I.C ASSIGNMENT #1: INDUCTOR AND CAPACITOR

Subject: I.C ASSIGNMENT # 1 CREATED BY: SYED AZEEM AHMED

ASSIGNMENT # 1

NAME : SYED AZEEM AHMED

CLASS : BE ( TELECOM ) Vth
SEMESTER

SUBJECT : INTEGRATED CIRCUIT

TEACHER : SIR ZAHID

CLASS: BE (TELECOM)Vth SEMESTER PAGE NUMBER 1


Inductor
An inductor is a passive electrical component that produces a voltage proportional to the
instantaneous change in current flowing through it:
V = L × dI/dt,

where V is the voltage generated, dI/dt is the rate of change of current, and L is a property of the
device called inductance. The SI unit of inductance is the henry (H).

Thus an inductor resists changes in current. A pure inductor does not offer any resistance to
direct current (an actual one does slightly), except when the current is switched on and off, then it
makes the change more gradual.

When a sinusoidal alternating current flows through an inductor, a sinusoidal alternating voltage
(or electromotive force, abbr. emf) is induced. The amplitude of the emf is related to the amplitude
of the current and to the frequency of the sinusoid by the following equation.

V = I × ωL

where ω is the angular frequency of the sinusoid defined in terms of the frequency f as

ω = 2πf

The term ωL is known as inductive reactance, which is denoted by the symbol XL and is the
positive imaginary component of impedance.

Construction

An inductor is usually constructed as a coil of conducting material, usually copper wire. A core of
ferrous material is sometimes used. Inductors can also be built on integrated circuits using the
same processes that are used to make computer chips. In these cases, aluminum is typically
used as the conducting material. (However it is rare that actual inductors are built on ICs, it is far
more common to use a circuit called a quot;gyratorquot; to make a capacitor appear to the IC as if it were
an inductor.)

This effect can be understood as follows: the current produces a magnetic field; a change in
current gives a change of this magnetic field; a changing magnetic field causes an electromotive
force in the conductor. An induction coil is closely related to electromagnets in structure, but used
for a different purpose—to store energy in a magnetic field.

Smaller inductors used for very high frequencies are sometimes made with a wire passing
through a ferrite cylinder or bead.

Capacitor

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This article is about the electronic component. For the physical phenomenon, see capacitance.
For an overview of various kinds of capacitors, see types of capacitor.



Capacitor

A simple demonstration of a parallel-plate capacitor

Type Passive

Invented Ewald Georg von Kleist (October 1745)

Electronic symbol

A capacitor or condenser is a passive electronic component consisting of a pair of conductors
separated by a dielectric. When a voltage potential difference exists between the conductors, an
electric field is present in the dielectric. This field stores energy and produces a mechanical force
between the plates. The effect is greatest between wide, flat, parallel, narrowly separated
conductors.

An ideal capacitor is characterized by a single constant value, capacitance, which is measured in
farads. This is the ratio of the electric charge on each conductor to the potential difference
between them. In practice, the dielectric between the plates passes a small amount of leakage
current. The conductors and leads introduce an equivalent series resistance and the dielectric
has an electric field strength limit resulting in a breakdown voltage.

The properties of capacitors in a circuit may determine the resonant frequency and quality factor
of a resonant circuit, power dissipation and operating frequency in a digital logic circuit, energy
capacity in a high-power system, and many other important system characteristics.

History

In October 1745, Ewald Georg von Kleist of Pomerania in
Germany found that charge could be stored by connecting a
generator by a wire to a volume of water in a hand-held
[
glass jar. Von Kleist's hand and the water acted as
conductors and the jar as a dielectric. Von Kleist found that
after removing the generator, touching the wire resulted in a
painful spark. In a letter describing the experiment, he said quot;I


]
would not take a second shock for the kingdom of France.quot; The following year, the Dutch
physicist Pieter van Musschenbroek invented a similar capacitor, which was named the Leyden
jar, after the University of Leyden where he worked Daniel Gralath was the first to combine
[
several jars in parallel into a quot;batteryquot; to increase the charge storage capacity.

