Transistor
Learning
In 1906, the eccentric American inventor Lee De
Forest developed a triode in a vacuum tube.
The transistor was successfully demonstrated on
December 23, 1947 at Bell Laboratories in Murray
Hill, New Jersey.
The three individuals credited with the invention of
the transistor were William Shockley, John Bardeen
and Walter Brattain.
The term transistor was derived from the words
TRANSfer and resISTOR.
Operation of the transistor - the transfer of an input signal current from a lowresistance circuit to a high-resistance circuit.
Transistor is used for amplification of input signals.
Construction of Transistor by using Diode: Two Diodes Connected Back to Back
E
C
B
E
PNP Transistor
In Common Base or grounded base configuration, the BASE
connection is common to both the input signal AND the output
signal with the input signal being applied between the base
and the emitter terminals.
The corresponding output signal is taken between the base
and the collector terminals with the base terminal grounded .
The input current flowing into the emitter is quite large as its
the sum of both the base current and collector current
respectively.
The collector current output is less than the emitter current
input resulting in a current gain for this type of circuit of "1"
(unity) or less, the common base configuration "attenuates"
the input signal.
In Common Emitter or grounded emitter configuration, the
EMITTER connection is common to both the input signal and
the output signal with the input signal applied between base
and emitter terminals.
The corresponding output signal is taken between the emitter
and the collector terminals with the base terminal grounded .
The current flowing out of the transistor must be equal to the
currents flowing into the transistor as the emitter current is
given as IE = IC + IB
The collector current output is higher than the base current
input resulting in a current gain for this type of circuit is greater
than 1.
The common emitter configuration "amplifies" the input signal.
In the Common Collector or grounded collector configuration,
the COLLECTOR is now common through the supply. The input
signal is connected directly to the base.
While the output is taken from the emitter load . This type of
configuration is commonly known as a Voltage Follower or
Emitter Follower circuit.
The emitter follower configuration is very useful for impedance
matching applications because of the very high input
impedance.
In the common collector configuration the load resistance is
situated in series with the emitter so its current is equal to that
of the emitter current.
Common Base Transistor Circuit
Common Emitter Transistor Circuit
Common Collector Transistor Circuit
Rin E
B
Emitter
Collector
P
N
Common Collector Configuration
NPN Transistor
Collector
P
Common Emitter Configuration
C
a) Two Diode Analogy
Emitter
Common Base Configuration
N
N
P
C
IE
VBE
IC
B
IB
C
Rin
RL
Vout
B
IB
IE
VBE
b) Construction
Base
Base
VCE
E
VCE
IE
IC
C
IE
E
IC
VBC
IC
C
Rin E
C
VCB
IB
VEB
c) Symbols
B
IB
B
The three elements of the two-junction transistor are:
(a) The EMITTER, which gives off or emits current carriers
(b) The BASE, which controls the flow of current carriers
(c) The COLLECTOR, which collects the current carriers.
Classification
As the Bipolar Transistor is a three terminal device, there are basically three possible
ways to connect it within an electronic circuit with one terminal being common to
both the input and output.
-
has Voltage Gain but no Current Gain.
has both Current and Voltage Gain.
has Current Gain but no Voltage Gain.
Regions of Transistor
Bipolar transistors have the ability to operate within three different regions:
Active Region - the transistor operates as an amplifier and Ic = ß.IB
Saturation - the transistor is "fully-ON" operating as a switch and Ic = I(saturation)
Cut-off - the transistor is "fully-OFF" operating as a switch and IC = 0
VBE
Vout
IC
C
Rin
B
IB
VBE
IE
Vout
RL
E
IB
VCE
E
VBE
Input Characterisitcs
IE
RL Vout
Input Characterisitcs
IB
(mA)
0.3
(mA)
VCB > 1V
IB
(m
A) 100
80
60
0.2
15
vCE > 1V
40
10
0.1
5
B
VCE
IE
20
vCB = 0
0
0
0.5
1.0
vEB (V)
Output Characteristics
IC
0
0.5
IC
(mA)
20
0.3
10
0.2
10
10
5
5
ICBO
5
10
IE = 0 mA
15
20 V (V)
CB
5
4
3
0.1
5
ICEO = 0
0
5
10
6
VCB
Output Characteristics
IB = 100m
A
80
5
15
15
2
IE
(mA) 6
0.4 mA
15
20
1
1.0
vBE (V)
Output Characteristics
20
(mA)
0
RL Vout
C
VBC
Input Characterisitcs
20
IE
VCE
E
IC
RL
B
IB
E
N
IB
Vout
RL
N
P
B
VCE=4V
Bipolar Transistor Configurations
VBE
Rin
VCE=2V
Transistors are classified as either NPN or PNP according to the arrangement of their
N and P materials.
An NPN transistor is formed by introducing a thin region of P-type material between
two regions of N-type material.
PNP transistor is formed by introducing a thin region of N-type material between two
regions of P-type material.
One PN junction is between the emitter and the base; the other PN junction is
between the collector and the base.
Area Wise: Collector has more area, Base has thin & Emitter has moderate area
Doping Status: Emitter: heavily doped, Base: lightly doped, Collector: moderately
doped
Common Base Configuration
Common Emitter Configuration
Common Collector Configuration
IE
VCB
C
VCE
Rin
VEB
IC
IC
N P N
History
4
60
3
40
2
20
1
ICEO
0
IB = 0
15
20
VCE (V)
0
1
2
3
4
5
6
VCE