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)
4-1 Transistor Structure (cont.)
 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
4-1 Transistor Structure (cont.)
 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
4-1 Transistor Structure (cont.)
• The base region is
lightly doped and very
thin compared to the
heavily doped emitter
and the moderately
doped collector region
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.)
 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.
4-2 Basic Transistor
Operation (cont.)
 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.
4-2 Basic Transistor
Operation (cont.)
 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
4-2 Basic Transistor
Operation (cont.)
• 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
4-2 Basic Transistor
Operation (cont.)
 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:
4-2 Basic Transistor Operation
(cont.)
Transistor Currents:
 The directions of the currents in npn transistor and pnp
transistor are shown in the figure.
4-2 Basic Transistor Operation
(cont.)
 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)
 The ratio of the dc collector current (IC) to the dc base
current (IB) is the dc beta
 Other name = dc current gain of transistor
 Range: 20 < DC < 200
 Usually designed as an equivalent hybrid (h)
parameter, hFE on transistor datasheet
 hFE = DC
 DC
IC

IB
(4.2)
4-3 Transistor Characteristic &
Parameters (cont.)
• DC Alpha (DC)
 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
Voltage
• dc base current, IB
• dc voltage at base with
respect to emitter, VBE
• dc emitter current, IE
• dc collector current, IC • Dc voltage at collector with
respect to base, VCB
• Dc voltage at collector with
respect to emitter, VCE
4-3 Transistor Characteristic &
Parameters (cont.)
Short Quiz: Draw location and direction of IB, IC, IE,
VBE, VCB & VCE.
• 5 minutes
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
VBB  VBE  I B RB
 Therefore, the dc base current is:
VBB  VBE
IB 
RB
(4-4)
……..(2)
4-3 Transistor Characteristic & Parameters (cont.)
Current and Voltage Analysis:
 The voltage across resistor RC is
VRC  VCC  VCE
 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.)
 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.
4-3 Transistor Characteristic &
Parameters (cont.)
 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.
4-3 Transistor Characteristic &
Parameters (cont.)
 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.
4-3 Transistor Characteristic &
Parameters (cont.)
 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.)
 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
4-3 Transistor Characteristic &
Parameters (cont.)
 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
4-3 Transistor Characteristic &
Parameters (cont.)
 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.)
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.)
Assignment
• Download/find/buy/copy any BJT
transistor datasheet and elaborate
on each parameter.
• Use handwriting / attach datasheet
• Due: Next friday
Basic Electronic
Chapter 4
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.