Chapter 1 INTRODUCTION

1
Chapter 1INTRODUCTION
2
Objectives

• Discuss basic operation of a diode

• Discuss the basic structure of atoms

• Discuss properties of insulators, conductors, and
  semiconductors

• Discuss covalent bonding

• Describe the properties of both p and n type
  materials

• Discuss both forward and reverse biasing of a p-n
  junction

3
Introduction
The basic function of a diode is to restrict
current flow to one direction.
Forward bias Current flows
Reverse Bias No current flows
4
Bohr model of an atom
As seen in this model, electrons circle the
nucleus. Atomic structure of a material
determines its ability to conduct or insulate.
5
Conductors, Insulators, and Semiconductors

• The ability of a material to conduct current is
  based on its atomic structure.

• The orbit paths of the electrons surrounding the
  nucleus are called shells.

• Each shell has a defined number of electrons it
  will hold. This is a fact of nature and can be
  determined by the formula, 2n2.

• The outer shell is called the valence shell.

• The less complete a shell is filled to capacity
  the more conductive the material is.

6
Conductors, Insulators, and Semiconductors
The valence shell determines the ability of
material to conduct current.
A Copper atom has only 1 electron in its valence
ring. This makes it a good conductor. It takes
2n2 electrons or in this case 32 electrons to
fill the valence shell.
A Silicon atom has 4 electrons in its valence
ring. This makes it a semiconductor. It takes 2n2
electrons or in this case or 18 electrons to
fill the valence shell.
7
Covalent Bonding
Covalent bonding is a bonding of two or more
atoms by the interaction of their valence
electrons.
8
Covalent Bonding
Certain atoms will combine in this way to form a
crystal structure. Silicon and Germanium atoms
combine in this way in their intrinsic or pure
state.
9
N-type and P-type Semiconductors
The process of creating N- and P-type materials
is called doping.
Other atoms with 5 electrons such as Antimony are
added to Silicon to increase the free electrons.
Other atoms with 3 electrons such as Boron are
added to Silicon to create a deficiency of
electrons or hole charges.
N-type
P-type
10
The Depletion Region
This creates the depletion region and has a
barrier potential. This potential cannot be
measured with a voltmeter but it will cause a
small voltage drop.
With the formation of the p and n materials
combination of electrons and holes at the
junction takes place.
11
Forward and Reverse Bias
Forward Bias
Reverse Bias
Voltage source or bias connections are to the p
material and to the n material. Bias must be
greater than .3 V for Germanium or .7 V for
Silicon diodes. The depletion region narrows.
Voltage source or bias connections are to the p
material and to the n material. Bias must be
less than the breakdown voltage. Current flow is
negligible in most cases. The depletion region
widens.
12
Forward Bias Measurements With Small Voltage
Applied
In this case with the voltage applied is less
than the barrier potential so the diode for all
practical purposes is still in a non-conducting
state. Current is very small.
13
Forward Bias Measurements With Applied Voltage
Greater Than the Barrier Voltage.
With the applied voltage exceeding the barrier
potential the now fully forward-biased diode
conducts. Note that the only practical loss is
the .7 Volts dropped across the diode.
14
Ideal Diode Characteristic Curve
In this characteristic curve we do not consider
the voltage drop or the resistive properties.
Current flow proportionally increases with
voltage.
15
Practical Diode Characteristic Curve
In most cases we consider only the forward bias
voltage drop of a diode. Once this voltage is
overcome the current increases proportionally
with voltage.This drop is particularly important
to consider in low voltage applications.
16
Complex Characteristic Curve of a Diode
The voltage drop is not the only loss of a diode.
In some cases we must take into account other
factors such as the resistive effects as well as
reverse breakdown.
17
Troubleshooting Diodes
Testing a diode is quite simple, particularly if
the multimeter used has a diode check function.
With the diode check function a specific known
voltage is applied from the meter across the
diode.
With the diode check function a good diode will
show approximately .7 V or .3 V when forward
biased.
When checking in reverse bias the full applied
testing voltage will be seen on the display. Note
some meters show an infinite (blinking) display.
18
Troubleshooting Diodes
An ohmmeter can be used to check the forward and
reverse resistance of a diode if the ohmmeter has
enough voltage to force the diode into
conduction. Of course, in forward- biased
connection, low resistance will be seen and in
reverse-biased connection high resistance will be
seen.
19
Troubleshooting Diodes
Open Diode In the case of an open diode no
current flows in either direction which is
indicated by the full checking voltage with the
diode check function or high resistance using an
ohmmeter in both forward and reverse connections.
Shorted Diode In the case of a shorted diode
maximum current flows indicated by a 0 V with the
diode check function or low resistance with an
ohmmeter in both forward and reverse connections.
20
Diode Packages
Diodes come in a variety of sizes and shapes. The
design and structure is determined by what type
of circuit they will be used in.
21
Summary

• Diodes, transistors, and integrated circuits are
  all made of semiconductor material.

• P-materials are doped with trivalent impurities

• N-materials are doped with pentavalent
  impurities.

• P and N type materials are joined together to
  form a PN junction.

• A diode is nothing more than a PN junction.

• At the junction a depletion region is formed.
  This creates barrier that requires approximately
  .3 V for a Germanium and .7 V for Silicon for
  conduction to take place.

22
Summary

• A diode conducts when forward-biased and does
  not conduct when reverse biased.

• When reversed-biased, a diode can only withstand
  so much applied voltage. The voltage at which
  avalanche current occurs is called reverse
  breakdown voltage.

• There are three ways of analyzing a diode. These
  are ideal, practical, and complex. Typically we
  use a practical diode model.