Benjamin Franklin investigated the Leyden jar, and proved that the charge was stored on the
]
glass, not in the water as others had assumed. Leyden jars began to be made by coating the
inside and outside of jars with metal foil, leaving a space at the mouth to prevent arcing between
[ ]
the foils. The earliest unit of capacitance was the 'jar', equivalent to about 1 nanofarad

Leyden jar or flat glass plate construction was used exclusively up until about 1900, when the
invention of wireless (radio) created a demand for standard capacitors, and the steady move to
]
higher frequencies required capacitors with lower inductance A more compact construction
began to be used of a flexible dielectric sheet such as oiled paper sandwiched between sheets of
[citation needed]
metal foil, rolled or folded into a small package.

Early capacitors were also known as condensers, a term that is still occasionally used today. It
was coined by Alessandro Volta in 1782 (derived from the Italian condensatore), with reference to
the device's ability to store a higher density of electric charge than a normal isolated conductor.
Most non-English European languages still use a word derived from quot;condensatorequot;.

Theory of operation

Charge separation in a parallel-plate capacitor causes an internal electric field. A dielectric
(orange) reduces the field and increases the capacitance.

[4]
A capacitor consists of two conductors separated by a non-conductive region. The non-
conductive substance is called the dielectric medium, although this may also mean a vacuum or a
semiconductor depletion region chemically identical to the conductors. A capacitor is assumed to
be self-contained and isolated, with no net electric charge and no influence from an external
[5]
electric field. The conductors thus contain equal and opposite charges on their facing surfaces,
and the dielectric contains an electric field. The capacitor is a reasonably general model for
electric fields within electric circuits.

An ideal capacitor is wholly characterized by a constant capacitance C, defined as the ratio of
[4]
charge ±Q on each conductor to the voltage V between them:

Sometimes charge buildup affects the mechanics of the capacitor, causing the capacitance to
vary. In this case, capacitance is defined in terms of incremental changes:



In SI units, a capacitance of one farad means that one coulomb of charge on each conductor
[6]
causes a voltage of one volt across the device.

Energy storage

Work must be done by an external influence to move charge between the conductors in a
capacitor. When the external influence is removed, the charge separation persists and energy is
stored in the electric field. If charge is later allowed to return to its equilibrium position, the energy
is released. The work done in establishing the electric field, and hence the amount of energy
[7]
stored, is given by:

Current-voltage relation

The current i (t ) through a component in an electric circuit is defined as the rate of change of the
charge q (t ) that has passed through it. Physical charges cannot pass through the dielectric layer
of a capacitor, but rather build up in equal and opposite quantities on the electrodes: as each
electron accumulates on the negative plate, one leaves the positive plate. Thus the accumulated
charge on the electrodes is equal to the integral of the current, as well as being proportional to
[8]
represent the initial voltage v (t0). This is the integral form of the capacitor equation,

.

[9]
Taking the derivative of this, and multiplying by C, yields the derivative form,

.

The dual of the capacitor is the inductor, which stores energy in the magnetic field rather than the
electric field. Its current-voltage relation is obtained by exchanging current and voltage in the
capacitor equations and replacing C with the inductance L.



DC circuits

A simple resistor-capacitor circuit demonstrates charging of a capacitor.

A series circuit containing only a resistor, a capacitor, a switch and a constant DC source of
[10]
voltage V0 is known as a charging circuit. If the capacitor is initially uncharged while the switch
is open, and the switch is closed at t = 0, it follows from Kirchhoff's voltage law that

Taking the derivative and multiplying by C, gives a first-order differential equation,

At t = 0, the voltage across the capacitor is zero and the voltage across the resistor is V0. The
initial current is then i (0) =V0 /R. With this assumption, the differential equation yields

where τ0 = RC is the time constant of the system.

