Chapter-6. Baipolar Junction Transistors BJT.ppt

It deals with baipolar juction transistors theory's, basic charactrestics, application in electronic devices and

1. Adigrat University College Of
Engineering and Technology
BJT power point
Prepared By Tesfay G.(BSc.)
Dec,2016

2. Chapter 6
BIPOLAR JUNCTION
TRANSISTORS (BJTs)

3. INTRODUCTION
What is transistor?
A three-terminal device whose output current,
voltage and/or power are controlled by its input.
Commonly used in audio application as an
amplifier, in switching application as a switch
and in power supply voltage and current
regulator circuit.
2 basic transistor types: BJT and FET
These two transistor differ in their operating
characteristic and their internal construction.

4. 1. BJT STRUCTURE
The BJT is constructed with three doped semiconductor regions
separated by two pn junctions.
The three region are called emitter (E),base (B) and collector (C)
The BJT have 2 types:
1. Two n region separate by a p region – called npn
2. Two p region separated by a n region – called pnp
The pn junction joining the base region and the emitter region is
called the base-emiter junction
The pn junction joining the base region and the collector region
is call base-collector junction
The base region is lightly doped and very thin compared to the
heavily doped emitter and the moderately doped collector
region

5. 1. BJT STRUCTURE

6. 1. BJT STRUCTURE
BJT schematic symbol
The arrow on schematic symbol is important
because:
Identify the component terminal
The arrow is always drawn on the emitter terminal.
The terminal opposite emitter is collector and the
center terminal is base.
The arrow always points toward n-type material
If the arrow point toward base, transistor is pnp
type. If it points toward emitter, transistor is npn
type.

7. 1. BJT STRUCTURE
Transistor terminal current

8. 1. BJT STRUCTURE
Transistor Currents:
The directions of the currents in npn transistor and pnp transistor
are shown in the figure.
The emitter current (IE) is the sum of the collector current (IC) and
the base current (IB)
IB << IE or IC
The capital letter – dc value
Transistor is a current-controlled device - the value of collector and
emitter currents are determined by the value of base current.
An increase or decrease in value of I
B causes similar change in
values of I
C
and I
E
.
CBE III 
BDCC
II
Current gain (β)  factor
by which current
increases from base of
transistor to its collector.

9. 1. BJT STRUCTURE
Transistor Voltages:
V
CC
– collector supply voltage. This is a power supply
voltage applied directly to collector of transistor.
V
BB
– base supply voltage. this is dc voltage used to bias
base of transistor.
V
EE – emitter supply voltage. dc biasing voltage and in
many cases, VEE is simply a ground connection.

10. 1. BJT STRUCTURE
Transistor Voltages:
V
C – dc voltage measured from collector terminal of
component to ground
V
B – dc voltage measured from base terminal to ground.
V
E – dc voltage measured from emitter terminal to ground.

11. 1. BJT STRUCTURE
Transistor Voltages:
V
CE
– dc voltage measured from collector to emitter
terminal of transistor.
V
BE – dc voltage measured from base to emitter terminal
of transistor.
V
CB – dc voltage measured from collector to base terminal
of transistor.

12. 2. BJT OPERATION
To operate the transistor properly, the two pn
junction must be correctly biased with external
dc voltages.
The figure shown the proper bias arrangement
for both npn and pnp transistor for active
operation as an amplifier.

13. 2. BJT OPERATION
Transistor is made of 3 separate semiconductor
materials that joined together to form two pn
junction.
Point at which emitter and base are joined
forms a single pn junction  base-emitter
junction
Collector-base junction  point where base and
collector meet.

14. 2. BJT OPERATION
Cutoff region
Both transistor
junctions are reverse
biased.
With large depletion
region between C-B
and E-B, very small
amount of reverse
current, I
CEO passes
from emitter to
collector and can be
neglected.
So, V
CE = V
CC

15. 2. BJT OPERATION
Saturation region
Both transistor junctions are
forward-biased.
I
C reaches its maximum
value as determined by V
CC
and total resistance in C-E
circuit.
I
C is independently from
relationship of β and I
B
.
V
BE is approximately 0.7V
and V
CE
< V
BE
.
EC
CC
C
RR
V
I



16. 2. BJT OPERATION
Active region
BE junction is forward biased
and the BC junction is reverse
biased.
All terminal currents have
some measurable value.
The magnitude of I
C depends
on the values of β and I
B.
V
BE is approximately near to
0.7V and V
CE falls in ranges
V
BE<V
CE<V
CC.

