uA80 uA90 uA
Saturation
Cutoff
Q-Point
30
Class ‘B’ Amplifier Curve
IC
VCE
IB
0 uA
10 uA
20 uA
30 uA
40 uA
50 uA
60 uA
70 uA
80 uA
90 uA
Saturation
Cutoff
Q-Point
31
Class ‘AB’ Amplifier Curve
IC
VCE
IB
0 uA
10 uA
20 uA
30 uA
40 uA
50 uA
60 uA
70 uA
80 uA
90 uA
Saturation
Cutoff
Q-Point
Can be used for guitar distortion.
32
Class ‘C’ Amplifier Curve
IC
VCE
IB
0 uA
10 uA
20 uA
30 uA
40 uA
50 uA
60 uA
70 uA
80 uA
90 uA
Saturation
CutoffQ-Point
33
Transistor Fundamentals
Semiconductor devices that have-three or more elements are called transistors. The term transistor was derived from the words transfer and resistor. This term best describes the operation of the transistor.
There are many different types of transistors, but their basic theory of operation is all the same. The three elements of the two-junction transistor are:
(1)Emitter, which gives off, or emits," current carriers (electrons or holes); (2) Base, which controls the flow of current carriers; and
(3) Collector, which collects the current carriers.
Introduction to BJT Small Signal
Analysis
Bipolar Junction Transistor (BJT)
• BJTs are made up of alternating layers of n and p semiconductor materials joined metallurgically
• Two types of BJTs:– pnp-type: Principal conduction by positive
holes– npn-type:Principal conduction by negative
electrons
Bipolar Junction Transistor (BJT)
• BJT is a three terminal device: – Emitter (E)– Collector (C)– Base (B)
: Base-Emitter voltage
: Collector-Emitter voltage
: Collector Current
: Base Current
: Emitter Current
BE
CE
C
B
E
v
v
i
i
i
Bipolar Junction Transistor (BJT)
• Applying KCL to BJT:– Finding two currents can yield the third
one
• Three operating modes of the BJT transistors:– Active mode
– Cutoff mode
– Saturation mode
E C Bi i i
Active Mode• In active mode collector current is controlled by
base current and base-emitter voltage is constant:
voltage threshold:
gaincurrent forward :
V
Vv
ii
BE
BC
: forward current gain
: threshold voltage
C B
BE
i i
v V
V
39
RRee model model• Fails to account the output impedance Fails to account the output impedance
level of device and feedback effect from level of device and feedback effect from output to inputoutput to input
Hybrid equivalent modelHybrid equivalent model• Limited to specified operating condition Limited to specified operating condition
in order to obtain accurate resultin order to obtain accurate result
DisadvantageDisadvantage
40
• The transistor can be employed as an amplifying device. That is, the output sinusoidal signal is greater than the input signal or the ac input power is greater than ac input power.
• How the ac power output can be greater than the input ac power?
Amplification in the AC domain
41
Amplification in the AC domain
Conservation; output Conservation; output power of a system power of a system cannot be large than its cannot be large than its input and the efficiency input and the efficiency cannot be greater than 1cannot be greater than 1
The input dc plays the The input dc plays the important role for the important role for the amplification to amplification to contribute its level to the contribute its level to the ac domain where the ac domain where the conversion will become conversion will become as as ηη=P=Po(ac)o(ac)/P/Pi(dc)i(dc)
42
• The superposition theorem is applicable for the analysis and design of the dc & ac components of a BJT network, permitting the separation of the analysis of the dc & ac responses of the system.
• In other words, one can make a complete dc analysis of a system before considering the ac response.
• Once the dc analysis is complete, the ac response can be determined using a completely ac analysis.
Amplification in the AC Amplification in the AC domaindomain
43
BJT Transistor Model• Use equivalent circuit• Schematic symbol for the device can be replaced
by this equivalent circuits.• Basic methods of circuit analysis is applied.• DC levels were important to determine the Q-point• Once determined, the DC level can be ignored in
the AC analysis of the network.• Coupling capacitors & bypass capacitor were
chosen to have a very small reactance at the frequency of applications.
44
The AC equivalent of a network isobtained by:1. Setting all DC sources to zero & replacing them
by a short-circuit equivalent.2. Replacing all capacitors by a short-circuit
equivalent.3. Removing all elements bypassed by short-
circuit equivalent.4. Redrawing the network.
BJT Transistor ModelBJT Transistor Model
45
46
47
Example
48
Example
49
Example
50
The re transistor model
• Common Base PNP Configuration
51
Common Base PNP Configuration
• Transistor is replaced by a single diode between E & B, and control current source between B & C
• Collector current Ic is controlled by the level of emitter current Ie.