23
Chapter 2Diode Applications
24
Objectives

• Explain and analyze the operation of both half
  and full wave rectifiers

• Explain and analyze filters and regulators and
  their characteristics

• Explain and analyze the operation of diode
  limiting and clamping circuits

• Explain and analyze the operation of diode
  voltage multipliers

• Interpret and use a diode data sheet

• Troubleshoot simple diode circuits

25
Introduction
The basic function of a DC power supply is to
convert an AC voltage to a smooth DC voltage.
26
Half Wave Rectifier
A half wave rectifier(ideal) allows conduction
for only 180 or half of a complete cycle.
The output frequency is the same as the input.
The average VDC or VAVG Vp/?
27
Half Wave Rectifier
Peak inverse voltage is the maximum voltage
across the diode when it is in reverse bias.
The diode must be capable of withstanding this
amount of voltage.
28
Transformer-Coupled Input
Transformers are often used for voltage change
and isolation.
The turns ratio of the primary to secondary
determines the output versus the input.
The fact that there is no direct connection
between the primary and secondary windings
prevents shock hazards in the secondary circuit.
29
Full-Wave Rectifier
A full-wave rectifier allows current to flow
during both the positive and negative half cycles
or the full 360º. Note that the output frequency
is twice the input frequency.
The average VDC or VAVG 2Vp/?.
30
Full-Wave RectifierCenter-Tapped
This method of rectification employs two diodes
connected to a center-tapped transformer.
The peak output is only half of the transformers
peak secondary voltage.
31
Full-Wave Center Tapped
Note the current flow direction during both
alternations. Being that it is center tapped, the
peak output is about half of the secondary
windings total voltage.
Each diode is subjected to a PIV of the full
secondary winding output minus one diode voltage
drop. PIV2Vp(out) 0.7V
32
The Full-Wave Bridge Rectifier
The full-wave bridge rectifier takes advantage of
the full output of the secondary winding.
It employs four diodes arranged such that current
flows in the same direction through the load
during each half of the cycle.
33
The Full-Wave Bridge Rectifier
The PIV for a bridge rectifier is approximately
half the PIV for a center-tapped
rectifier. PIVVp(out) 0.7V
Note that in most cases we take the diode drop
into account.
34
Power Supply Filters And Regulators
As we have seen, the output of a rectifier is a
pulsating DC. With filtration and regulation this
pulsating voltage can be smoothed out and kept to
a steady value.
35
Power Supply Filters And Regulators
A capacitor-input filter will charge and
discharge such that it fills in the gaps
between each peak. This reduces variations of
voltage. The remaining voltage variation is
called ripple voltage.
36
Power Supply Filters And Regulators
The advantage of a full-wave rectifier over a
half-wave is quite clear. The capacitor can more
effectively reduce the ripple when the time
between peaks is shorter.
37
Power Supply Filters And Regulators
Being that the capacitor appears as a short
during the initial charging, the current through
the diodes can momentarily be quite high. To
reduce risk of damaging the diodes, a surge
current limiting resistor is placed in series
with the filter and load.
38
Power Supply Filters And Regulators
Regulation is the last step in eliminating the
remaining ripple and maintaining the output
voltage to a specific value. Typically this
regulation is performed by an integrated circuit
regulator. There are many different types used
based on the voltage and current requirements.
39
Power Supply Filters And Regulators
How well the regulation is performed by a
regulator is measured by its regulation
percentage. There are two types of regulation,
line and load. Line and load regulation
percentage is simply a ratio of change in voltage
(line) or current (load) stated as a
percentage. Line Regulation (?VOUT/?VIN)100 Lo
ad Regulation (VNL VFL)/VFL)100
40
Diode Limiters
Limiting circuits limit the positive or negative
amount of an input voltage to a specific value.
This positive limiter will limit the output to
VBIAS .7V
41
Diode Limiters
The desired amount of limitation can be attained
by a power supply or voltage divider. The amount
clipped can be adjusted with different levels of
VBIAS.
This positive limiter will limit the output to
VBIAS .7V
The voltage divider provides the VBIAS . VBIAS
(R3/R2R3)VSUPPLY
42
Diode Clampers
A diode clamper adds a DC level to an AC voltage.
The capacitor charges to the peak of the supply
minus the diode drop. Once charged, the capacitor
acts like a battery in series with the input
voltage. The AC voltage will ride along with
the DC voltage. The polarity arrangement of the
diode determines whether the DC voltage is
negative or positive.
43
Voltage Multipliers
Clamping action can be used to increase peak
rectified voltage. Once C1 and C2 charges to the
peak voltage they act like two batteries in
series, effectively doubling the voltage output.
The current capacity for voltage multipliers is
low.
44
Voltage Multipliers
The full-wave voltage doubler arrangement of
diodes and capacitors takes advantage of both
positive and negative peaks to charge the
capacitors giving it more current capacity.
Voltage triplers and quadruplers utilize three
and four diode-capacitor arrangements
respectively.
45
The Diode Data Sheet
The data sheet for diodes and other devices gives
detailed information about specific
characteristics such as the various maximum
current and voltage ratings, temperature range,
and voltage versus current curves. It is
sometimes a very valuable piece of information,
even for a technician. There are cases when you
might have to select a replacement diode when the
type of diode needed may no longer be available.
46
Troubleshooting
Our study of these devices and how they work
leads more effective troubleshooting. Efficient
troubleshooting requires us to take logical steps
in sequence. Knowing how a device, circuit, or
system works when operating properly must be
known before any attempts are made to
troubleshoot. The symptoms shown by a defective
device often point directly to the point of
failure. There are many different methods for
troubleshooting. We will discuss a few.
47
Troubleshooting
Here are some helpful troubleshooting techniques

• Power Check Sometimes the obvious eludes the
  most proficient troubleshooters. Check for fuses
  blown, power cords plugged in, and correct
  battery placement.

• Sensory Check What you see or smell may lead
  you directly to the failure or to a symptom of a
  failure.

• Component Replacement Educated guesswork in
  replacing components is sometimes effective.

