B1 M4.1 PDF
B1 M4.1 PDF
B1 M4.1 PDF
MODULE 4: ELECTRONIC FUNDAMENTALS
Sub Module 4.1–SEMICONDUCTORS
Sub Module 4.2 –PRINTED CIRCUIT BOARDS
Sub Module 4.3 –SERVOMECHANISM
List of Amendments
Sub-Module &
Amendment No. Issue Date: Date Inserted: Inserted By: Date Removed: Removed By:
Pages:
MODULE 4
SEMICONDUCTORS
INTEGRATED CIRCUITS ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐ 45 INTRODUCTION ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐ 55
INTRODUCTION
In this fast developing society, electronics has come to stay as Importance
the most important branch of engineering. Electronic devices Electronics has gained much importance due to its numerous
are being used in almost all the industries for quality control and applications in industry. The electronic devices are capable of
automation and they are fast replacing the present vast army of performing (yet not limited to) the following functions:
workers engaged in processing and assembling in the factories.
Great strides taken in the industrial applications of electronics Rectification
during the recent years have demonstrated that this versatile
The conversion of A.C. into D.C. is called rectification.
tool can be of great importance in increasing production,
Electronic devices can convert A.C. power into D.C. power
efficiency and control.
(See Fig 4.1) with very high efficiency. This D.C. supply can
The rapid growth of electronic technology offers a formidable be used for charging storage batteries, field supply of D.C.
challenge to the beginner, who may be almost paralyzed by the generators, electroplating etc.
mass of details. However, the mastery of fundamentals can
simplify the learning process to a great extent. The purpose of
this chapter is to present the elementary knowledge in order to
enable the readers to follow the subsequent chapters.
ELECTRONICS
Amplification Oscillators
The process of raising the strength of a weak signal is Electronic devices can convert D.C. power into A.C. power
known as amplification. Electronic devices can accomplish of any frequency and vice versa. When performing the
the job of amplification and thus act as amplifiers (See Fig conversion from D.C to A.C, they are known as oscillators.
4.2). The amplifiers are used in a wide variety of ways. For The oscillators are used in a wide variety of ways. For
example, an amplifier is used in a radio set where the weak example, electronic high frequency heating is used for
signal is amplified so that it can be heard loudly. annealing and hardening.
Fig 4.1.2
Control
Electronic devices find wide applications in automatic
control. For example, speed of a motor, voltage across a
refrigerator etc. can be automatically controlled with the
help of such devices.
N-type Semiconductor
FIGURE 4.1.4
(i) Many new free electrons called Majority Carriers are N-type Conductivity
produced by the addition of pentavalent impurity.
The current conduction in an n-type semiconductor is
predominantly by the Majority Carriers free electrons i.e.
(ii) Thermal energy of room temperature still generates
negative charges and is called n-type or electron type
a few hole-electron pairs. However, the number of
conductivity. To understand n-type conductivity, refer to Fig.
free electrons provided by the pentavalent impurity
4.1.6. When p.d. is applied across the n-type semiconductor,
far exceeds the number of holes. It is due to this
the free electrons (donated by impurity) in the crystal will be
predominance of electrons over holes due to which
directed towards the positive terminal, constituting electric
current. As the current flow through the crystal is by free
holes are called Minority carriers.
electrons which are carriers of negative charge, therefore, this
type of conductivity is called negative or n-type conductivity. It
may be noted that conduction is similar as in ordinary metals
like copper.
called a hole. Therefore, for each gallium atom added, one hole
is created. A small amount of gallium provides millions of holes.
Fig. 4.1.8 shows the energy band description of the p-type
semiconductor. The addition of trivalent impurity has produced a
large number of holes in the valence band. However, there are
a few conduction band electrons due to thermal energy
associated with room temperature. It is due to the
Fig. 4.1.6 predominance of holes over free electrons that Holes are called
Majority Carriers and the material is called p-type
semiconductor (p stands for positive).
P-type Semiconductor
When a small amount of trivalent impurity is added to a pure
semiconductor, it is called p-type semiconductor. The addition
of trivalent impurity provides a large number of holes in the
semiconductor. Typical examples of trivalent impurities are
Gallium (Atomic No. 31), Bismuth and indium (Atomic No. 49).
Such impurities which produce p-type semiconductor are known
as acceptor impurities because the holes created can accept
the electrons.
