C
B
E
Transistors
• They are unidirectional current carrying
devices like diodes with capability to control
the current flowing through them
• Bipolar Junction Transistors (BJT) control
current by current
• Field Effect Transistors (FET) control current
by voltage
• They can be used either as switches or as
amplifiers
• A transistor allows you to control the current, not
just block it in one direction.
• A good analogy for a transistor is a pipe with an
adjustable gate.
• A transistor has three terminals.
• The main path for current is between the collector and emitter.
• The base controls how much current flows, just like the gate
controls the flow of water in the pipe.
BIPOLAR JUNCTION TRANSISTOR
• Two back to back P-N junctions
• Emitter
– Heavily doped
– Main function is to supply majority carriers to base
• Base
– Lightly doped as compared to emitter
– Thickness 10-6 m
• Collector
– Collect majority carriers from emitter through base
– Physically larger than the emitter region
E
N
P
B
N
C
E
P
N
B
P
C
The BJT – Bipolar Junction Transistor
The Two Types of BJT Transistors
npn
pnp
E
n
p
n
C
C
Cross Section
B
E
p
n
p
C
C
Cross Section
B
B
Schematic
Symbol
B
E
Schematic
Symbol
E
NPN Bipolar Junction
Transistor
LP4
6
PNP Bipolar Junction
Transistor
LP4
7
STRUCTURE
• The collector surrounds the emitter region, making it
almost impossible for the electrons injected into the
base region to escape being collected, thus making the
resulting value of α very close to unity, and so, giving
the transistor a large β
• Energy Band diagram of an unbiased transistor
– N-region moves down and P-region moves up due to
diffusion of majority carriers across junction.
– The displacement of band and carrier migration
stops when Fermi levels in the three regions are
equalized
Biasing of Transistor
– Base and emitter current when collector is open
• EB is forward biased- electron diffusion from
emitter to base and hole diffusion from base to
emitter
• Hence IB will be large and is equal to IE
• Collector is open so no current flows into collector
• Base and Collector current when the Emitter
is open (ICBO)
• CB is reverse biased- electron from base flow into
collector region and holes from collector flow
into base
• This current is known as reverse saturation
current
• The base current IB will be small and is equal to
ICBO
• Four Ways of Transistor biasing
– Both EB and CB junctions are fwd biased- Huge
current flows through base. The transistor is said to
be operating in Saturation region (mode)
– Both EB and CB junctions are reverse biased- The
transistor is said to be operating in cut off region
(mode)
– EB junction is fwd biased and CB junction is reverse
biased. The collector current is controlled by emitter
current or base current- The transistor is said to
operate in Active region (mode)
– EB junction in reverse biased and CB junction in fwd
biased- inverted region (mode)
Transistor Biasing-Active Region
When both Emitter and Collector are closed
• Emitter-base junction is forward biased
• Collector-base junction is reverse biased
• DC emitter supply voltage (VEE)- Negative terminal of
VEE is connected to emitter
• DC collector supply voltage (VCC)- Positive terminal of
VCC is connected to collector
• IB becomes very small and IC will be as large as IE
IE
VEE
N
P
IB
N
IC
VCC
Transistor currents
• Forward biasing from base to emitter narrows the BE
depletion region
• Reverse biasing from base to collector widens the
depletion CB region
• Conduction electrons diffuse into p-type base region
• Base is lightly doped and also very thin- so very few
electron combine with available hole and flow out of the
base as valence electrons (small base electron current)
IE
VEE
N
P
IB
N
IC
VCC
• Sufficient holes are not avail in base – remote possibility
of joining of electrons with holes
• Electron concentration is large on emitter side and nil
on collector
• Electrons swiftly move towards collector
• At CB junction they are acted upon by strong electric
field due to reverse bias and are swept into collector
Transistor currents
• Most of the electrons diffuse into CB depletion region
• These electrons are pulled across the reverse biased CB
junction by the attraction of the collector supply voltage
and form the collector electron current. Therefore
IE= IC + IB
• 1-2% of emitter current goes to supply base current and
98-99% goes to supply collector current
• Moreover, IE flows into the transistor and IB & IC flow out
of transistor
• Current flowing in is taken as positive and currents
flowing out are taken as negative
• The ratio of the number of electrons arriving at collector
to the number of electrons emitted by the emitter is called
base transportation factor 
Important Biasing Rule
• Both collector and base are positive with respect to
emitter
• But collector is more positive than base
• Different potentials have been designated by double
subscripts as shown in the figure
• VCB (Collector is more positive than base) and VBE (base
is more positive than emitter)
++ C
E
++ C
-
B
+
VCB
+
VBE
VCB
VBE
B
-E
Transistor circuit configuration
• There are of three types
– Common base (CB) OR grounded base
– Common emitter (CE) OR grounded emitter
– Common collector (CC) OR grounded collector
• Common is the term used to denote the electrode that is
common to the input and output circuits and it is
generally grounded
• Common-Base Biasing (CB) :
input = VBE & IE
•
output = VCB & IC
• Common-Emitter Biasing (CE):
•
input = VBE & IB
output = VCE & IC
• Common-Collector Biasing (CC):
•
input = VBC & IB
output = VEC & IE
Common Base Configuration
• Input signal applied between emitter & base
• Output is taken from collector & base
• Ratio of collector current to emitter current
is called dc alpha (dc) of a transistor
++
-
+
E
VBE
VCB
B
C
 dc
IC

