1. Bipolar Junction Transistors
BJTs B C E

2. 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

3. A good analogy for a transistor is a pipe with an adjustable gate.
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.

4. 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 B N P C E B P N C

5. The BJT – Bipolar Junction Transistor The Two Types of BJT Transistors
npn
pnp n p n p n p E C E C C C
Cross Section
Cross Section B B B B
Schematic Symbol
Schematic Symbol E E

6. NPN Bipolar Junction Transistor
LP4

7. PNP Bipolar Junction Transistor
LP4

8. 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

9. Biasing of Transistor 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

10. 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

11. 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)

12. IB becomes very small and IC will be as large as IE
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 N P
VEE
VCC IE IC IB

13. 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) N P
VEE
VCC IE IC IB

14. 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

15. 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 

16. 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 + B
VCB
VBE E C + B - ++
VBE
VCB

17. 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

18. is called dc alpha (dc) of a transistor
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 C + B - ++
VBE
VCB

19. 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

20. Common Emitter Configuration Relation between  and 
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 C E + B -
Relation between  and 
and

21. Common Collector Configuration
using
then
becomes or or or
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

22. Relation between transistor currents
From the figure C E + B -
Output current=(1+) x Input current
Relation between transistor currents
We know
and
because
We get
and

23. (1-)IE which becomes base current in the external circuit
Therefore
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

25. 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

26. BJT Input Characteristics
VCC E C + B
VBE
VCB IC IE
VEE R1 R2
VBE IE
2 mA
4 mA
6 mA
8 mA
0.7 V

27. 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
VCC E C + B
VBE
VCB IC IE
VEE R1 R2

28. 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 IE IC
VCB
Active Region
Cutoff
IE = 0

29. 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

30. Common Emitter Configuration Common Emitter(CE) Connection
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
VCC B C E
VBE
VCE IC IB
VBB R1 R2
Common Emitter(CE) Connection
LP4

31. 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

32. VBE IB
2 mA
4 mA
6 mA
8 mA
0.7 V

33. 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

34. 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
VCE IC
Active Region IB
Saturation Region
Cutoff Region
IB = 0
Region of Operation
Description
Active
Small base current controls a large collector current
Saturation
VCE(sat) ~ 0.2V, VCE increases with IC
Cutoff
Achieved by reducing IB to 0, Ideally, IC will also equal 0.

35. (Reverse saturation current)
Common Base
where
(Reverse saturation current)
Therefore, in general
Common Emitter

36. (Reverse saturation current) Relationship between dc and dc
where
Relationship between dc and dc
and

37. Common Base Formulas VCC E C B IC IE IB VEE RL RE and
VBE
VCB IC IE IB
VEE RL RE
and
Where VBE=0.3 V for Ge and 0.7 V for Si
Generally VEE>>VBE so IE=VEE/RE

38. Common Emitter Formulas
VCC E C B
VBE
VCE IC IB IE
VBB RL RB
and

39. DC  and DC   = Common-emitter current gain
 = Common-base current gain
 = IC  = IC
IB IE
The relationships between the two parameters are:
 =   = 
 
Note:  and  are sometimes referred to as dc and dc because the relationships being dealt with in the BJT are DC.

40. Using Common-Base NPN Circuit Configuration
BJT Example
Using Common-Base NPN Circuit Configuration C
Given: IB = 50  A , IC = 1 mA
Find: IE ,  , and 
Solution:
IE = IB + IC = 0.05 mA + 1 mA = 1.05 mA
= IC / IB = 1 mA / 0.05 mA = 20
 = IC / IE = 1 mA / 1.05 mA =
 could also be calculated using the value of  with the formula from the previous slide.
 =  = =  IC
VCB + _ IB B
VBE IE + _ E

41. Transistor as an amplifier

42. 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
VEE
VCC IE IC IB RL

43. 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

44. The collector current IC changes by
This current IC flows through RL causing a voltage drop
Hence as
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

45. Problems
In the CE Transistor circuit VBB= 5V, RBB= k, RCC = 1 k, VCC = 10V. Find IB, IC, VCE,  and the transistor power dissipation

46. By Applying KVL to the base emitter circuit
In the CE Transistor circuit shown earlier VBB= 5V, RBB= 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
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
By using this equation along with the iC / vCE characteristics of the base collector junction, iC = 4 mA, VCE = 6V

47. Input Characteristics Output Characteristics
Transistor power dissipation = VCEIC = 24 mW
We can also solve the problem without using the characteristics
if  and VBE values are known iC
10 mA
vCE
100 A
80 A
60 A
40 A
20 A iB
100 A 5V
vBE
Input Characteristics
Output Characteristics