Fig. 4.1.8
P-type Conductivity bond to another unlike the n-type where current conduction is
The current conduction in p-type semiconductor is by free electrons.
predominantly by Majority carriers holes or. Positive charges
HOLE CURRENT
and is called p-type or hole-type conductivity. To understand p-
At room temperature, some of the co-valet bonds in pure
type conductivity, refer to Fig. 4.1.9.
semiconductor break, setting up free electrons. Under the
influence of electric field, these free electrons constitute electric
current. At the same time, another current – the hole current –
also flows in the semiconductor. When a covalent bond is
broken due to thermal energy, the removal of one electron
leaves a vacancy i.e. a missing electron in the covalent bond.
This missing electron is called a hole or virtual charge which
acts as a positive charge. For one electron set free, one hole is
created. Therefore, thermal energy creates hole-electron pairs;
there being as many holes as the free electrons. The current
conduction by holes can be explained as follows:
It may be noted that hole current is due to the movement of Energy Band Description
valence electrons from one covalent bond to another bond. The hole current can be beautifully explained in terms of energy
bands. Suppose due to thermal energy, an electron leaves the
The reader may wonder why to call it a hole current when the
valence band to enter into the conduction band as shown in Fig.
conduction is again by electrons (of course valence electrons!).
4.22.This leaves a vacancy at L. Now the valence electron at M
The answer is that the basic reason for current flow is the
comes to fill the hole at L. The result is that hole disappears at
presence of holes in the co-valet bonds. Therefore, it is more
Land appears at M. Next, the valence electron at N moves into
appropriate to consider the current as the movement of holes. the hole at M. Consequently, hole is created at N. It is clear that
valence electrons move along the path PNML whereas holes
move in the opposite direction i.e. along the path LMNP.
Fig. 4.1.10
Fig. 4.1.11
Formation of PN-Junction
In actual practice, the characteristic properties of pn-junction will
not be apparent if a p-type block is just brought in contact with
n-type block. In fact, pn-junction is fabricated by special
techniques. One common method of making pn-junction is
called ALLOYING. In this method, a small block of indium
(trivalent impurity) is placed on an n-type germanium slab as
shown in Fig. 41.14 (i). The system is then heated to a
temperature of about 500ºC. The indium and some of the
germanium melt to form a small puddle of molten germanium-
indium mixture as shown in Fig. 4.1.14 (ii). The temperature is
then lowered and puddle begins to solidify. Under proper
conditions, the atoms of indium impurity will be suitably adjusted
Fig. 4.1.16
BIASING A PN-JUNCTION
In electronics, the term bias refers to the use of d.c. voltage to
establish certain operating conditions for an electronic device. In
relation to a pn junction, there are two bias conditions:
1. Forward biasing
2. Reverse biasing
1. Forward biasing.
When external d.c. voltage applied to the junction is in such a
direction that it cancels the potential barrier, thus permitting high
current flow, it is called forward biasing. To apply forward bias,
we connect positive terminal of the battery to p-type and
negative terminal to n-type as shown in Fig. 4.28.
(i) The free electrons from the negative terminal electric field which acts in the same direction as the field due to
continue to pour into the n-region while the free potential barrier. Therefore, the resultant field at the junction is
electrons in the n-region move towards the junction. strengthened and the barrier height is increased as shown in
Fig. 4.1.18.the increased potential barrier prevents the flow of
(ii) The electrons travel through the n-region as free- charge carriers across the junction. Thus, a high resistance path
electrons i.e. current in n-region is by free electrons is established for the entire circuit and hence the current does
not flow. With reverse bias to pn junction, the following points
(iii) When these electrons reach the junction, they are worth noting.
combine with holes and become valence electrons.
FIGURE 4.1.18
DIODE SYMBOL
A pn junction is known as a semi-conductor or *crystal diode.
The outstanding property of a semiconductor diode to conduct
current in one direction only permits it to be used as a rectifier.
A diode is usually represented by the schematic symbol shown
below.
From the graphs it can be seen that in the forward-biased This problem is solved by forcing equal voltage sharing by
condition, both diodes conduct the same amount of current and connecting a resistor across each diode as shown in figure
the forward voltage drop for each diode would be almost equal. 4.1.27.The values of R1 & R2 are selected in accordance with
In the reversed-biased condition, however, where each diode the reverse breakdown voltages so that it maintains constant
has to carry the same leakage current, the blocking voltage and same potential drop across both the diodes. Due to the
would differ significantly as shown in figure 4.1.26-b. equal voltage sharing the leakage current of each diode would
be different as shown in figure 2-b.