48
Troubleshooting
Signal tracing is the most popular and most
accurate. We look at signals or voltages through
a complete circuit or system to identify the
point of failure. This method requires more
thorough knowledge of the circuit and what things
should look like at the different points
throughout.
49
Troubleshooting
This is just one example of troubleshooting that
illustrates the effect of an open diode in this
half-wave rectifier circuit. Imagine what the
effect would be if the diode were shorted.
50
Troubleshooting
This gives us an idea of what would be seen in
the case of an open diode in a full-wave
rectifier. Note the ripple frequency is now half
of what it was normally. Imagine the effects of a
shorted diode.
51
Summary

• The basic function of a power supply to give us
  a smooth ripple free DC voltage from an AC
  voltage.

• Half-wave rectifiers only utilize half of the
  cycle to produce a DC voltage.

• Transformer Coupling allows voltage manipulation
  through its windings ratio.

• Full-Wave rectifiers efficiently make use of the
  whole cycle. This makes it easier to filter.

• The full-wave bridge rectifier allows use of the
  full secondary winding output whereas the
  center-tapped full wave uses only half.

52
Summary

• Filtering and Regulating the output of a
  rectifier helps keep the DC voltage smooth and
  accurate.

• Limiters are used to set the output peak(s) to a
  given value.

• Clampers are used to add a DC voltage to an AC
  voltage.

• Voltage Multipliers allow a doubling, tripling,
  or quadrupling of rectified DC voltage for low
  current applications.

53
Summary

• The Data Sheet gives us useful information and
  characteristics of device for use in replacement
  or designing circuits.

• Troubleshooting requires use of common sense
  along with proper troubleshooting techniques to
  effectively determine the point of failure in a
  defective circuit or system.

54
Chapter 3Special-Purpose Diodes
55
Objectives

• Describe the characteristics of a zener diode
  and analyze its operation

• Explain how a zener is used in voltage
  regulation and limiting

• Describe the varactor diode and its variable
  capacitance characteristics

• Discuss the operation and characteristics of
  LEDs and photodiodes

• Discuss the basic characteristics of the current
  regulator diode, the pin diode, the step-recovery
  diode, the tunnel diode, and the laser diode.

56
Introduction
The basic function of zener diode is to maintain
a specific voltage across its terminals within
given limits of line or load change. Typically it
is used for providing a stable reference voltage
for use in power supplies and other equipment.
This particular zener circuit will work to
maintain 10 V across the load.
57
Zener Diodes
A zener diode is much like a normal diode, the
exception being is that it is placed in the
circuit in reverse bias and operates in reverse
breakdown. This typical characteristic curve
illustrates the operating range for a zener. Note
that its forward characteristics are just like a
normal diode.
58
Zener Diodes
The zener diodes breakdown characteristics are
determined by the doping process. Low voltage
zeners less than 5V operate in the zener
breakdown range. Those designed to operate more
than 5 V operate mostly in avalanche breakdown
range. Zeners are available with voltage
breakdowns of 1.8 V to 200 V.
This curve illustrates the minimum and maximum
ranges of current operation that the zener can
effectively maintain its voltage.
59
Zener Diodes
As with most devices, zener diodes have given
characteristics such as temperature coefficients
and power ratings that have to be considered. The
data sheet provides this information.
60
Zener Diode Applications
Regulation In this simple illustration of zener
regulation circuit, the zener diode will adjust
its impedance based on varying input voltages and
loads (RL) to be able to maintain its designated
zener voltage. Zener current will increase or
decrease directly with voltage input changes. The
zener current will increase or decrease inversely
with varying loads. Again, the zener has a finite
range of operation.
61
Zener Limiting
Zener diodes can used for limiting just as normal
diodes. Recall in previous chapter studies about
limiters. The difference to consider for a zener
limiter is its zener breakdown characteristics.
62
Varactor Diodes
A varactor diode is best explained as a variable
capacitor. Think of the depletion region a
variable dielectric. The diode is placed in
reverse bias. The dielectric is adjusted by
bias changes.
63
Varactor Diodes
The varactor diode can be useful in filter
circuits as the adjustable component.
64
Optical Diodes
The light-emitting diode (LED) emits photons as
visible light. Its purpose is for indication and
other intelligible displays. Various impurities
are added during the doping process to vary the
color output.
65
Optical Diodes
The seven segment display is an example of LEDs
use for display of decimal digits.
66
Optical Diodes
The photodiode is used to vary current by the
amount of light that strikes it. It is placed in
the circuit in reverse bias. As with most diodes
when in reverse bias, no current flows when in
reverse bias, but when light strikes the exposed
junction through a tiny window, reverse current
increases proportional to light intensity.
67
Other Diode Types
Current regulator diodes keeps a constant current
value over a specified range of forward voltages
ranging from about 1.5 V to 6 V.
68
Other Diode Types
The Schottky diodes significant characteristic
is its fast switching speed. This is useful for
high frequencies and digital applications. It is
not a typical diode in that it does not have a
p-n junction. Instead, it consists of a
heavily-doped n-material and metal bound
together.
69
Other Diode Types
The pin diode is also used in mostly microwave
frequency applications. Its variable forward
series resistance characteristic is used for
attenuation, modulation, and switching. In
reverse bias it exhibits a nearly constant
capacitance.
70
Other Diode Types
The step-recovery diode is also used for fast
switching applications. This is achieved by
reduced doping at the junction.
71
Other Diode Types
The tunnel diode has negative resistance. It will
actually conduct well with low forward bias. With
further increases in bias it reaches the negative
resistance range where current will actually go
down. This is achieved by heavily-doped p and n
materials that creates a very thin depletion
region.
72
Other Diode Types
The laser diode (light amplification by
stimulated emission of radiation) produces a
monochromatic (single color) light. Laser diodes
in conjunction with photodiodes are used to
retrieve data from compact discs.
73
Troubleshooting
Although precise power supplies typically use IC
type regulators, zener diodes can be used alone
as a voltage regulator. As with all
troubleshooting techniques we must know what is
normal.
A properly functioning zener will work to
maintain the output voltage within certain limits
despite changes in load.
74
Troubleshooting
With an open zener diode, the full unregulated
voltage will be present at the output without a
load. In some cases with full or partial loading
an open zener could remain undetected.
75
Troubleshooting
With excessive zener impedance the voltage would
be higher than normal but less than the full
unregulated output.
76
Summary

• The zener diode operates in reverse breakdown.