17. 3. BJT CHARACTERISTICS & PARAMETERS
DC Beta ( ) and DC Alpha ( ):
The ratio of the dc collector current (IC) to the dc base current (IB) is
the dc beta
( ) = dc current gain of transistor
Range value : 20< <200
Usually designed as an equivalent hybrid (h) parameter, on
transistor data sheet –
The ratio of the dc collector current (IC) to the dc emitter current (IE)
is the dc alpha ( ) – less used parameter in transistor circuits
Range value-> 0.95< <0.99 or greater , but << 1 (Ic< IE )
DC
DC
DC

DC

FEh
DCFE
h
DC
DC
B
C
DC
I
I

E
C
DC
I
I


18. 3. BJT CHARACTERISTICS & PARAMETERS
Current and Voltage Analysis:
The current and voltage can be identified as follow:
Current: Voltage:
dc base current, dc voltage at base with respect to emitter,
dc emitter current, dc voltage at collector with respect to base,
dc collector current, dc voltage at collector with respect to emitter,
BI
E
I
C
I
CE
V
CBV
BE
V
Transistor current & voltage
reverse-biased the
base-collector junction
forward-biased the
base-emitter junction

19. 3. BJT CHARACTERISTICS & PARAMETERS
Current and Voltage Analysis:
When the BE junction is forward-biased, like a forward biased
diode and the voltage drop is
Since the emitter is at ground (0V), by Kirchhoff’s voltage law,
the voltage across is: …….(1)
Also, by Ohm’s law: ……..(2)
From (1) ->(2) :
Therefore, the dc base current is:
VV
BE 7.0
BR BEBBR
VVV
B

BBR RIV
B

BBBEBB
RIVV 
B
BEBB
B
R
VV
I



20. 3. BJT CHARACTERISTICS & PARAMETERS
Current and Voltage Analysis:
The voltage at the collector with respect to the grounded
emitter is:
Since the drop across is:
The dc voltage at the collector with respect to the emitter is:

where
The dc voltage at the collector with respect to the base is:
CRCCCE
VVV 
CCCCCE RIVV 
BECECB
VVV 
CR CCRC RIV
BDCC II

21. Example 1
Determine I
B, I
C, I
E, V
CE and V
CB in the circuit
below. The transistor has a β
DC=150.

22. Solution Example 1
VmAVRIVV
CCCCCE 55.3)100)(5.64(10 
When BE junction is FB, act as normal diode. So,
V
BE=0.7V.
The base current,
Collector current,
Emitter current,
Solve for V
CE and V
CB.
VVVV
BECECB
85.27.055.3 
A
kR
VV
I
B
BEBB
B
430
10
7.05






mAAII
BDCC
5.64)430(150  
mAAmAIII
BCE
9.644305.64  

23. 3. BJT CHARACTERISTICS & PARAMETERS
Collector Characteristic Curve:
Using a circuit as shown in below, we can generate a set of
collector characteristic curve that show how the collector
current, Ic varies with the VCE voltage for specified values of
base current, IB.
variable voltage

24. Collector characteristic curve:
3. BJT CHARACTERISTICS & PARAMETERS

25. 3. BJT CHARACTERISTICS & PARAMETERS
Collector Characteristic Curve:
Assume that VBB is set to produce a certain value of IB and VCC is
zero.
At this condition, BE junction and BC junction are forward biased
because the base is approximately 0.7V while the emitter and the
collector are zero.
IB is through the BE junction because of the low impedance path to
ground, therefore IC is zero.
When both junctions are forward biased – transistor operate in
saturation region.
As VCC increase, VCE is increase gradually, IC increase – indicated by
point A to B.
IC increase as VCC is increased because VCE remains less than 0.7V
due to the forward biased BC junction.
When VCE exceeds 0.7V, the BC becomes reverse biased and the
transistor goes into the active or linear region of its operation.

26. 3. BJT CHARACTERISTICS & PARAMETERS
Collector Characteristic Curve:
Once BC junction is RB, IC levels off and remains constant for given
value of IB and VCE continues to increase.
Actually IC increases slightly as VCE increase due to widening of the
BC depletion region
This result in fewer holes for recombination in the base region which
effectively caused a slight increase in indicated in point
B and C.
When VCE reached a sufficiently high voltage, the reverse biased BC
junction goes into breakdown.
The collector current increase rapidly – as indicated at the right point
C
The transistor cannot operate in the breakdown region.
When IB=0, the transistor is in the cutoff region although there is a
very small collector leakage current as indicated – exaggerated on
the graph for purpose of illustration.
BDCC
II