For same IS the BV differs
For same BV the IS differs.
The relationship between the resistors for equal voltage sharing Hence using equation (1) under conditions of equal voltage
is developed below sharing yields
For steady state current sharing, the circuit of figure 3a, with
series resistors is used. The values of R1 & R2 are selected
according to the difference in characteristics of the two diodes in
respect of their reverse breakdown voltages and the resistances
are selected in manner to make their reverse breakdown
voltages equal so that both the diodes share equal amount of
reverse current. If somehow current in any branch increases Figure 4.1.28
than a specific value, the drop across branch Resistance also
The dynamic current sharing circuit has an advantage over
increases causing an increase of the branch temperature. As
steady state current sharing circuit in the sense that due to
diode is of negative temperature coefficient thereby cumulative
some fault if current I1 increases than unlike steady state, this
increase of current takes place until the diode is burnt, therefore
induces an emf in L2 in a direction as to decrease Z2 thereby
only steady state stability is possible.
causing I2 to increase, as a result the load on D1 decreases and
Dynamic current sharing is achieved with the use of coupled the circuit continues to work without the burning of D1; which
inductors as indicated by figure 3b. otherwise if not fitted with L1 & L2 in the circuit would have
burnt with regenerative increase of current in D1 coz of increase
in temperature
DIODE TESTING
The condition of a semiconductor diode can be determined
quickly using
Fig. 4.1.29 Fig 4.1.33: Half wave rectifier
RECTIFIER DIODES
HALF WAVE RECTIFIER
The important thing to notice about the half-wave rectifier is this: The current through each diode is half the dc load current.
It has converted the ac input voltage to a pulsating dc voltage.
In other words, the load voltage is always positive or zero, IL = I1 + I2
depending on which half cycle it's in. Stated another way, the
ES= E1 + E2 for center tap E1 = E2 thereby
load current is always in the same direction. This conversion
from ac to dc is known as rectification.
However for positive half cycle the D1 conducts & D2 is open, Introduction
the potential Drop across D2 is such that polarity of pd across The silicon controlled rectifier (abbreviated as SCR) is a three-
RL and E2 adds up across D2. Thus by KVL terminal semiconductor switching device which is probably the
most important circuit element after the diode and the transistor.
E1 + E2 – PIV = 0
Invented in 1957, an SCR can be used as a controlled switch to
PIV = 2 E1 = ES i.e twice the E1 or E2 value. perform various functions such as rectification, inversion and
regulation of power flow. The SCR has assumed paramount
importance in electronics because it can be produced in
versions to handle currents up to several thousand amperes
and voltages up to more than 1 kV.
The SCR has appeared in the market under different names
such as thyristor, thyrode transistor. It is a unidirectional power
switch and is being extensively used in switching D.c. and ac.,
rectifying a.c. to give controlled d.c. output, converting d.c. into
a.c. etc. the various characteristics of silicon controlled rectifiers
and their increasing applications in power electronics are
discussed below.
A silicon controlled rectifier is a semiconductor device that acts
as a true electronic switch. It can change alternating current into
direct current and at the same time can control the amount of
power fed to the load. Thus SCR combines the features of a
rectifier and a transistor.
Conclusion
The following conclusions are drawn from the working of SCR:
(i) An SCR has two states i.e. either it does not conduct or
it conducts heavily. There is no state in between.
Therefore, SCR behaves like a switch.
(ii) There are two ways to turn on the SCR. The first method
is to keep the gate open and make the supply voltage
equal to the break over voltage. The second method is
to operate SCR with supply voltage less than break over
voltage and then turn it on by means of a small voltage
(typically 1.5 V,30 mA) applied to the gate.
(iii) Applying small positive voltage to the gate is the normal
way to close an SCR because the break over voltage is
usually much greater than supply voltage.
(iv) To open the SCR(i.e. to make it non-conducting), reduce
the supply voltage to zero.