• A zener diode maintains a nearly constant
  voltage across its terminals over a specified
  range of currents.

• Line regulation is the maintenance of a specific
  voltage with changing input voltages.

• Load regulation is the maintenance of a specific
  voltage for different loads.

• There are other diode types used for specific RF
  purposes such as varactor diodes (variable
  capacitance), Schottky diodes (high speed
  switching), and PIN diodes (microwave attenuation
  and switching).

77
Summary

• Light emitting diodes (LED) emit either infrared
  or visible light when forward-biased.

• Photodiodes exhibit an increase in reverse
  current with light intensity.

• The laser diode emits a monochromatic light

78
Chapter 4Bipolar Junction Transistors
79
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

• Discuss how a transistor can be used as an
  amplifier or a switch

• Troubleshoot various failures typical of
  transistor circuits

80
Introduction
A transistor is a device that can be used as
either an amplifier or a switch. Lets first
consider its operation in a simpler view as a
current controlling device.
81
Basic Transistor Operation
Look at this one circuit as two separate
circuits, the base-emitter(left side) circuit and
the collector-emitter(right side) circuit. Note
that the emitter leg serves as a conductor for
both circuits.The amount of current flow in the
base-emitter circuit controls the amount of
current that flows in the collector circuit.
Small changes in base-emitter current yields a
large change in collector-current.
82
Transistor Structure
With diodes there is one p-n junction. With
bipolar junction transistors (BJT), there are
three layers and two p-n junctions. Transistors
can be either pnp or npn type.
83
Transistor Characteristics and Parameters
As previously discussed, base-emitter current
changes yield large changes in collector-emitter
current. The factor of this change is called
beta(?). ? IC/IB
84
Transistor Characteristics and Parameters
There are three key dc voltages and three key dc
currents to be considered. Note that these
measurements are important for troubleshooting.
IB dc base current IE dc emitter current IC
dc collector current VBE dc voltage across
base-emitter junction VCB dc voltage across
collector-base junction VCE dc voltage from
collector to emitter
85
Transistors Characteristics and Parameters
For proper operation, the base-emitter junction
is forward-biased by VBB and conducts just like a
diode. The collector-base junction is reverse
biased by VCC and blocks current flow through
its junction just like a diode.
Remember that current flow through the
base-emitter junction will help establish the
path for current flow from the collector to
emitter.
86
Transistor Characteristics and Parameters
Analysis of this transistor circuit to predict
the dc voltages and currents requires use of
Ohms law, Kirchhoffs voltage law and the beta
for the transistor. Application of these laws
begins with the base circuit to determine the
amount of base current. Using Kirchhoffs voltage
law, subtract the .7 VBE and the remaining
voltage is dropped across RB. Determining the
current for the base with this information is a
matter of applying of Ohms law. VRB/RB IB
The collector current is determined by
multiplying the base current by beta.
.7 VBE will be used in most analysis examples.
87
Transistor Characteristics and Parameters
What we ultimately determine by use of
Kirchhoffs voltage law for series circuits is
that in the base circuit VBB is distributed
across the base-emitter junction and RB in the
base circuit. In the collector circuit we
determine that VCC is distributed proportionally
across RC and the transistor(VCE).
88
Transistor Characteristics and Parameters
Collector characteristic curves give a graphical
illustration of the relationship of collector
current and VCE with specified amounts of base
current. With greater increases of VCC , VCE
continues to increase until it reaches breakdown,
but the current remains about the same in the
linear region from .7V to the breakdown voltage.
89
Transistor Characteristics and Parameters
With no IB the transistor is in the cutoff region
and just as the name implies there is practically
no current flow in the collector part of the
circuit. With the transistor in a cutoff state
the the full VCC can be measured across the
collector and emitter(VCE)
90
Transistor Characteristics and Parameters
Current flow in the collector part of the circuit
is, as stated previously, determined by IB
multiplied by ?. However, there is a limit to how
much current can flow in the collector circuit
regardless of additional increases in IB.
91
Transistor Characteristics and Parameters
Once this maximum is reached, the transistor is
said to be in saturation. Note that saturation
can be determined by application of Ohms law.
IC(sat)VCC/RC The measured voltage across the
now shorted collector and emitter is 0V.
92
Transistor Characteristics and Parameters
The dc load line graphically illustrates IC(sat)
and cutoff for a transistor.
93
Transistor Characteristics and Parameters
The beta for a transistor is not always constant.
Temperature and collector current both affect
beta, not to mention the normal inconsistencies
during the manufacture of the transistor. There
are also maximum power ratings to consider. The
data sheet provides information on these
characteristics.
94
Transistor Amplifier
Amplification of a relatively small ac voltage
can be had by placing the ac signal source in the
base circuit. Recall that small changes in the
base current circuit causes large changes in
collector current circuit. The small ac voltage
causes the base current to increase and decrease
accordingly and with this small change in current
the collector current will mimic the input only
with greater amplitude.
95
Transistor Switch
A transistor when used as a switch is simply
being biased so that it is in cutoff (switched
off) or saturation (switched on). Remember that
the VCE in cutoff is VCC and 0 V in saturation.
96
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
Ohms law and Kirchhoffs voltage and current
laws is imperative. With live circuits it is most
practical to troubleshoot with voltage
measurements.
97
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.
98
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.
99
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.
100
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.
101
Summary

• The bipolar junction transistor (BJT) is
  constructed of three regions base, collector,
  and emitter.

