1. BIPOLAR JUNCTION TRANSISTORS (BJTs)
Chapter 4
BIPOLAR JUNCTION
TRANSISTORS (BJTs)

2. 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.

3. OBJECTIVES
Describe the basic structure of the bipolar junction transistor (BJT)
Explain and analyze basic transistor bias and operation
Discuss the parameters and characteristics of a transistor and how they apply to transistor circuits

4. LECTURE OUTLINE 1. BJT structure 2. Basic BJT operations
3. BJT Characteristics and Parameters
4. BJT as an amplifier
5. BJT as a switch
6. Troubleshooting
Summary

5. 1. BJT STRUCTURE

6. 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

7. 1. BJT STRUCTURE

8. 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.

9. 1. BJT STRUCTURE
Transistor terminal current

10. 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 IB causes similar change in values of IC and IE.
Current gain (β)  factor by which current increases from base of transistor to its collector.

11. 1. BJT STRUCTURE Transistor Voltages:
VCC – collector supply voltage. This is a power supply voltage applied directly to collector of transistor.
VBB – base supply voltage. this is dc voltage used to bias base of transistor.
VEE – emitter supply voltage. dc biasing voltage and in many cases, VEE is simply a ground connection.

12. 1. BJT STRUCTURE Transistor Voltages:
VC – dc voltage measured from collector terminal of component to ground
VB – dc voltage measured from base terminal to ground.
VE – dc voltage measured from emitter terminal to ground.

13. 1. BJT STRUCTURE Transistor Voltages:
VCE – dc voltage measured from collector to emitter terminal of transistor.
VBE – dc voltage measured from base to emitter terminal of transistor.
VCB – dc voltage measured from collector to base terminal of transistor.

14. 2. BJT OPERATION

15. 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.

16. 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.

17. 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, ICEO passes from emitter to collector and can be neglected.
So, VCE = VCC

18. 2. BJT OPERATION Saturation region
Both transistor junctions are forward-biased.
IC reaches its maximum value as determined by VCC and total resistance in C-E circuit.
IC is independently from relationship of β and IB.
VBE is approximately 0.7V and VCE < VBE.

19. 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 IC depends on the values of β and IB.
VCE is approximately near to 0.7V and VCE falls in ranges VBE<VCE<VCC.

20. 2. BJT OPERATION Transistor Operating Regions: 1.Cutoff region:
leakage current is
neglected
Transistor Operating Regions:
1.Cutoff region:
Both transistor junctions are reverse biased
All terminal current are approximately equal
to zero. Since ICEO neglected, VCE = VCC
2.Active region:
The BE junction is forward biased and the BC junction is reverse biased
All terminal currents have some measurable value
The magnitude of IC depends on the values of and IB
VCE is approximately near to 0.7V and VCE falls in ranges VBE<VCE<VCC
3.Saturation:
Both transistor junctions are forward biased
IC reaches its maximum values- determine by
the component in the CE circuit, and independent
of the values of and IB
VBE is approximately 0.7V and VCE < VBE

21. 3. BJT CHARACTERISTICS & PARAMETERS

22. 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 : < <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 )

23. 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,
forward-biased the
base-emitter junction
reverse-biased the
base-collector junction
Transistor current & voltage

24. 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:

25. 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:

26. Example 1
Determine IB, IC, IE, VCE and VCB in the circuit below. The transistor has a βDC=150.

27. Solution Example 1
When BE junction is FB, act as normal diode. So, VBE=0.7V.
The base current,
Collector current,
Emitter current,
Solve for VCE and VCB.

28. 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

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

30. 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.

31. 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.

32. 3. BJT CHARACTERISTICS & PARAMETERS
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.

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

34. Solution Example 2 First, determine IC(sat),
Now, see if IB is large enough to produce IC(sat),
With specific βDC, this base current is capable of producing IC greater than IC(sat). Thus, transistor is saturated and IC = 11.5mA is never reached. If further increase IB, IC remains at its saturation value.

35. 3. BJT CHARACTERISTICS & PARAMETERS
More About beta, :
-Important parameter for BJT
-Varies both IC & temperature
-Keeping the junction temperature
constant, IC cause
-Further increase in IC beyond this
max. point cause to decrease
Maximum Transistor Ratings:
-Specified on manufacturer’s data sheet
-Given for VCE,VBE,VBC,IC & power dissipation
-The product of VCE and IC must not exceed the max. power dissipation
-Both VCE and IC cannot be max. at the same time.