27. DC Load Line:
Cutoff and saturation can be illustrated in relation to
the collector characteristic curves by the use of a load line.
DC load line drawn on the connecting cutoff and saturation
point.
The bottom of load line is ideal
cutoff where IC=0 & VCE=VCC.
The top of load line is saturation
where IC=IC(sat) & VCE =VCE(sat)
In between cutoff and saturation
is the active region of transistor’s
operation.
3. BJT CHARACTERISTICS & PARAMETERS

28. Example 2
Determine whether or not the transistor in figure
below is in saturation. Assume V
CE(sat) = 0.2V

29. Solution Example 2
First, determine I
C(sat),
Now, see if I
B is large enough to produce I
C(sat),
With specific β
DC, this base current is capable of
producing I
C greater than I
C(sat). Thus, transistor is
saturated and I
C = 11.5mA is never reached. If
further increase I
B, I
C remains at its saturation
value.
mA
kR
VV
I
C
satCECC
satC
8.9
0.1
2.010)(
)(






mA
kR
VV
I
B
BEBB
B 23.0
10
7.03






mAII
BDCC
5.11)23.0(50 

30. 4. BJT AS AN AMPLIFIER
BDCC II
•Transistor amplify current
because
•I
B
is very small, so I
C
I
≈
E
.
•Amplification of a small ac
voltage by placing the ac
signal source in the base
circuit.
•V
in
is superimposed on the
DC bias voltage V
BB by
connecting them in series
with base resistor R
B
.
•Small changes in the base
current circuit causes
large changes in collector
current circuit.

31. Voltage gain:
resistance emitter ac internal 're
Ce
cRIV
b
c
V
V
V
A
Cc
c
RIV
ee
Ce
b
c
V
rI
RI
V
V
A
'

e
C
V
r
R
A
'

e
cII
•Ac emitter current is Ie Ic = Vb / r’e.
≈

•Ac collector voltage, Vc equals ac voltage drop across Rc.
•Since , ac collector voltage is
•Vb is considered as ac input voltage where V
b=V
in - I
bR
B. Vc as the transistor
ac output voltage. The ratio of Vc to Vb is ac voltage gain, Av of the circuit.
•Substituting I
e
R
C
for Vc and I
e
r’
e
for V
b
, yields:
4. BJT AS AN AMPLIFIER

32. 5. BJT AS A SWITCH
A transistor when used as a switch is simply being biased
so that it is in:
1.cutoff (switched off)
2.saturation (switched on)

33. Conditions in Cutoff
CC
cutoffCE VV 
)(
C
satCECC
satC
R
VV
I
)(
)(


DC
satC
B
I
I

)(
(min)
Conditions in Saturation
Since V
CE(sat) is very small compared to
V
CC, it can be neglected.
Neglect leakage current and all currents
are zero. BE junction is reverse biased.
5. BJT AS A SWITCH

34. Example 3
a)For the transistor circuit in below figure, what is
V
CE when V
IN=0v?
b)What minimum value of I
B
is required to
saturate this transistor if β
DC is 200?
c)Calculate the maximum value of R
B when
V
IN=5V.

35. Solution Example 3
a)When V
IN=0V, the transistor is in cutoff (act as
open switch), so V
CE(cutoff)=V
CC = 10V.
b)Since V
CE(sat) is neglected (assumed 0V),
This is the value of I
B
necessary to drive transistor to
point of saturation.
c) When transistor is ON, V
BE
=0.7V. The voltage
across RB is
V
RB
=V
IN
– V
BE
= 5 – 0.7 = 4.3V
By Ohm’s Law, the maximum value of RB is:

A
mAI
I
mA
k
V
R
V
I
DC
satC
B
C
CC
satC


50
200
10
10
0.1
10
)(
(min)
)(




 k
I
V
R
B
RB
B
86
50
3.4
(min)
(max)


36. Summary
 The bipolar junction transistor (BJT) is constructed of
three regions: base, collector, and emitter.
 The BJT has two p-n junctions, the base-emitter
junction and the base-collector junction.
 The two types of transistors are pnp and npn.
 For the BJT to operate as an amplifier, the base-
emitter junction is forward biased and the collector-base
junction is reverse biased (transistor in active region).
 Of the three currents I
B is very small in comparison to
I
E and I
C.
 Beta is the current gain of a transistor. This the ratio
of I
C
/I
B
.

37. Summary
 A transistor can be operated as an electronics
switch.
 When the transistor is off it is in cutoff condition (no
current).
 When the transistor is on, it is in saturation condition
(maximum current).
 Beta can vary with temperature and also varies from
transistor to transistor.