IE
OR
I C   dc I E
• The subscript dc on  signifies that this ratio is defined
from dc values of IC and IE
• There is also an ac  which refers to the ratio of change
in collector current to the change in emitter current
• For all practical purposes dc= ac= 
• IE is taken as positive (flowing into transistor) and IC is
taken as negative (flowing out of transistor)
•  is the measure of quality of a transistor- higher its
values, better is the transistor
• Value ranges from 0.95 to 0.999
Common Emitter Configuration
• The input signal is applied between the base and
emitter and the output signal is taken out from the
collector and the emitter
• Ratio of collector current to base current is called dc
beta (dc) of a transistor
IC

IB
OR
I C  I B
C
Relation between  and 
IC
and  
IB
-
IC

IE
B
+
E
 IE

 IB
using
I B I E I C then   I C becomes
IB
IC

I E  IC
IC / I E


or  
I E / I E  IC / I E 1  
 1      or    1    or    / 1   
Common Collector Configuration
• The input signal is applied between the base and
collector and the output signal is taken out from the
emitter-collector circuit
• Ratio of emitter current to base current is
I E I E IC 


.
 
 1   
I B I C I B   / 1   
From the figure
C
Output current=(1+) x Input current
-
I EI B I CI BI B 1   I B
B
+
Relation between transistor currents
I E:I B:I C
We know
I C  I B  I E
and
IB 
because


1   
IC


I E  / 1   I E
I

 E  1   I E


1 

and  
1
1
We get 1   
1   
E
Therefore
I E: (1   ) I E: I E
1 : (1   ) : 
• This shows that emitter current initiated by the
forward biased emitter base junction is split
into two parts
• (1-)IE which becomes base current in the
external circuit
• IE which becomes collector current in the
external circuit
Static Characteristics
• Common Base Static characteristics
–Input characteristics. IE varies with VBE
when voltage VCB is held constant
• VCB is adjusted with the help of R1
• VBE is increased and corresponding values of IE are
noted
• The plot gives input characteristics
• Similar to the forward characteristics of P-N diode
• This characteristics is used to find the input
resistance of the transistor. Its value is given by the
reciprocal of its slope
Rin= VBE /  IE
BJT Input Characteristics
VEE
C IC
E
+
IE
R2
VCB
VBE
R1
B
IE
8
mA
6
mA
4
mA
2
mA
0.7 V
VBE
VCC
Static Characteristics
• Common Base Static characteristics
– Output characteristics. IC varies with VCB when IE is held
constant
•
•
•
•
VBE is adjusted with the help of R2 and IE is held constant
VCB is increased and corresponding values of IC are noted
The plot gives output characteristics
Then IE is increased to a value little higher and whole process is
repeated
• The output resistance of the transistor is given by
Rout= VCB /  IC
VEE
C IC
E
+
IE
R2
VCB
VBE
B
R1
VCC
BJT Output Characteristics
 IC flows even when VCB=0 for different values of
IE(due to internal junction voltage at CB junction)
 IC flows even when IE=0 (Collector leakage current or
reverse saturation current ICBO)
The output resistance is very high (500k)
Saturation Region
IC
Active
Region
IE
Cutoff
IE = 0
VCB
Static Characteristics
– It can be seen that IC flows even when VCB is zero
– It is due to the fact that electrons are being injected
into base due to forward biased E-B junction and are
collected by collector due to action of internal junction
voltage at C-B junction
– Another important feature is that a small amount of
collector current flows even when the emitter current IE
is zero called collector leakage current (ICBO)
– When VCB is permitted to increase beyond a certain
value, IC increases rapidly due to avalanche breakdown
• This characteristics may be used to find ac
ac =IC/ IE
Common Emitter Configuration
• Transistor is biased in active region
• Called CE because emitter is common to both VBB and VCC
• VBB forward biases the EB junction and VCC reverse biases the CB
IB
VBB
C IC
B
E
R2
VCE
VBE
VCC
R1
B
Common Emitter(CE) Connection
LP4
30
Static Characteristics
• Common Emitter Static
characteristics
– Input characteristics. IB varies with VBE when
voltage VCE is held constant
•
•
•
•
•
VCE is adjusted with the help of R1
VBE is increased and corresponding values of IB are noted
The plot gives input characteristics
Procedure is repeated for different (constant) values of VCE
This characteristics is used to find the input resistance of the
transistor. Its value is given by the reciprocal of its slope
Rin= VBE /  IB
IB
8 mA
6 mA
4 mA
2 mA
0.7 V
VBE
Static Characteristics
• Common Emitter Static characteristics
– Output characteristics. IC varies with VCE
when IB is held constant
• IB is held constant
• VCE is increased and corresponding values of IC are
noted
• The plot gives output characteristics
• Then IB is increased to a value little higher and
whole process is repeated
• The output resistance in this case is very less as
compared to CB circuit and is given by
Rout= VCE /  IC
 As VCE increases from zero, IC rapidly increases to saturation level for a fixed
value of IB
 IC flows even when IB=0 (Collector leakage current or reverse saturation
current ICEO), the transistor is said to be cutoff
When VCB is permitted to increase beyond a certain value, IC increases rapidly
due to avalanche breakdown
 This characteristics may be used to find ac
ac =IC/ IB
IC
Region of Description
Operation
Active
Small base current
controls a large
collector current
Active
Region
IB
Saturation VCE(sat) ~ 0.2V, VCE
increases with IC
Cutoff
Achieved by reducing
IB to 0, Ideally, IC will
also equal 0.
VCE
Saturation Region
Cutoff Region
I =0
Common Base
I E  IC  I B
 dc
where
IC