36. 3. BJT CHARACTERISTICS & PARAMETERS
Derating :
-Specified at 25°C, for higher temp, PD(max) is less.
-Data sheet often give derating factor for determining at > 25°C
-Example: derating factor of 2mW/°C indicates that the max. power
dissipation is reduced 2mW for each degree increase in temperature.

37. Data Sheets
Data sheets give manufacturer’s specifications for maximum operating conditions, thermal, and electrical characteristics. For example, an electrical characteristic is βDC, which is given as hFE. The 2N3904 shows a range of β’s on the data sheet from 100 to 300 for IC = 10 mA.

38. 4. BJT AS AN AMPLIFIER

39. 4. BJT AS AN AMPLIFIER Transistor amplify current because
IB is very small, so IC ≈ IE.
Amplification of a small ac voltage by placing the ac signal source in the base circuit.
Vin is superimposed on the DC bias voltage VBB by connecting them in series with base resistor RB.
Small changes in the base current circuit causes large changes in collector current circuit.
Fig 4-20a & b (stacked)

40. 4. BJT AS AN AMPLIFIER Voltage gain:
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 Vb=Vin - IbRB. Vc as the transistor
ac output voltage. The ratio of Vc to Vb is ac voltage gain, Av of the circuit.
Substituting IeRC for Vc and Ier’e for Vb, yields:

41. 5. BJT AS A SWITCH

42. 5. BJT AS A SWITCH
A transistor when used as a switch is simply being biased so that it is in:
cutoff (switched off)
saturation (switched on)
Fig 4-22

43. 5. BJT AS A SWITCH Neglect leakage current and all currents
Conditions in Cutoff
Neglect leakage current and all currents
are zero. BE junction is reverse biased.
Conditions in Saturation
Since VCE(sat) is very small compared to
VCC, it can be neglected.

44. Example 3
For the transistor circuit in below figure, what is VCE when VIN=0v?
What minimum value of IB is required to saturate this transistor if βDC is 200?
Calculate the maximum value of RB when VIN=5V.

45. Solution Example 3
When VIN=0V, the transistor is in cutoff (act as open switch), so VCE(cutoff)=VCC = 10V.
Since VCE(sat) is neglected (assumed 0V),
This is the value of IB necessary to drive transistor to point of saturation.
c) When transistor is ON, VBE=0.7V. The voltage across RB is
VRB=VIN – VBE = 5 – 0.7 = 4.3V
By Ohm’s Law, the maximum value of RB is:

46. 6. TROUBLESHOOTING

47. 6. Troubleshooting
Troubleshooting a live transistor circuit requires us to be familiar with known good voltages, but some general rules do apply. Certainly a solid fundamental understanding of Ohm’s law and Kirchhoff’s voltage and current laws is imperative. With live circuits it is most practical to troubleshoot with voltage measurements.

48. Correct voltage measurement
6. Troubleshooting
Possible faults are open bias resistors, open or resistive connections, shorted connections and open or short internal to the transistor itself.
Voltage measurements that are typically low are caused by a point that not “electrically connected to ground”. This called a floating point. This is typically indicative of an open.
More in-depth discussion of typical failures are discussed within the textbook.
Fig 4-30 bias circuit
Correct voltage measurement

49. 6. Troubleshooting
Testing a transistor can be viewed more simply if you view it as testing two diode junctions. Forward bias having low resistance and reverse bias having high resistance.
Fig 4-32a&b

50. 6. Troubleshooting
The diode test function of a multimeter is more reliable than using an ohmmeter. Make sure to note whether it is an npn or pnp and polarize the test leads accordingly.
Fig DMM test

51. 6. Troubleshooting
In addition to the traditional DMMs there are also transistor testers. Some of these have the ability to test other parameters of the transistor, such as leakage and gain. Curve tracers give us even more detailed information about a transistors characteristics.

52. 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 IB is very small in comparison to IE and IC.
Beta is the current gain of a transistor. This the ratio of IC/IB.

53. 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.