Transcript Chapter 4 - UniMAP Portal

Chapter 4
Bipolar Junction
Transistors (BJTs)
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
4-1 Transistor 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
4-1 Transistor Structure (cont.)
4-2 Basic Transistor 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.
4-2 Basic Transistor Operation (cont.)
npn transistor operation:
 The forward bias from base to emitter narrow the BE depletion region, and the
reverse bias from base to collector widens the BC depletion region.
 The heavily doped n-type emitter region is teeming with conduction-band (free )
electrons that easily diffuse through BE junction into the p-type base region
where they become minority carriers.
 The base region is lightly doped and very thin so that it has a very limited number
of holes.
 Thus only a small percentage of all the electrons flowing through the BE junction
can combine with the available holes in the base.
 A few recombined electrons flow out of the base lead as valence electrons,
forming the small base electron current.
 Most of electrons from the emitter diffuse into the BC depletion region.
 Once in this region they are pulled through the reverse-biased BC junction by the
electric field set up by the force of attraction between the positive and negative
ions.
 The electron now move through the collector region out through the collector lead
into the positive terminal of the collector voltage source.
 The operation of pnp transistor is the same as for the npn except that the roles of
electrons and holes, the bias voltage polarities and the current directions are all
reversed.
4-2 Basic Transistor Operation (cont.)
Illustration of BJT action:
C
B
E
4-2 Basic Transistor Operation (cont.)
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)
I E  I B  IC
 IB << IE and IC
 The capital letter – dc value
(4-1)
4-3 Transistor Characteristic & Parameters
DC Beta (  DC ) and DC Aplha ( DC ):
 The ratio of the dc collector current (IC) to the dc base current (IB) is the
dc beta
(  DC ) = dc current gain of transistor
 Range value :
20<
 DC <200
 Usually designed as an equivalent hybrid (h) parameter,
transistor data sheet – hFE   DC
 DC
IC

IB
hFE on
(4-2)
 The ratio of the dc collector current (IC) to the dc emitter current (IE) is the
dc alpha (  DC ) – less used parameter in transistor circuits
 Range value-> 0.95<
 DC <0.99 or greater , but << 1 (Ic< IE )
 DC
IC

IE
4-3 Transistor Characteristic & Parameters (cont.)
Current and Voltage Analysis:
 The current and voltage can be identified as following:
 Current:
IB
dc emitter current, I E
dc collector current, I C
dc base current,
forward-biased the
base-emitter junction
Voltage:
VBE
dc voltage at collector with respect to base, VCB
dc voltage at collector with respect to emitter, VCE
dc voltage at base with respect to emitter,
reverse-biased the
base-collector junction
Transistor current & voltage
4-3 Transistor Characteristic & Parameters (cont.)
Current and Voltage Analysis:
 When the BE junction is forward-biased, like a forward biased
diode and the voltage drop is
(4-3)
VBE  0.7V
 Since the emitter is at ground (0V), by Kirchhoff’s voltage law, the
voltage across RB is: VR  VBB  VBE …….(1)
B
 Also, by Ohm’s law:
 From (1) ->(2) :
VRB  I B RB
……..(2)
VBB  VBE  I B RB
 Therefore, the dc base current is:
VBB  VBE
IB 
RB
(4-4)
4-3 Transistor Characteristic & Parameters (cont.)
Current and Voltage Analysis:
 The voltage at the collector with respect to the grounded emitter is
VCE  VCC  VRC
 Since the drop across RC is: VRC  I C RC
 The dc voltage at the collector with respect to the emitter is:
VCE  VCC  I C RC
where
(4-5)
I C   DC I B
 The dc voltage at the collector with respect to the base is:
VCB  VCE  VBE
(4-6)
4-3 Transistor Characteristic & Parameters (cont.)
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
4-3 Transistor Characteristic & Parameters (cont.)
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.
 The 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 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.
 In this time, IC levels off and remains constant for given value of IB and VCE
continues to increase.
4-3 Transistor Characteristic & Parameters (cont.)
Collector Characteristic Curve:
 Actually, IC increase very 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 I C   DC I B
 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.
4-3 Transistor Characteristic & Parameters (cont.)
Transistor Operating Regions:
leakage current is
neglected
1.Cutoff region:
• Both transistor junctions are reverse biased
• All terminal current are approximately equal
to zero
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 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
4-3 Transistor Characteristic & Parameters (cont.)
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.
4-3 Transistor Characteristic & Parameters (cont.)
More About beta,  DC , hFE:
-Important parameter for BJT
-Varies both IC & temperature
-Keeping the junction temperature
constant, IC
cause  DC
-Further increase in IC beyond this
max. point cause  DC 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.
IC 
PD (max)
VCE
4-3 Transistor Characteristic & Parameters (cont.)
Derating PD (max) :
-Specified at 25°C
-Data sheet often give derating factor for determining PD (max) at > 25°C
-Example: derating factor of 2mW/°C indicates that the max. power
dissipation is reduced 2mW for each degree increase in temperature.
Transistor Data Sheet:
-See Figure 4-20, pg. 179
-Max. VCEO = 40V – indicated that the voltage is measured from C to E
with the B is open
-The max. IC is 200mA
-
 DC for several values of IC
-VCE(sat) is 0.2V max for IC(sat) = 10mA
4-3 Transistor Characteristic & Parameters (cont.)
4-4 Transistor as an Amplifier
• 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.
I C   DC I B
• Small changes in the base
current circuit causes large
changes in collector current
circuit.
Voltage gain
Vc
AV 
Vb
Vc  IcRC
Vc IeRC
AV  
Vb Iere'
RC
AV 
re '
I c  Ie
re' internal ac emitter resistance
4.5 Transistor 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)
Conditions in Cutoff
VCE ( cutoff )  VCC
Conditions in Saturation
IC ( sat) 
VCC  VCE ( sat)
RC
IB (min) 
IC ( sat)
DC
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.
Troubleshooting
Opens in the external resistors or connections of the base or the
circuit collector circuit would cause current to cease in the collector
and the voltage measurements would indicate this.
Internal opens within the transistor itself
could also cause transistor operation to
cease.
Erroneous voltage measurements that
are typically low are a result of point that
is not “solidly connected”. This called a
floating point. This is typically
indicative of an open.
More in-depth discussion of typical
failures are discussed within the
textbook.
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 infinite resistance.
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.
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.
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.
 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.
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.