2. Contents
• Transistor (BJT) Structure
• Transistor characteristics and parameters
• DC operating point
• Transistor as an amplifier
• Transistor as a switch
• MOSFET
• Operational Amplifier
15 October 2023 2
3. Introduction
• The semiconductor device like a diode cannot amplify a
signal, therefore its application area is limited.
• The next development of semiconductor device after diode is a
BJT (bipolar junction transistor).
• It is a three terminal device. The terminals are – collector,
emitter, and base. Out of which the base is a control terminal.
• A signal of small amplitude applied to the base is available in
the “magnified” form at the collector of the transistor.
• Thus the large power signal is obtained from a small power
signal.
15 October 2023 3
5. Why is it called transistor ?
• The term transistor was derived from the words
TRANSFER & RESISTOR.
• Transfers input signal current from a low resistance
path to a high resistance path.
15 October 2023 5
7. The BJT – Bipolar Junction Transistor
Normally Emitter layer is heavily doped, Base layer is lightly doped and Collector
layer has Moderate doping.
npn pnp
n p n
E
B
C p n p
E
B
C
Cross Section Cross Section
B
C
E
Schematic
Symbol
B
C
E
Schematic
Symbol
15 October 2023 7
9. Number of P-N junctions and equivalent circuit
N
P
E
B
P
N
B
C
E
Emitter
C
Collector
B
Base
15 October 2023 9
10. An unbiased Transistor – Depletion region
• For an unbiased transistor no external power supplies are
connected to it
P
Junction
JEB
Emitter collector
N
Base
Junction
JCB
N
Depletion
region
Depletion
region
-
-
-
-
-
+
+
+
+
+
-
-
-
-
-
+
+
+
+
+
-
-
-
-
-
-
-
-
-
-
15 October 2023 10
11. Transistor biasing in the active region
Sr.
No.
Region of
operation
Base emitter
junction
Collector base
junction
application
1 Cutoff region Reverse
biased
Reverse
biased
transistor is OFF
2 Saturation
region
Forward
biased
Forward
biased
transistor is ON
3 Active
region
Forward
biased
Reverse
biased
Amplifier
15 October 2023 11
12. Transistor operation in the active region P-N-P
P
Junction
JEB
Emitter collector
N
Base
Junction
JCB
VEE
RE
+
-
RC
VCC
-
holes emitted
holes collected
Conventional
current
conventional
current
+
P P
N
IE = IC + IB
15 October 2023 12
13. Transistor configuration
• Depending on which terminal is made common to input and
output port there are three possible configurations of the
transistor. They are as follows:
• Common base configuration
• Common emitter configuration
• Common collector configuration
15 October 2023 13
14. Transistor operation in the active region N-P-N
common base configuration
P
Junction
JEB
Emitter collector
N
Base
Junction
JCB
N
VEE
RE
+
-
RC
VCC
+
-
Electron emitted
Electron collected
Emitter electron
current
Direction
Conventional
Current IC (INJ)
Direction
Conventional
Current IB
Direction
Conventional
Current IE
(injected collector current)
15 October 2023 14
15. Transistor operation in the active region N-P-N
common base configuration
P
JEB
Emitter collector
N
Base
JCB
N
Depletion
region
Depletion
region
-
-
-
-
-
+
+
+
+
+
-
-
-
-
-
+
+
+
+
+
-
-
-
-
-
-
-
-
-
-
RC
VCC
+
-
IC=ICBO
ICBO
Is a collector to base leakage current
With open emitter
ICBO is a reverse saturation
Current flowing due to the
Minority carriers between
Collector and base when the
Emitter is open. ICBO flows due to the reverse
Biased collector base junction.