• For the ac response the diode can be replaced by its equivalent ac resistance.
52
Common Base PNP Configuration• The ac resistance of
a diode can be determined by the equation;
Where ID is the dc current through the diode at the Q-point.
EI
mVre
26
53
• Input impedance is relatively small and output impedance quite high.
• range from a few Ω to max 50 Ω
• Typical values are in the M Ω
CBi reZ
CBZo
Common Base PNP Configuration
54
The common-base characteristics
55
Voltage Gain
re
RA
re
R
rI
RI
V
VA
rI
ZI
ZIV
RI
RI
RIV
LV
L
ee
Le
i
OV
ee
ie
iii
Le
LC
Loo
:gain voltage
: ageinput volt
)(
: tageoutput vol
56
Current Gain
1
i
e
e
e
C
i
oi
A
I
I
I
I
I
IA
• The fact that the polarity of the Vo as determined by the current IC is the same as defined by figure below.
• It reveals that Vo and Vi are in phase for the common-base configuration.
57
Common Base PNP Configuration
Approximate model for a common-base npn transistor configuration
58
Example 1: For a common-base configuration in figurebelow with IE=4mA, =0.98 and AC signal of 2mV isapplied between the base and emitter terminal:a) Determine the Zi b) Calculate Av if RL=0.56kc) Find Zo and Ai
e
b b
c
ec I αI
IcIe
common-base re equivalent cct
re
59
Solution:
5.6m4
m26
I
26mr Za)
Eei
43.845.6
)k56.0(98.0
r
RA b)
e
Lv
98.0I
IA
Ω Zc)
i
oi
o
60
Example 2: For a common-base configuration in previous example with Ie=0.5mA, =0.98 and AC signal of 10mV is applied, determine:a) Zi b) Vo if RL=1.2k c) Av d)Ai e) Ib
20m5.0
m10
I
V Za)
:Solution
e
ii
88mV5(1.2k)0.98(0.5m)
RIRIV b) LeLco
8.58m10
m588
V
VA c)
i
ov
98.0A d) i
A10
)98.01(m5.0
)1(m5.0
I-I
I-II e)
ee
ceb
Common Emitter NPN Configuration
• Base and emitter are input terminal
• Collector and emitter are output terminals
61
Common Emitter NPN Configuration
• Substitute re
equivalent circuit
• Current through diode
62
bc II
bbe
bbbce
III
IIIII
)1(
63
• Input impedance
ei
ei
b
ebi
eb
eei
b
be
i
ii
rZ
rZ
I
rIZ
rI
rIV
I
V
I
VZ
; 1an greater thusually
)1(
)1( that so
)1(
:ageinput volt
:impedanceinput
64
The output graph
65
bI
c
e
bIi=Ib
re model for the C-E transistor configuration
rero
e
0AbI
c
e
bIi=Ib
rero
e
Vs=0V
= 0A
oZ
impedance)high cct,(open ΩZ
the thusignored is r if
rZ
o
o
oo
Output impedance Zo
66e
LV
eb
Lb
i
oV
eb
iii
Lb
Lco
Loo
r
RA
rI
RI
V
VA
rI
ZIV
RI
RIV
RIV
that so
:ageinput volt
: tageoutput vol
i
b
b
b
C
i
oi
A
I
I
I
I
I
IA
Voltage GainVoltage Gain Current GainCurrent Gain
re model for common-emitter
67
68
Example 3: Given =120 and IE(dc)=3.2mA for a common-emitter configuration with ro= , determine:
a) Zi b)Av if a load of 2 k is applied c) Ai with the 2 k load
975)125.8(120rZ
125.8m2.3
m26
I
26mr a)
ei
Ee
:Solution
15.246125.8
k2
r
Rb)A
e
Lv
120I
IA c)
i
oi
69
Example 4: Using the npn common-emitter configuration, determine the following if =80, IE(dc)=2 mA and ro=40 k
a) Zi b) Ai if RL =1.2k c) Av if RL=1.2k
k04.1)13(80rZ
13m2
m26
I
26mr a)
ei
Ee
:Solution
bI
cbIi=Ib
re model for the C-E transistor configuration
rero
e
RL
Io
70
67.77
)80(k2.1k40
k40
Rr
r
IRr
)I(r
A
Rr
)I(rI
I
I
I
IiAb)
(cont)Solution
Lo
o
b
Lo
bo
i
Lo
boL
b
L
i
o
6.8913
k40k2.1
r
rRvAc)
e
oL
71
Common Collector Configuration
• For the CC configuration, the model defined for the common-emitter configuration is normally applied rather than defining a model for the common-collector configuration.
END
THANKS FOR LISTERNING