A further development of the standard LED package is the Because these displays are composed of linear segments (that
seven segment numerical indicator and the sixteen segment is, there are no curls or twists which can be produced), some
alpha-numeric indicator. In these devices, the PN junctions are anomalies could exist between similarly formed letters or
elongated into a rectangular format and the light is emitted in a numbers. Any combination which may introduce a
bar shape. The letter or number which a multi-segment display misinterpretation is usually not specified in the equipment
is required to produce is formed from a combination of manual which covers the interpretation of the display. As an
illuminated segments. Fig 2a shows the layout of the example, the number 1 and the letter I could easily be read one
constituent light emitting diodes which are used in seven for the other, and the distinction will be shown in the display
segment displays and Fig 2b shows the layout for sixteen dictionary.
VARISTORS
The name is a portmanteau of variable resistor. A varistor is
also known as voltage-dependent resistor (VDR).
Hazards
While an varistor is designed to conduct significant power for
very short durations (about 8 to 20 microseconds), such as
caused by lightning strikes, it typically does not have the
capacity to conduct sustained energy. Under normal utility
voltage conditions, this is not a problem. However, certain types
NOTES:
TRANSISTORS
the transistor. The circuit representation of the PNP and NPN
Introduction transistors are shown in the figure.
In 1951 William Shockley invented the first Bipolar Junction
Transistor. The term is a derivative of Transfer & Resistance,
implying that it transfers resistance from output to input.
BIASED TRANSISTOR
Transistor Circuit notations:
A Double Subscript notation is used to identify Potential Drops
and EMF sources with the transistor circuits. The details are as
under:
VCE = VC
VBE = VB.
Transistor Currents Therefore a reverse biased collector base junction does not
prevent the diffusion of these carriers in the base. These
The arrow on the emitter specifies the conventional direction of
majority carriers injected from the emitter into the base travel
current when the emitter base junction is forward biased.
towards the collector and get collected on it thus forming the
For PNP transistor, the emitter base junction J1 is forward collector current IC. However, a very few of these carriers
biased, i.e. the (+) ve terminal of the battery is connected to the injected into the base recombine with the majority carriers of the
emitter (P side) and the (-) ve terminal to the base (N side). lightly doped base region. That is, holes injected from the
Holes on the emitter cross in to the base while electrons on the emitter of a PNP will combine with the electrons in its base and
base cross in to the emitter. r. electrons injected from the emitter of a NPN will combine with
the holes in its base. This gives rise to another small current in
In the active mode of operation of the transistor, the emitter the base terminal. This base current IB is very small compared
base junction is forward biased and the collector base junction to the collector current Ic. Sometimes the collector current can
is reverse biased. Thereby the majority carriers in J1 for PNP be taken as nearly equal in magnitude to the emitter current IE.
will be holes and for NPN are electrons. The base layer being
Since the base current IB is small it can be neglected. these emitter electrons INEare pulled up, by the positive potential
of the collector voltage Vcc through the base region to the
Therefore, IE is really equal to the sum of the base current IB and
collector current IC, and we can write, collector region, without meeting up with base holes and hence
forming the collector current IC or INC1As we know that P-type
materials contain more holes but they are neutral in charge and
as some of the electrons from emitter unite with holes to form
IE = IB + IC……………………… (1)
negatively charged atoms, the number of holes is reduced. The
Thus, the actual directions of IB and Ic also can be determined by flow of IB thereby requires persistently formation of new holes to
the direction of the arrow on the emitter. replace the ones lost due to recombination process at the base.
As IB<< IE thereby for practical reasons IE ≈ IC. As an idea, if 100
If IB is made zero by making the base lead open circuit, the
charge carriers compose emitter current then 98 charge carriers
number of holes quickly reduce to the point where the overall
will flow as Collector Current and only 02 charge carriers as the
charge of the base is highly negative. Then –ve charge carriers
Base current.
in the emitter region will no longer move towards the base,
because the negative charge of the base region would repel
them. Hence the collector current IC becomes zero.
IDEAL CHARACTERISTICS
In an IDEAL Transistor, the leakage currents ICEO& ICBO are By varying the value of IB we vary the value of the IC, because
considered zero. the strength of IB determines how fast new holes can form.