As the capacitor reaches equilibrium with the source voltage, the voltage across the resistor and
the current through the entire circuit decay exponentially. The case of discharging a charged
capacitor likewise demonstrates exponential decay, but with the initial capacitor voltage replacing
V0 and the final voltage being zero.

AC circuits

Impedance, the complex sum of reactance and resistance, describes the phase difference and
the ratio of amplitudes between sinusoidally varying voltage and sinusoidally varying current at a
given frequency. Fourier analysis allows any signal to be constructed from a spectrum of
frequencies, whence the circuit's reaction to the various frequencies may be found. The
reactance and impedance of a capacitor are respectively



where j is the imaginary unit and ω is the angular velocity of the sinusoidal signal. The - j phase
indicates that the AC voltage V = Z I lags the AC current by 90°: the positive current phase
corresponds to increasing voltage as the capacitor charges, zero current corresponds to
instantaneous constant voltage, etc.

Note that impedance decreases with increasing capacitance and increasing frequency. This
implies that a higher-frequency signal or a larger capacitor results in a lower voltage amplitude
per current amplitude—an AC quot;short circuitquot; or AC coupling. Conversely, for very low frequencies,
the reactance will be high, so that a capacitor is nearly an open circuit in AC analysis—those
frequencies have been quot;filtered out.quot;

Capacitors are different from resistors and inductors in that the impedance is inversely
proportional to the defining characteristic, i.e. capacitance.

Parallel plate model

Dielectric is placed between two conducting plates, each of area A and with a separation of d.

The simplest capacitor consists of two parallel conductive plates separated by a dielectric with
permittivity ε. The model may also be used to make qualitative predictions for other device
geometries. The plates are considered to extend uniformly over an area A and a charge density
±ρ = ±Q/A exists on their surface. Assuming that the width of the plates is much greater than their
separation d, the electric field near the centre of the device will be uniform with the magnitude E =
ρ/ε. The voltage is defined as the line integral of the electric field between the plates

Solving this for C = Q/V reveals that capacitance increases with area and decreases with
separation

.

The capacitance is therefore greatest in devices made from materials with a high permittivity.


What do resistors do?
Resistors limit current. In a typical application, a resistor is connected in series with an LED:

Enough current flows to make the LED light up, but not so much that the LED is damaged. Later
in this Chapter, you will find out how to calculate a suitable value for this resistor. (LEDs are
described in detail in Chapter 5.)

The 'box' symbol for a fixed resistor is popular in the UK and Europe. A 'zig-zag' symbol is used in
America and Japan:

Resistors are used with transducers to make sensor subsystems. Transducers are electronic
components which convert energy from one form into another, where one of the forms of energy
is electrical. A light dependent resistor, or LDR, is an example of an input transducer.
Changes in the brightness of the light shining onto the surface of the LDR result in changes in its
resistance. As will be explained later, an input transducer is most often connected along with a
resistor to to make a circuit called a potential divider. In this case, the output of the potential
divider will be a voltage signal which reflects changes in illumination.

Microphones and switches are input transducers. Output transducers include loudspeakers,
filament lamps and LEDs. Can you think of other examples of transducers of each type?

In other circuits, resistors are used to direct current flow to particular parts of the circuit, or may
be used to determine the voltage gain of an amplifier. Resistors are used with capacitors
(Chapter 4) to introduce time delays.

Most electronic circuits require resistors to make them work properly and it is obviously important
to find out something about the different types of resistor available, and to be able to choose the
correct resistor value, in , , or M , for a particular application.


.
Fixed value resistors

The diagram shows the construction of a carbon film resistor:

During manufacture, a thin film of carbon is deposited onto a small ceramic rod. The resistive
coating is spiralled away in an automatic machine until the resistance between the two ends of
the rod is as close as possible to the correct value. Metal leads and end caps are added, the
resistor is covered with an insulating coating and finally painted with coloured bands to indicate
the resistor value.