ICBO is neglected as compared to IC
15 October 2023 15
16. Current relations in CB configuration
• Current amplification factor ( αdc)
• the current amplification factor is the ratio of collector current
due to the injection of total emitter current
IC = IC(INJ) + ICBO
αdc = IC(INJ) / IE
IC(INJ) = αdc IE
But ICBO is negligibly small
Therefore the current amplification factor
IC = αIE + ICBO
IC = αIE
α= IC / IE
15 October 2023 16
17. Characteristics of a transistor in CE configuration
• It is a graph of input current (IB)
versus input voltage (VBE) at a
constant output voltage (VCE). N
N
P
C
E
B
JC
JE
+
-
-
+
RB
IC
IB
IE
VCE
constant
VBE
VBB
VCC
N-P-N Transistor
VBE
IB
(μA)
VCE = 4V 10V
0 0.7 1 2
The value of dynamic input resistance “Ri”
is low for CE
ΔIB
ΔVBE Ri=ΔVBE/ΔIB
VCE Constant
Input characteristics
15 October 2023 17
18. B E C
I/P I Vs i/p V Keep o/p vtg const
o/p I vs o/p V keep i/p I const
Ib vs Vbe keep Vce cont
Ic vs Vce keep Ib const
15 October 2023 18
19. Characteristics of a transistor in CE configuration
• It is a graph of output current (Ic)
versus output voltage (VCE) at a
constant input current (IB)
E
C
N-P-N Transistor
-
+
RE
B
+
-
RB
IC
IE
VCC
VCE
VBE
VBB
VCE
IB = 0
4
3
2
1
IC
(mA)
1 2 3 4
Cutoff region
IB = 2μA
IB = 4μA
IB = 3μA
IB = 4μA
Saturation
region
Active
region
βdc = IC /IB
Output characteristics
15 October 2023 19
20. Sr. No. Parameter CB CE CC
1 Common terminal
between input and
output
Base Emitter Collector
Conduction Angle 0 o 180 o 0 o
2 Input current IE IB IB
3 Output current IC IC IE
4 Current gain αDC = IC/IE
Less than one
βDC = IC/IB
High
γ = IE/IB
HIGH
5 Input Voltage Veb Vbe Vbc
6 Output voltage Vcb Vce Vec
7 Current gain Less than
unity
High High
15 October 2023 20
21. Sr. No. Parameter CB CE CC
8 Input resistance Very low (20Ω) Low (1KΩ) High(500kΩ)
9 Output resistance Very high (1M) High(40kΩ) Low (50Ω)
10 Application As
preamplifier
Audio
amplifier
Impedance
matching
11. Voltage gain Medium Large Less than 1
15 October 2023 21
22. Transistor Biasing
• What is meant by dc biasing of a transistor ?
• Depending on the application, a transistor is to be operated in
any of the three regions of operation namely cutoff, active and
saturation region.
• To operate the transistor in these regions the two junctions of a
transistor should be forward or reverse bias
15 October 2023 22
23. RC
RE CE
R2
R1
+VCC
C1
C2
VO
Vi
Signal to be
Amplified RL
Amplified signal
output Signal
R1 & R2 are Biasing
Resistor
C1 & C2 are
Coupling
Capacitors
Bypass Capacitor
Single Stage RC Coupled CE Amplifier
15 October 2023 23
24. BJT Switch
• When operated in
saturation, the BJT
acts as a closed
switch.
• When operated in
cutoff, the BJT acts as
an open switch.
15 October 2023 24
26. FIELD-EFFECT TRANSISTORS ( FET)
• FETs are the uni polar devices because, unlike BJTs
that use both electron and hole current, they operate
only with one type of charge carrier.
• The two main types of FET’s are -
Junction Field Fffect Transistor (JFET) and
Metal Oxide Semiconductor Field Effect Transistor
(MOSFET)
15 October 2023 26
29. MOSFET (IGFET)
• The MOSFET (metal oxide semiconductor field effect
transistor) is the category of FET.
• The MOSFET differs from the JFET in that it has no PN
junction structure; instead, the gate of the MOSFET is
insulated from the channel by a silicon dioxide (Sio2) layer.
• Two basic types of MOSFETS are :
Depletion ( D ) MOSFET and
Enhancement ( E ) MOSFET
• Because of the insulated gate, these devices are also called
as IGFET.
15 October 2023 29
30. ENHANCEMENT MOSFET ( E-MOSFET)
MOSFET was
invented by
Atalla & Dawon
at Bell Labs in
1959
15 October 2023 30
33. BJT MOSFET
It is a current controlled device. It is a voltage controlled device.
It is a bipolar device (Current flows due
to both majority & minority carriers).
It is a unipolar device (Current flows
due to only majority carriers).
Thermal Runaway can damage the BJT Thermal Runaway does not take place
Input resistance (Ri) is very low. Output resistance (Ro) is very high.
Transfer characteristics are linear in
nature.
Transfer characteristics are non-linear in
nature.
BJT is More sensitive than MOSFET MOSFET is less Sensitive
AC Voltage Gain is HIGH AC Voltage Gain is Less
Bigger in size. Smaller in size.
Regions of operation: Saturation – ON
Switch , Cut off – OFF Switch
Active – Amplifier
Regions of operation: Ohmic – ON
Switch ,Saturation – Amplifier ,
Cut off – OFF Switch
It is more noisy. It is less noisy.
Switching speed is less. Switching speed is high.
Symbol Symbol
15 October 2023 33
34. Operational
Amplifier
An operational amplifier (often op-amp or opamp) is
a DC coupled high-gain electronic voltage amplifier with
a differential input and usually, a single-ended output
15 October 2023 34
35. • Op-amp is basically a multistage amplifier which is uses
a number of amplifier stages interconnected to each
other in a complicated manner.
• The amplifier which could be configured to perform a
variety of operations such as amplification, addition,
subtraction, differentiation and integration.
• Hence the name is operational amplifier (OP-AMP)
• IC 741 is extremely popular and was used in a variety of
applications.