When the J1 is forward biased by VBB in CE configuration, then Negative charge carriers will move from emitter to base at such
a rate that holes are neutralized as fast as new holes are
the majority carriers electrons ( INE )from the emitter region will formed.
move toward and into the base region & the +ve charge carriers
holes in the base region move toward and into the emitter
region. Near J1 some of the free electrons meet and combine
with the base holes to form –ve charge atoms which flow as the
base current IBor (INE – INC1), however, Since holes are relatively
few in number and the base region is very thin, thereby most of
Sometimes you may find it necessary to determine if a given Refer to the table below which will identify the transistor
transistor is an NPN or PNP device. This identification can also
be performed with an ohmmeter. As in testing a transistor with
an ohmmeter, you must know the polarity of the voltage at the
Ohmmeter lead to +Ohmmeter lead
ohmmeter leads. Normally, the internal ohmmeter battery will
be connected so that a positive potential will appear at the red Resistance emitter
to emitter
or plus lead of the ohmmeter and a negative potential appears
high PNP NPN
at the black or minus lead. In some ohmmeters this is not true,
however. Check your unit by referring to the ohmmeter circuit. low NPN PNP
In order to forward bias a PN junction, you must apply a bias
As an example of how to use the table, consider the case of a
voltage to it so that the cathode (N) is negative and the anode
high resistance reading when the ohmmeter negative (-) lead is
(P) is positive. You must reverse the polarity to reverse bias the
connected to the emitter. The high resistance indicates reverse
junction. With this information and the knowledge of your
bias on the emitter-base junction. Therefore the negative (-)
ohmmeter operation, you can identify a transistor as being PNP
lead must be on the P section of the junction (the emitter) and
or NPN. The procedure below tells you how:
the + lead on an N section (the base). Therefore, we have a
PNP transistor.
Identify the transistor leads. Locate the emitter and
base connections NOTE: When using an analogue Multimeter switched to ‘ohms’
Set your ohmmeter to the RX 10 or R X 100 range the red lead becomes the negative and vice versa. When using
an electronic multimeter the ‘diode’ range must be used.
Connect the ohmmeter to the emitter and base leads of Polarity of the input leads is normal i.e. red is positive, black is
negative
INTEGRATED CIRCUITS
Integrated Circuit
INTRODUCTION
An integrated circuit is one in which circuit components such as
The circuits discussed so far in the text consisted of separately transistors, diodes, resistors, capacitors etc. are automatically
manufactured components (e.g. resistors, capacitors, diodes, part of a small semiconductor chip. An integrated circuit
transistors etc.) joined by wires or plated conductors on printed consists of a number of circuit components (e.g. transistors,
boards. Such circuits are known as discrete circuits because diodes, resistors etc.) and their inter connections in a single
each component added to the circuit is discrete (i.e. distinct or small package to perform a complete electronic function. These
separate) from the others. Discrete circuits have two main components are formed and connected within a small chip of
disadvantages. Firstly, in a large circuit (e.g. TV circuit, semiconductor material. The following points are worth noting
computer circuit) there may be hundreds of components and about integrated circuits:
consequently discrete assembly would occupy a large space.
Secondly, there will be hundreds of soldered points posing a (i) In an IC, the various components are automatically part
considerable problem of reliability. To meet these problems of of a small semi-conductor chip and the individual
space conservation and reliability, engineers started a drive for components cannot be removed or replaced. This is in
miniaturized circuits. This led to the development of contrast to discrete assembly in which individual
microelectronics in the late 1950s.Microelectronics is the branch components can be removed or replaced if necessary.
of electronics engineering which deals with micro-circuits. A
micro-circuit is simply a miniature assembly of electronic (iii) The size of an IC is extremely small. In fact, ICs are so
components. One type of such circuit is the integrated circuit, small that you normally need a microscope to see the
generally abbreviated as IC. An integrated circuit has various connections between the components. Fig. 23.1 shows
components such as resistors, capacitors, diodes, transistors a typical semi-conductor chip having dimensions 0.2 mm
etc. fabricated on a small semiconductor chip. How circuits ×0.2 mm ×0.001 mm. It is possible to produce circuits
containing hundreds of components are fabricated on a small containing many transistors, diodes, resistors etc. on the
semiconductor chip to produce an IC is a fascinating feat of surface of this small chip.
microelectronics. This has not only fulfilled the ever-increasing
demand of industries for electronic equipment of smaller size, (ii) No components of an IC are seen to project above the
lighter weight and low power requirements, but it has also surface of the chip. This is because all the components
resulted in high degree of reliability. In this chapter, we shall are formed within the chip.
focus our attention on the various aspects of integrated circuits.
Since logic gates operate using digital data, all input and output
signals will be composed of 1s or 0s. Typically, the symbol 1
represents “ON” or voltage positive. The symbol0 represents
“OFF” or voltage negative. Voltage negative is often referred to
as zero voltage or the circuit’s ground.