Carbon film resistors are cheap and easily available, with values within ±10% or ±5% of their
marked, or 'nominal' value. Metal film and metal oxide resistors are made in a similar way, but
can be made more accurately to within ±2% or ±1% of their nominal value. There are some
differences in performance between these resistor types, but none which affect their use in simple
circuits.

Wirewound resistors are made by winding thin wire onto a ceramic rod. They can be made
extremely accurately for use in multimeters, oscilloscopes and other measuring equipment. Some
types of wirewound resistors can pass large currents wihtout overheating and are used in power
supplies and other high current circuits.

.
Colour code

How can the value of a resistor be worked out from the colours of the bands? Each colour
represents a number according to the following scheme:

Number Colour

0 black

1 brown

2 red

3 orange

4 yellow

5 green


6 blue

7 violet

8 grey

9 white

The first band on a resistor is interpreted as the FIRST DIGIT of the resistor value. For the
resistor shown below, the first band is yellow, so the first digit is 4:

The second band gives the SECOND DIGIT. This is a violet band, making the second digit 7. The
third band is called the MULTIPLIER and is not interpreted in quite the same way. The multiplier
tells you how many noughts you should write after the digits you already have. A red band tells
you to add 2 noughts. The value of this resistor is therefore 4 7 0 0 ohms, that is, 4 700 , or
4.7 . Work through this example again to confirm that you understand how to apply the
colour code given by the first three bands.

The remaining band is called the TOLERANCE band. This indicates the percentage accuracy of
the resistor value. Most carbon film resistors have a gold-coloured tolerance band, indicating that
the actual resistance value is with + or - 5% of the nominal value. Other tolerance colours are:

Tolerance Colour

±1% brown

±2% red

±5% gold

±10% silver

When you want to read off a resistor value, look for the tolerance band, usually gold, and hold the
resistor with the tolerance band at its right hand end. Reading resistor values quickly and
accurately isn't difficult, but it does take practice!

CLASS: BE (TELECOM)Vth SEMESTER PAGE NUMBER
10


DIODE
History of the Diode

Thermionic rectifiers were discovered in 1873 by Frederick Guthrie, and later rediscovered by
Thomas Edison in 1880, while crystal rectifiers were discovered in 1874 by Karl Braun. It wasn't
until 1919 that rectifiers were renamed diodes by William Eccles, although power diodes are still
called rectifiers today. The name diode comes from the Greek for quot;two pathquot; (di and odos).

How a Diode Works

A junction area, known as the depletion layer, forms around the boundary between two different
semiconductors. The boundary is usually created by doping one half of a silicon substrate with a
chemical. The substrate remains conductive but the junction is non-conductive, due to the
potential difference created when charge carriers (electrons and holes) diffuse through the
boundary. When a voltage of reverse polarity to this potential difference is applied, the charge
carriers join and current flows, a process known as recombination.

Schottky and Zener diodes are slightly different. Schottky diodes have a metal and semiconductor
junction. This allows fast switching between the conducting and non-conducting states because
there is no recovery time, unlike regular diodes which need time to change. A Zener diode works
like a regular diode, but also allows current to flow in the other direction if the voltage exceeds the
quot;Zener voltagequot;.

A diode has two terminals: cathode and anode. On circuit diagrams, the diode symbol is an arrow
head and a perpendicular line. The arrow represents the anode, and the line represents the
cathode, as it does on the diode case. Current flows through the diode when the cathode is made
negative, and the anode is made positive. However, the current stops if the polarity is reversed.

Types of Diodes

There are many different types, including light-emitting diodes, and peltier diodes. The major
development in recent years has been the organic light-emitting diode (OLED). They are made
from plastic, and are used to make thin video screens which have better visual quality than LCD
or plasma screens.