• The BJT has two pn 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.

102
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.

103
Chapter 5 Transistor Bias Circuits
104
Objectives

• Discuss the concept of dc biasing of a
  transistor for linear operation

• Analyze voltage-divider bias, base bias, and
  collector-feedback bias circuits.

• Basic troubleshooting for transistor bias
  circuits

105
Introduction
For the transistor to properly operate it must be
biased. There are several methods to establish
the DC operating point. We will discuss some of
the methods used for biasing transistors as well
as troubleshooting methods used for transistor
bias circuits.
106
The DC Operating Point
The goal of amplification in most cases is to
increase the amplitude of an ac signal without
altering it.
107
The DC Operating Point
For a transistor circuit to amplify it must be
properly biased with dc voltages. The dc
operating point between saturation and cutoff is
called the Q-point. The goal is to set the
Q-point such that that it does not go into
saturation or cutoff when an a ac signal is
applied.
108
The DC Operating Point
Recall that the collector characteristic curves
graphically show the relationship of collector
current and VCE for different base currents. With
the dc load line superimposed across the
collector curves for this particular transistor
we see that 30 mA of collector current is best
for maximum amplification, giving equal amount
above and below the Q-point. Note that this is
three different scenarios of collector current
being viewed simultaneously.
109
The DC Operating Point
With a good Q-point established, lets look at
the effect a superimposed ac voltage has on the
circuit. Note the collector current swings do not
exceed the limits of operation(saturation and
cutoff). However, as you might already know,
applying too much ac voltage to the base would
result in driving the collector current into
saturation or cutoff resulting in a distorted or
clipped waveform.
110
Voltage-Divider Bias
Voltage-divider bias is the most widely used type
of bias circuit. Only one power supply is needed
and voltage-divider bias is more stable(?
independent) than other bias types. For this
reason it will be the primary focus for study.
111
Voltage-Divider Bias
Apply your knowledge of voltage-dividers to
understand how R1 and R2 are used to provide the
needed voltage to point A(base). The resistance
to ground from the base is not significant enough
to consider in most cases. Remember, the basic
operation of the transistor has not changed.
112
Voltage-Divider Bias
In the case where base to ground resistance(input
resistance) is low enough to consider, we can
determine it by the simplified equation RIN(base)
?DCRE We can view the voltage at point A of
the circuit in two ways, with or without the
input resistance(point A to ground) considered.
113
Voltage-Divider Bias
For this circuit we will not take the input
resistance into consideration. Essentially we are
determining the voltage across R2(VB) by the
proportional method. VB (R2/R1 R2)VCC
114
Voltage-Divider Bias
We now take the known base voltage and subtract
VBE to find out what is dropped across RE.
Knowing the voltage across RE we can apply Ohms
law to determine the current in the
collector-emitter side of the circuit. Remember
the current in the base-emitter circuit is much
smaller, so much in fact we can for all practical
purposes we say that IE approximately equals IC.
IE IC
115
Voltage-Divider Bias
Although we have used npn transistors for most of
this discussion, there is basically no difference
in its operation with exception to biasing
polarities. Analysis for each part of the circuit
is no different than npn transistors.
116
Base Bias
This type of circuit is very unstable since its ?
changes with temperature and collector current.
Base biasing circuits are mainly limited to
switching applications.
117
Emitter Bias
This type of circuit is independent of ? making
it as stable as the voltage-divider type. The
drawback is that it requires two power supplies.
Two key equations for analysis of this type of
bias circuit are shown below. With these two
currents known we can apply Ohms law and
Kirchhoff's law to solve for the voltages.
IB IE/? IC IE -VEE-VBE/RE RB/?DC
118
Collector-Feedback Bias
Collector-feedback bias is kept stable with
negative feedback, although it is not as stable
as voltage-divider or emitter. With increases of
IC, less voltage is applied to the base. With
less IB ,IC comes down as well. The two key
formulas are shown below.
IB VC - VBE/RB IC VCC - VBE/RC RB/?DC
119
Troubleshooting
Shown is a typical voltage divider circuit with
correct voltage readings. Knowing these voltages
is a requirement before logical troubleshooting
can be applied. We will discuss some of the
faults and symptoms.
120
Troubleshooting
R1 Open With no bias the transistor is in
cutoff. Base voltage goes down to 0 V. Collector
voltage goes up to 10 V(VCC). Emitter
voltage goes down to 0 V.
121
Troubleshooting
Resistor RE Open Transistor is in cutoff. Base
reading voltage will stay approximately the
same. Collector voltage goes up to 10
V(VCC). Emitter voltage will be approximately the
base voltage .7 V.
122
Troubleshooting
Base Open Internally
Transistor is in cutoff. Base voltage stays
approximately the same. Collector voltage goes up
to 10 V(VCC). Emitter voltage goes down to 0 V.
123
Troubleshooting
Open BE Junction
Transistor is in cutoff. Base voltage stays
approximately the same. Collector voltage goes up
to 10 V(VCC) Emitter voltage goes down to 0 V.
124
Troubleshooting
Open BC Junction
Base voltage goes down to 1.11 V because of more
base current flow through emitter. Collector
voltage goes up to 10 V(VCC). Emitter voltage
will drop to .41 V because of small current flow
from forward-biased base-emitter junction.
125
Troubleshooting
RC Open
Base voltage goes down to 1.11 V because of more
current flow through the emitter. Collector
voltage will drop to .41 V because of current
flow from forward-biased collector-base
junction. Emitter voltage will drop to .41 V
because of small current flow from forward-biased
base-emitter junction.
126
Troubleshooting
R2 Open
Transistor pushed close to or into
saturation. Base voltage goes up slightly to
3.83V because of increased bias. Emitter voltage
goes up to 3.13V because of increased
current. Collector voltage goes down because of
increased conduction of transistor.
127
Summary

• The purpose of biasing is to establish a stable
  operating point (Q-point).