Inverter
Inverters provide the complement function by utilizing the
switching characteristics of a transistor. Referring to figure 1, a
high (1) on input A biases the transistor into conduction,
reflecting a low (0) onto output Z. A low (0) at A cuts off the
transistor and Vcc is the potential at Z.
Figure 2
0 0 0
0 0 1
0 1 0
0 1 1
1 0 0
1 0 1
1 1 0
1 1 1
Figure 4
Figure 5
AND Gate
Complete the truth table for the AND gate shown in figure 6.
An AND gate utilizes the same circuitry as a NAND gate with an
additional stage for inversion. As illustrated in figure 5, the
output of an AND gate is high only when all inputs are high.
With inputs A and B high, transistors Ql and Q2 conduct,
biasing Q3 at cut-off.
A B C Z
0 0 0
0 0 1
0 1 0
0 1 1
1 0 0
1 0 1
1 1 0
Figure 7
1 1 1
Figure 6 Complete the truth table for the NOR gate shown in figure 8.
NOR Gate
The output of a NOR gate is high only when all inputs all low. As
illustrated in figure 7, if any input A, B, or C is high, the
corresponding transistor is biased into conduction, reflecting a
low at Z. Only when all inputs are low will all transistors be cut
off, applying Vcc to Z.
0 0 0
0 0 1
0 1 0
0 1 1
1 0 0
Figure 9
1 0 1
1 1 0
Complete the truth table for the OR gate in figure below.
1 1 1
Figure 8
OR Gate
The OR gate circuitry is similar to the NOR gate with the
addition of an inverter stage. Referring to figure 9, Z is high
when any input is high. If one or more of the inputs are high, Q4
is cut off and Vcc is the potential of Z. Applying a low to all
inputs biases Q4 into conduction, reflecting a low to Z.
A B C Z
0 0 0
0 0 1
0 1 0
0 1 1
1 0 0
1 0 1
1 1 0
1 1 1
Figure 10
NOTES:
Zero Output Impedance: The output impedance of Zero Phase Shift: Any circuit containing either
operational amplifiers is extremely low and they can capacitance and/or inductance will cause phase shifts.
cope with changes in load impedance by supplying more
(or less) current without any significant change in the Zero Distortion: An ideal amplifier will give a perfect
output voltage. Acts like an Ideal voltage Source. amplified version of its input. All amplifiers will distort a
signal, however the more you pay the better the
Zero Offset: This means ideally that the output should distortion figures.
be zero when the inputs are shorted together or
OUTPUT VOLTAGE OF AN OP AMP
grounded, but practically opamp shows a small output
voltage termed as output offset voltage. However, this
effect can be cancelled out by supplying a small ‘offset’ The output of an opamp is differential and its gain is denoted by
voltage to zero the output – often by a simple ‘Ad’.
potentiometer adjustment external to the integrated Vo (V1-V2)
circuit. Vo = Ad (V1 – V2 ) = Ad Vd
Maximum output voltage obtainable at a given frequency: For example an audio amplifier may be required to amplify
signals within the frequency range of 20 Hz up to 20 KHz.
The amplifier can be used at higher frequencies than that Between these two frequencies the gain of the amplifier should
calculated above but only if the output amplitude is reduced so be substantially flat. The 20 Hz and 20KHz frequencies are
that the maximum slew rate of the amplifier is not exceeded. As called the lower and upper cutoff frequencies and represent the
an example, let us reconsider the above amplifier with an input frequencies at which the power output of the amplifier has fallen
signal at a frequency of 1MHz. The maximum Vm is given by: to 50% of its mid band value, in other words the output is 3dB
down. The Bandwidth is therefore the difference between the
upper and lower cutoff frequencies.
Vm
SR
3000000 volts / sec ond
2f 2 1000000 sec ond
Specifications for the gain of devices usually give the open loop
gain and apply only to DC or low frequency input signals. The
response of all devices falls off with frequency, and
specifications usually quote the frequency at which the gain has
fallen to 1 i.e. to 0dB. This frequency is typically of the order of Some manufacturers publish graphs similar to the one in Fig 6
a few megahertz. which shows how gain falls off with frequency; others quote the
frequency for unity gain and the fall-off of gain in dB/decade or
in dB/octave from which the graph may be constructed.
Fig 6: Response and Bandwidth