Light-emitting diodes (LEDs) produce light in a process called electroluminescence. When an
electron meets a hole at the junction, it drops to a lower energy level and releases a photon of a
certain wavelength. These photons are the light emitted by the LED, and their color (wavelength)
depends on the materials used in the diode. These materials include gallium nitride which
produces green light, and diamond which produces ultraviolet light. A special type of LED is the
laser diode, used in CD/DVD players and optical fiber networks. The photodiode behaves in the
opposite way to an LED, by creating a current when photons are absorbed from light striking it's
surface.

Peltier diodes absorb heat on one side of the junction, and emit it from the other side. This
transfer effect allows them to be used as thermoelectric heat pumps. However, they have such
low efficiency (under 10%) that they are only used when the benefits of a solid state device justify
their inefficiency. They are commonly used to cool other electronic components where
mechanical cooling would be impractical.

11

Uses of Diodes

A diode is a semiconductor device which allows current to flow
through it in only one direction. Although a transistor is also a
semiconductor device, it does not operate the way a diode does. A
diode is specifically made to allow current to flow through it in only
one direction.
Some ways in which the diode can be used are listed here.
A diode can be used as a rectifier that converts AC (Alternating
Current) to DC (Direct Current) for a power supply device.
Diodes can be used to separate the signal from radio frequencies.
Diodes can be used as an on/off switch that controls current.
This symbol is used to indicate a diode in a circuit diagram.

The meaning of the symbol is (Anode) (Cathode).
Current flows from the anode side to the cathode side.

Although all diodes operate with the same general principle, there are different types suited to
different applications. For example, the following devices are best used for the applications
noted.

Voltage regulation diode (Zener Diode)
The circuit symbol is .
It is used to regulate voltage, by taking advantage of the fact that Zener diodes tend to
stabilize at a certain voltage when that voltage is applied in the opposite direction.
Light emitting diode

This type of diode emits light when current flows through it in the forward direction.
(Forward biased.)
Variable capacitance diode
The current does not flow when applying the voltage of the opposite direction to the diode.
In this condition, the diode has a capacitance like the capacitor. It is a very small
capacitance. The capacitance of the diode changes when changing voltage. With the
change of this capacitance, the frequency of the oscillator can be changed.

12


The graph on the right shows
the electrical characteristics of
a typical diode.

When a small voltage is
applied to the diode in the
forward direction, current
flows easily.
Because the diode has a
certain amount of resistance,
the voltage will drop slightly
as current flows through the
diode. A typical diode causes
a voltage drop of about 0.6 -
1V (VF) (In the case of silicon
diode, almost 0.6V)
This voltage drop needs to be
taken into consideration in a circuit which uses many diodes in series. Also, the amount of
current passing through the diodes must be considered.

When voltage is applied in the reverse direction through a diode, the diode will have a great
resistance to current flow.
Different diodes have different characteristics when reverse-biased. A given diode should be
selected depending on how it will be used in the circuit.
The current that will flow through a diode biased in the reverse direction will vary from several
mA to just A, which is very small.

The limiting voltages and currents permissible must
be considered on a case by case basis. For
example, when using diodes for rectification, part of
the time they will be required to withstand a reverse
voltage. If the diodes are not chosen carefully, they
will break down.

Rectification / Switching / Regulation Diode

The stripe stamped on one end of the diode shows indicates the polarity of the diode.
The stripe shows the cathode side.
The top two devices shown in the picture are diodes used for rectification. They are made to
handle relatively high currents. The device on top can handle as high as 6A, and the one below
it can safely handle up to 1A.

13

However, it is best used at about 70% of its rating because this current value is a maximum
rating.
The third device from the top (red color) has a part number of 1S1588. This diode is used for
switching, because it can switch on and off at very high speed. However, the maximum current
it can handle is 120 mA. This makes it well suited to use within digital circuits. The maximum
reverse voltage (reverse bias) this diode can handle is 30V.
The device at the bottom of the picture is a voltage regulation diode with a rating of 6V. When
this type of diode is reverse biased, it will resist changes in voltage. If the input voltage is
increased, the output voltage will not change. (Or any change will be an insignificant amount.)
While the output voltage does not increase with an increase in input voltage, the output current
will.
This requires some thought for a protection circuit so that too much current does not flow.
The rated current limit for the device is 30 mA.
Generally, a 3-terminal voltage regulator is used for the stabilization of a power supply.
Therefore, this diode is typically used to protect the circuit from momentary voltage spikes. 3
terminal regulators use voltage regulation diodes inside.