• The Q-point is the best point for operation of a
  transistor for a given collector current.

• The dc load line helps to establish the Q-point
  for a given collector current.

• The linear region of a transistor is the region
  of operation within saturation and cutoff.

128
Summary

• Voltage-divider bias is most widely used because
  it is stable and uses only one voltage supply.

• Base bias is very unstable because it is ?
  dependent.

• Emitter bias is stable but require two voltage
  supplies.

• Collector-back is relatively stable when
  compared to base bias, but not as stable as
  voltage-divider bias.

129
Chapter 6BJT Amplifiers
130
Objectives

• Understand the concept of amplifiers

• Identify and apply internal transistor parameters

• Understand and analyze common-emitter,
  common-base, and common-collector amplifiers

• Discuss multistage amplifiers

• Troubleshoot amplifier circuits

131
Introduction
One of the primary uses of a transistor is to
amplify ac signals. This could be an audio signal
or perhaps some high frequency radio signal. It
has to be able to do this without distorting the
original input.
132
Amplifier Operation
Recall from the previous chapter that the purpose
of dc biasing was to establish the Q-point for
operation. The collector curves and load lines
help us to relate the Q-point and its proximity
to cutoff and saturation. The Q-point is best
established where the signal variations do not
cause the transistor to go into saturation or
cutoff. What we are most interested in is the ac
signal itself. Since the dc part of the overall
signal is filtered out in most cases, we can view
a transistor circuit in terms of just its ac
component.
133
Amplifier Operation
For the analysis of transistor circuits from both
dc and ac perspectives, the ac subscripts are
lower case and italicized. Instantaneous values
use both italicized lower case letters and
subscripts.
134
Amplifier Operation
The boundary between cutoff and saturation is
called the linear region. A transistor which
operates in the linear region is called a linear
amplifier. Note that only the ac component
reaches the load because of the capacitive
coupling and that the output is 180º out of phase
with input.
135
Transistor Equivalent Circuits
We can view transistor circuits by use of
resistance or r parameters for better
understanding. Since the base resistance, rb is
small it normally is not considered and since the
collector resistance, rc is fairly high we
consider it as an open. The emitter resistance,
rc is the main parameter that is viewed.
You can determine rc from this simplified
equation. rc 25 mV/IE
136
Transistor Equivalent Circuits
The two graphs best illustrate the difference
between ?DC and ?ac. The two only differ slightly.
137
Transistor Equivalent Circuits
Since r parameters are used throughout the rest
of the textbook we will not go into deep
discussion about h parameters. However, since
some data sheets include or exclusively provide h
parameters these formulas can be used to convert
them to r parameters.
re hre/hoe rc hre 1/hoe rb hie - (1
hfe)
138
The Common-Emitter Amplifier
The common-emitter amplifier exhibits high
voltage and current gain. The output signal is
180º out of phase with the input. Now lets use
our dc and ac analysis methods to view this type
of transistor circuit.
139
The Common Emitter AmplifierDC Analysis
The dc component of the circuit sees only the
part of the circuit that is within the boundaries
of C1, C2, and C3 as the dc will not pass through
these components. The equivalent circuit for dc
analysis is shown. The methods for dc analysis
are just are the same as dealing with a
voltage-divider circuit.
140
Common Emitter AmplifierAC Equivalent Circuit
The ac equivalent circuit basically replaces the
capacitors with shorts, being that ac passes
through easily through them. The power supplies
are also effectively shorts to ground for ac
analysis.
141
Common Emitter AmplifierAC Equivalent Circuit
We can look at the input voltage in terms of the
equivalent base circuit (ignore the other
components from the previous diagram). Note the
use of simple series-parallel analysis skills for
determining Vin.
142
Common Emitter Amplifier AC Equivalent Circuit
The input resistance as seen by the input voltage
can be illustrated by the r parameter equivalent
circuit. The simplified formula below is used.
Rin(base) ?acre
The output resistance is for all practical
purposes the value of RC.
143
Common Emitter AmplifierAC Equivalent Circuit
Voltage gain can be easily determined by dividing
the ac output voltage by the ac input voltage. Av
Vout/Vin Vc/Vb Voltage gain can also be
determined by the simplified formula below. Av
RC/re
144
Common Emitter AmplifierAC Equivalent Circuit
Taking the attenuation from the ac supply
internal resistance and input resistance into
consideration is included in the overall gain.
Av (Vb/Vs)Av or Av Rin(total)/Rs
Rin(total)
145
The Common-Emitter Amplifier
The emitter bypass capacitor helps increase the
gain by allowing the ac signal to pass more
easily. The XC(bypass) should be about ten times
less than RE.
146
The Common-Emitter Amplifier
The bypass capacitor makes the gain unstable
since transistor amplifier becomes more dependent
on IE. This effect can be swamped or somewhat
alleviated by adding another emitter
resistor(RE1).
147
The Common-Collector Amplifier
The common-collector amplifier is usually
referred to as the emitter follower because there
is no phase inversion or voltage gain. The output
is taken from the emitter. The common-collector
amplifiers main advantages are its high current
gain and high input resistance.
148
The Common-Collector Amplifier
Because of its high input resistance the
common-collector amplifier used as a buffer to
reduce the loading effect of low impedance loads.
The input resistance can be determined by the
simplified formula below.
Rin(base) ? ?ac(re Re)
149
The Common-Collector Amplifier
The output resistance is very low. This makes it
useful for driving low impedance loads.
The current gain(Ai) is approximately ?ac.
The voltage gain is approximately 1.
The power gain is approximately equal to the
current gain(Ai).
150
The Common-Collector Amplifier
The darlington pair is used to boost the input
impedance to reduce loading of high output
impedance circuits. The collectors are joined
together and the emitter of the input transistor
is connected to the base of the output
transistor. The input impedance can be determined
the formula below. Rin ?ac1?ac2Re
151
The Common-Base Amplifier
The common-base amplifier has high voltage gain
with a current gain no higher than 1. It has a
low input resistance making it ideal for low
impedance input sources. The ac signal is applied
to the emitter and the output is taken from the
collector.
152
The Common-Base Amplifier
The common-base voltage gain(Av) is approximately
equal to Rc/re
The current gain is approximately 1.
The power gain is approximately equal to the
voltage gain.
The input resistance is approximately equal to
re.
The output resistance is approximately equal to
RC.
153
Multistage Amplifiers
Two or more amplifiers can be connected to
increase the gain of an ac signal. The overall
gain can be calculated by simply multiplying each
gain together. Av Av1Av2Av3
154
Multistage Amplifiers
Gain can be expressed in decibels(dB). The
formula below can be used to express gain in
decibels. A v(dB) 20logAv Each stages gain
can now can be simply added together for the
total.
155
Multistage Amplifiers
The capacitive coupling keeps dc bias voltages
separate but allows the ac to pass through to the
next stage.
156
Multistage Amplifiers
The output of stage 1 is loaded by input of stage
2. This lowers the gain of stage 1. This ac
equivalent circuit helps give a better
understanding how loading can effect gain.
157
Multistage Amplifiers
Direct coupling between stage improves low
frequency gain. The disadvantage is that small
changes in dc bias from temperature changes or
supply variations becomes more pronounced.
158
Troubleshooting
Troubleshooting techniques for transistor
amplifiers is similar to techniques covered in
Chapter 2. Usage of knowledge of how an amplifier
works, symptoms, and signal tracing are all
valuable parts of troubleshooting. Needless to
say experience is an excellent teacher but having
a clear understanding of how these circuits work
makes the troubleshooting process more efficient
and understandable.
159
Troubleshooting
The following slide is a diagram for a two stage
common-emitter amplifier with correct voltages at
various points. Utilize your knowledge of
transistor amplifiers and troubleshooting
techniques and imagine what the effects would be
with various faulty componentsfor example, open
resistors, shorted transistor junctions or
capacitors. More importantly, how would the
output be affected by these faults? In
troubleshooting it is most important to
understand the operation of a circuit. What
faults could cause low or no output? What faults
could cause a distorted output signal?
160
Troubleshooting
161
Summary