 I C   dc I E
IE
I E  0, I C  I CBO
Therefore, in general
(Reverse saturation current)
I C   dc I E  I CBO
Common Emitter
I E  IC  I B
 dc
IC

 I C   dc I B
IB
I B  0, I C  I CEO
where
(Reverse saturation current)
I C  dc I B  I CEO
Relationship between dc and dc


1   
and


1   
Common Base Formulas
IE
C IC
E
RE
VEE
VBE
IB
RL
VCB
VCC
B
VEE  I E RE  VBE
VEE  VBE
 0  IE 
RE
and
VCC  I C RL  VCB  0  VCB  VCC  I C RL
Where VBE=0.3 V for Ge and 0.7 V for Si
Generally VEE>>VBE so IE=VEE/RE
Common Emitter Formulas
IB
C
E
RB
VBB
IE
VBE
IC
RL
VCE
VCC
B
VBB  I B RB  VBE
VBB  VBE
 0  IE 
RB
and
VCC  I C RL  VCE  0  VCE  VCC  I C RL
DC  and DC 
 = Common-emitter current gain
 = Common-base current gain
 = IC
 = IC
IB
IE
The relationships between the two parameters are:
=

+1
=

1-
Note:  and  are sometimes referred to as dc and dc
because the relationships being dealt with in the BJT
are DC.
BJT Example
Using Common-Base NPN Circuit Configuration
C
Given: IB = 50  A , IC = 1 mA
VCB
Find:
IE ,  , and 
IB
B
VBE
IC
+
_
Solution:
+
_
IE
IE = IB + IC = 0.05 mA + 1 mA = 1.05 mA
 = IC / IB = 1 mA / 0.05 mA = 20
E
 = IC / IE = 1 mA / 1.05 mA = 0.95238
 could also be calculated using the value of
 with the formula from the previous slide.
=

= 20 = 0.95238
+1
21
Transistor as an amplifier
Transistor as an amplifier
• An electronic circuit that causes an increase in the
voltage or power level of a signal
• It is defined as the ratio of the output signal voltage to
the input signal voltage
OutputVoltage vo
G

InputVoltage
vi
IE
VEE
IC
IB
VCC
RL
 In the figure we see that an output voltage is developed
across RL
 The dc voltage VEE is a fixed voltage and causes a dc
current IE to flow through EB junction
 When the ac voltage Vi is super-imposed on VEE, the
emitter base voltage varies with time
 Say if VEE =10V and the peak voltage of Vi is is 1V, the
EB voltage swings from 9V to 11V
 The causes corresponding variations in IE and IC which
gives Vo
 The emitter variation due to EB voltage variation can be
expressed as
vi
I E 
ri
 The collector current IC changes by
I C   dc I E
 This current IC flows through RL causing a voltage
drop
vo  I C RL
vo   dc I E RL
vo   dc vi / ri RL
 Hence
G  vo / vi   dc RL / ri  as  dc  1
 Where ri is very small (100 ) and RL is of the order of
kilo-ohms. It means Vo is larger than Vi indicating that
the transistor has amplified small Vi to a larger Vo
Problems
• In the CE Transistor circuit VBB= 5V, RBB=
107.5 k, RCC = 1 k, VCC = 10V. Find IB, IC,
VCE,  and the transistor power dissipation
In the CE Transistor circuit shown earlier VBB= 5V, RBB= 107.5 k,
RCC = 1 k, VCC = 10V. Find IB, IC, VCE,  and the transistor power
dissipation using the characteristics as shown below
By Applying KVL to the base emitter circuit
VBB  VBE
IB 
RBB
By using this equation along with the
iB / vBE characteristics of the base
emitter junction, IB
= 40 A
By Applying KVL to the collector
emitter circuit
VCC  VCE
IC 
R CC
By using this equation along with the iC / vCE characteristics
of the base collector junction, iC
= 4 mA, VCE = 6V
I C 4m A
 
 100
I B 40A
Transistor power dissipation = VCEIC = 24 mW
We can also solve the problem without using the characteristics
if  and VBE values are known
iC
iB
100 A
10 mA
100 A
80 A
60 A
40 A
20 A
0
0
5V vBE
Input Characteristics
vCE
Output Characteristics