14


Diode bridge

Rectification diodes are used to make DC
from AC. It is possible to do only 'half wave
rectification' using 1 diode. When 4 diodes
are combined, 'full wave rectification'
occurrs.
Devices that combine 4 diodes in one
package are called diode bridges. They are
used for full-wave rectification.

The photograph on the left shows two examples of diode bridges.

The cylindrical device on the right in the photograph has a current limit of 1A. Physically, it is 7
mm high, and 10 mm in diameter.
The flat device on the left has a current limit of 4A. It is has a thickness of 6 mm, is 16 mm in
height, and 19 mm in width.

15


The photograph on the right
shows a large, high-power diode bridge.
It has a current capacity of 15A. The peak reverse-bias voltage is 400V.
Diode bridges with large current capacities like this one, require a heat sink. Typically, they are
screwed to a piece of metal, or the chasis of device in which they are used. The heat sink
allows the device to radiate excess heat.
As for size, this one is 26 mm wide on each side, and the height of the module part is 10 mm.

Light Emitting Diode ( LED )

Light emitting diodes must be choosen according to how
they will be used, because there are various kinds.
The diodes are available in several colors. The most
common colors are red and green, but there are even blue
ones.

The device on the far right in the photograph combines a red
LED and green LED in one package. The component lead in the middle is common to both
LEDs. As for the remaing two leads, one side is for the green, the other for the red LED. When
both are turned on simultaneously, it becomes orange.

When an LED is new out of the package, the polarity of the device can be determined by
looking at the leads. The longer lead is the Anode side, and the short one is the Cathode side.

The polarity of an LED can also be determined using a resistance meter, or even a 1.5 V
battery.

When using a test meter to determine polarity, set the meter to a low resistance measurement
range. Connect the probes of the meter to the LED. If the polarity is correct, the LED will glow.
If the LED does not glow, switch the meter probes to the opposite leads on the LED. In either
case, the side of the diode which is connected to the black meter probe when the LED glows, is
the Anode side. Positive voltage flows out of the black probe when the meter is set to measure
resistance.

It is possible to use an LED to obtain a fixed voltage.
The voltage drop (forward voltage, or VF) of an LED is comparatively stable at just about 2V.

16

I explain a circuit in which the voltage was stabilized with an LED in
quot;Thermometer of bending apparatus-2quot;.

Shottky barrier diode

Diodes are used to rectify alternating current into direct current. However, rectification will not
occur when the frequency of the alternating current is too high. This is due to what is known as
the quot;reverse recovery characteristic.quot;
The reverse recovery characteristic can be explained as follows:
IF the opposite voltage is suddenly applied to a forward-biased diode, current will continue to
flow in the forward direction for a brief moment. This time until the current stops flowing is called
the Reverse Recovery Time. The current is considered to be stopped when it falls to about 10%
of the value of the peak reverse current.
The Shottky barrier diode has a short reverse recovery time, which makes it ideally suited to
use in high frequency rectification.

The shottky barrier diode has the following characteristics.
The voltage drop in the forward direction is low.
The reverse recovery time is short.
However, it has the following disadvantages.
The diode can have relatively high leakage current.
The surge resistance is low.
Because the reverse recovery time is short, this diode is often used for the switching regulator
in a high frequency circuit.

BJT TRNASISTOR

A bipolar junction transistor, (BJT) is very versatile. It can be used in many ways, as an amplifier,
a switch or an oscillator and many other uses too. Before an input signal is applied its operating
conditions need to be set. This is achieved with a suitable bias circuit, some of which I will
describe. A bias circuit allows the operating conditions of a transistor to be defined, so that it will
operate over a pre-determined range. This is normally achieved by applying a small fixed dc
voltage to the input terminals of a transistor.