• Most transistors amplifiers are designed to
  operate in the linear region.

• Transistor circuits can be view in terms of its
  ac equivalent for better understanding.

• The common-emitter amplifier has high voltage
  and current gain.

• The common-collector has a high current gain and
  voltage gain of 1. It has a high input impedance
  and low output impedance.

162
Summary

• The common-base has a high voltage gain and a
  current gain of 1. It has a low input impedance
  and high output impedance

• Multistage amplifiers are amplifier circuits
  cascaded to increased gain. We can express gain
  in decibels (dB).

• Troubleshooting techniques used for individual
  transistor circuits can be applied to multistage
  amplifiers as well.

163
Chapter 7Field Effect Transistors
164
Objectives

• Explain the operation and characteristics of
  junction field effect transistors (JFET)

• Understand JFET parameters

• Discuss and analyze how JFETs are biased

• Explain the operation and characteristics of
  metal oxide semiconductor field effect
  transistors (MOSFET)

• Discuss and analyze how MOSFET are biased

• Troubleshoot FET circuits.

165
Introduction
Field effect transistors control current by
voltage applied to the gate. The FETs major
advantage over the BJT is high input resistance.
Overall, the purpose of the FET is the same as
that of the BJT.
166
The JFET
The junction field effect transistor, like a BJT,
controls current flow. The difference is the way
this is accomplished. The JFET uses voltage to
control the current flow. As you will recall the
transistor uses current flow through the
base-emitter junction to control current. JFETs
can be used as an amplifier just like the BJT.
VGG voltage levels control current flow in the
VDD, RD circuit.
167
The JFET
The terminals of a JFET are the source, gate, and
drain. A JFET can be either p channel or n
channel.
168
The JFET
The current is controlled by a field that is
developed by the reverse biased gate-source
junction (gate is connected to both sides). With
more VGG (reverse bias) the field (in white)
grows larger. This field or resistance limits the
amount of current flow through RD. With low or no
VGG current flow is at maximum.
169
JFET Characteristics and Parameters
Lets first take a look at the effects with a VGS
of 0V. ID increases proportionally with increases
of VDD (VDS increases as VDD is increased). This
is called the ohmic region (point A to B).
170
JFET Characteristics and Parameters
The point when ID ceases to increase regardless
of VDD increases is called the pinch-off voltage
(point B). This current is called maximum drain
current (IDSS). Breakdown (point C) is reached
when too much voltage is applied. This of course
is undesirable, so JFETs operation is always well
below this value.
171
JFET Characteristics and Parameters
From this set of curves you can see with
increased voltage applied to the gate the ID is
limited and of course the pinch-off voltage is
lowered as well.
172
JFET Characteristics and Parameters
We know that as VGS is increased ID will
decrease. The point that ID ceases to increase is
called cutoff. The amount of VGS required to do
this is called the cutoff voltage (VP). The field
(in white) grows such that it allows practically
no current to flow through.
It is interesting to note that pinch-off voltage
(VGS(off)) and cutoff voltage (VP) are both the
same value but opposite polarity.
173
JFET Characteristics and Parameters
The transfer characteristic curve illustrates the
control VGS has on ID from cutoff (V GS(off) ) to
pinch-off (VP). Note the parabolic shape. The
formula below can be used to determine drain
current. ID IDSS(1 - VGS/VGS(off))2
174
JFET Characteristics and Parameters
Forward transfer conductance of JFETs is
sometimes considered. It is the changes in ID
based on changes in VGS.
Input resistance for a JFET is high since the
gate-source junction is reverse-biased. However,
the capacitive effects can offset this advantage
particularly at high frequencies.
Drain-to-source resistance is the ratio of
changes of VDS to ID.
175
JFET Biasing
Just as we learned that the bipolar junction
transistor must be biased for proper operation,
the JFET must also be biased for operation. Lets
look at some of the methods for biasing JFETs. In
most cases the ideal Q-point will be the middle
of the transfer characteristic curve, which is
about half of the IDSS.
176
JFET Biasing
Self-bias is the most common type of biasing
method for JFETs. Notice there is no voltage
applied to the gate. The voltage to ground from
here will always be 0V. However, the voltage from
gate to source (VGS) will be positive for n
channel and negative for p channel keeping the
junction reverse-biased. This voltage can be
determined by the formulas below. ID IS for all
JFET circuits. (n channel) VGS IDRS (p
channel) VGS -IDRS
177
JFET Biasing
Setting the Q-point requires us to determine a
value of RS that will give us the desired ID and
VGS.. The formula below shows the relationship.
RS VGS/ID To be able to do that we must