15 October 2023 35
38. Op-amp symbols and packages.
Thomas L. Floyd
Electronic Devices, 6e and Electronic
Devices: Electron Flow Version, 4e
15 October 2023 38
39. Ideal differential amplifier
• An ideal differential amplifier is expected to amplify the
differential signal present between its two input signal.
• It is also the basic stage of an integrated Op-amp with
differential input.
3
5
Vd
Vo = V1 – V2
+
+
-
-
Ideal
Differential
Amplifier
15 October 2023 39
41. Ideal
Differential
Amplifier (Ad=10)
3
5
Vd
Vo =Ad(V1 – V2)
+
+
-
-
Differential gain -
• Vo = Ad ( V1 – V2 )
Where Ad is called as the differential gain.
• The differential gain can be defined as the gain with which the
differential amplifier amplifies the differential signal.
Vo = Ad Vd as Vd = V1 – V2
Gain Ad = Vo / Vd decibel Vo=10 Vd 2V
Ad (dB) =10 log10 [ Vo / Vd ]
15 October 2023 41
42. Ideal
Differential
Amplifier
3
5
Vd
Vo = Ac(V1 – V2)
+
+
-
-
Common mode signal
• A common signal to both the input terminals ( i.e. V1=V2=V)
is called as common mode signal.
• Ac=V0/(Vc)=
• The output voltage produced by an ideal differential amplifier
is zero for the common mode signal.
15 October 2023 42
43. Block diagram of a typical OP-AMP
Input
Stage
Intermediate
stage
Level
shifting
stage
Output
Stage
Non-inverting
input
Inverting
input
+
-
Output
Dual input
Balanced
Output
Differential
amplifier
Dual input
unbalanced
Output
Differential
amplifier
Such as
Emitter follower
Using constant
Current source
Complementary
Symmetry
Push-pull
amplifier
15 October 2023 43
45. Ideal
Differential
Amplifier
V2
V1
Vd
Vo = A(V1 – V2)
+
+
-
-
Common mode rejection ratio (CMRR)
• CMRR is defined as the ratio of differential gain Ad and
common mode gain Ac. It is denoted by letter “ρ”
• CMRR = ρ = Ad / Ac
• =10log10(Ad/Ac) in decibel
• Ideally CMRR should be infinite and practically it should be as
high as possible.
15 October 2023 45
47. Important characteristics of OP-AMP IC 741
Sr. No. Characteristics Value for IC 741 Ideal value
1 Input resistance Ri 2 MΩ
2 Output resistance Ro 75 Ω 0
3 Voltage gain Av 2 X 105
4 Bandwidth BW 1 MHz
5 CMRR 90 dB
6 Slew rate S 0.5 V/μSec
7 Input offset voltage 2 mV 0
8 PSRR 150 μV/V 0
9 Input bias current
(Ib1+ib2)/2
50 nA 0
10 Input offset current
Ib1-Ib2
6 nA 0
8
8
8
8
8
15 October 2023 47
49. OP-AMP
-
+
VS
V1
RF
Vo
R1
+
-
V2
Vd
IB2 = 0
+ -
-
+
AV =
8
input
t
t
0
0
VS
VO
Expression for the closed loop voltage gain (AVF)
AVF = - RF / R1
vin = 2V Rf=10K R1=2k V0= A*Vin = -5*2 =-10
The negative sign indicates that there is a phase shift of 1800
Between the input and output voltages.
I
The Inverting Amplifier
15 October 2023 49
50. OP-AMP
-
+
VS
V1
RF
Vo
R1
+
-
V2
I2 = 0
+ -
-
+
AV =
8
input
t
t
0
0
VS
VO
I1 = 0
As input impedance of ideal Op-amp is infinite, the current
entering into both the input terminals of
Op-amp will have zero values. (I1 = I2 = 0 )
The Non-Inverting Amplifier
15 October 2023 50
51. The Voltage follower (unity gain buffer)
OP-AMP
-
+
VS
V1
RF = 0
Vo
+
-
V2
I2 = 0
+ -
-
+
AV =
8
I1 = 0
When R1 is infinite and RF = 0 the non-inverting amplifier
gets converted into a voltage follower or unity gain.
Av=1+Rf/R1
R1 =
8
15 October 2023 51
52. Conclusion
• Read the Instruction Manuals of equipment i.e. Car,
Washing m/c, Microwave oven, Cell phone, Laptop etc.
• BJT is used rarely.
• MOSFET is matured technology & used everywhere.
• MOSFET ckts have low dissipations, high swing &
integration.
• Device / Ckt / Chip / Application designers are well
respected. Less effect of recession.
• Classrooms may diminish; Hands on has only meaning.
• Knowledge of E&TC is must for every branch.
• Opamp is hot topic forever !
15 October 2023 52