first determine the VGS and ID from the either
the transfer characteristic curve or more
practically from the formula below. The data
sheet provides the IDSS and VGS(off). VGS is the
desired voltage to set the bias. ID IDSS(1 -
VGS/VGS)2
178
JFET Biasing
Since midpoint biasing is most common, lets
determine how this is done. The values of RS and
RD determine the approximate midpoint bias. Half
of IDSS would be ID that is midpoint. The VGS to
establish this can be determined by the formula
below. VGS ? VGS(off)/3.4
179
JFET Biasing
The value of RS needed to establish the computed
VGS can be determined by the previously
discussed relationship below. RS VGS/ID
The value of RD needed can be determined by
taking half of VDD and dividing it by ID. RD
(VDD/2)/ID
180
JFET Biasing
Remember the purpose of biasing is to set a point
of operation (Q-point). In a self-biasing type
JFET circuit the Q-point is determined by the
given parameters of the JFET itself and values of
RS and RD. Setting it at midpoint on the drain
curve is most common. One thing not mentioned in
the discussion was RG . Its value is arbitrary
but it should be large enough to keep the input
resistance high.
181
JFET Biasing
The transfer characteristic curve along with
other parameters can be used to determine the
midpoint bias Q-point of a self-biased JFET
circuit. First determine the VGS at IDSS from
the formula below. VGS -IDRS Where the two
lines intersect gives us the ID and VGS
(Q-point) needed for midpoint bias. Note that
load line extends from VGS(off)(ID 0A) to VP(ID
IDSS)
182
JFET Biasing
Voltage-divider bias can also be used to bias a
JFET. R1 and R2 are used to keep the gate-source
junction in reverse bias. Operation is no
different from self-bias. Determining ID, VGS for
a JFET voltage-divider circuit with VD given can
be calculated with the formulas below.
ID VDD - VD/RD VS IDRS VG (R2/R1
R2)VDD VGS VG - VS
183
JFET Biasing
In using the transfer characteristic curve to
determine the approximate Q-point we must
establish the two points for the load line. The
first point is ID 0 and VGS (note that VGS VG
when ID 0). VGS VG (R2/R1 R2)VDD The
second point is ID when VGS is 0. ID VG/RS
184
JFET Biasing
Transfer characteristics can vary for JFETs of
the same type. This would adversely affect the
Q-point. The voltage-divider bias is less
affected by this than self-bias. This is an
undesirable problem that in extreme cases would
require trying several of the same type until you
find one that works within the desired range of
operation.
185
The MOSFET
The metal oxide semiconductor field effect
transistor (MOSFET) is the second category of
FETs. The chief difference is that there is no
actual pn junction as the p and n materials are
insulated from each other. MOSFETs are static
sensitive devices and must be handled by
appropriate means. There are depletion MOSFETs
(D-MOSFET) and enhancement MOSFETs (E-MOSFET).
Note the difference in construction. The
E-MOSFET has no structural channel.
186
The MOSFET
The D-MOSFET can be operated in depletion or
enhancement modes. To be operated in depletion
mode, the gate is made more negative effectively
narrowing the channel or depleting the channel of
electrons.
187
The MOSFET
To be operated in the enhancement mode the gate
is made more positive, attracting more electrons
into the channel for better current flow.
Remember we are using n channel MOSFETs for
discussion purposes. For p channel MOSFETs,
polarities would change.
188
The MOSFET
The E-MOSFET or enhancement MOSFET can operate in
only the enhancement mode. With a positive
voltage on the gate the p substrate is made more
conductive.
189
The MOSFET
The lateral double diffused MOSFET (LDMOSFET) and
the V-groove MOSFET (VMOSFET) are specifically
designed for high power applications. Dual gate
MOSFETs have two gates, which helps control
unwanted capacitive effects at high frequencies.
190
MOSFET Characteristics and Parameters
Since most of the characteristics and parameters
of MOSFETs are the same as JFETs we will cover
only the key differences.
191
MOSFET Characteristics and Parameters
For the D-MOSFET we have to also consider its
enhancement mode. Calculating ID with given
parameters in the enhancement mode and depletion
mode is the same. Note this equation is no
different for ID than JFETs and that the transfer
characteristics are similar except for its effect
in the enhancement mode. ID IDSS(1 -
VGS/VGS(off) )2
Remember n and p channel polarity differences.
192
MOSFET Characteristics and Parameters
The E-MOSFET for all practical purposes does not
conduct until VGS reaches the threshold voltage
(VGS(th)). ID when it is when conducting can be
determined by the formulas below. The constant K
must first be determined. ID(on) is a data sheet
given value. K ID(on) /(VGS - VGS(th))2 ID
K(VGS - VGS(th))2
193
MOSFET Biasing
The three ways to bias a MOSFET are zero-bias,
voltage-divider bias, and drain-feedback bias.
For D-MOSFET, zero biasing as the name implies
has no applied bias voltage to the gate. The
input voltage swings it into deplet