Bias design can take a mathematical approach or can be simplified using transistor characteristic
curves. The characteristic curves predict the performance of a BJT. There are three curves, an
input characteristic curve, a transfer characteristic curve and an output characteristic curve. Of
these curves, the most useful for amplifier design is the output characteristics curve. The output

17

characteristic curves for a BJT are a graph displaying the output voltages and currents for
different input currents. The linear

(straight) part of the curve needs is utilized for an amplifier or oscillator. For use as a
switch, a transistor is biased at the extremities of the graph, these conditions are known as quot;cut-
offquot; and quot;saturationquot;.

Output Characteristic Curves
For each transistor configuration, common emitter, common base and emitter follower the output
curves are slightly different. A typical output characteristic for a BJT in common emitter mode are
shown below :-

After the initial bend, the curves approximate a straight line. The slope or gradient of each line
represents the output impedance, for a particular input base current. So what has all this got to do
with biasing ? Take, for example the middle curve. The collector emitter voltage is displayed up to
20 volts. Let's assume that we have a single stage amplifier, working in common emitter mode,
and the supply voltage is 10 volts. The output terminal is the collector, the input is the base,
where do you set the bias conditions? The answer is anywhere on the flat part of the graph.
However, imagine the bias is set so that the collector voltage is 2 volts. What happens if the
output signal is 4 volts peak to peak ? Depending on whether the transistor used is a PNP or
NPN, then one half cycle will be amplified cleanly, the other cycle will approach the limits of the
power supply and will quot;clipquot;. This is shown below :

The above diagram shows a 4 volt peak to peak waveform with clipping on the positive half cycle.
This is caused by setting the bias at a value other than half the supply voltage.

18


The lower diagram shows the same amplifier, but here the bias is set so that collector voltage is
half the value of the supply voltage. Hence, it is a good idea to set the bias for a single stage
amplifier to half the supply voltage, as this allows maximum output voltage swing in both
directions of an output waveform.

Input Characteristic Curves
Before describing the bias circuits, it is worthwhile looking at a typical input characteristic curve
for a small signal BJT. The following is the input characteristic for a transistor in common emitter
mode, it is a plot of input base emitter voltage verses base current. It is shown with both x and y
axis slightly zoomed.

The base emitter voltage, Vbe is quoted in most text books as either 0.6 V or 0.7 V Both values
are an approximation, and as can be seen from the above graph the value of Vbe varies between
this range. For small signal work with base currents of 50uA or below a value for Vbe of 0.6 volts
is a reasonable quote. For higher base currents, a Vbe of 0.7 V is a better approximation. In fact,
in a large power transistor, the Vbe value can be even higher. The value of Vbe also varies widely
with temperature change.

Simple Bias Circuit
The simplest bias circuit is shown below. It consists only of a fixed bias resistor and load resistor.
The BJT is operating in common emitter mode. The dc current gain or beta, hFE is the ratio of dc
collector current divided by dc base current. The BJT is a BC107A. The values of Rb and Rc can
be determined by either mathematical approach or by using the output characteristic curves for
the BC107A.

19


Quiescent Point (Q-Point)
The point Vo in the diagram above is where the output signal would be taken. For simplicity, the
input signal and coupling capacitors have been omitted. For minimum distortion and clipping it is
desirable to bias this point to half the supply voltage, 10 volts dc in this case. This is also known
as the quiescent point. The ac output signal would then be superimposed on the dc bias voltage.
The Q-point is sometimes indicated on the output characteristics curves for a transistor amplifier.
The quiescent point also refers to the dc conditions (bias conditions) of a circuit without an input
signal.

Q-Point Value
I have mentioned that setting the Q-point to half the supply voltage is a good idea. It gives a
circuit the highest margin for overload. However, any amplifier will clip if the input amplitude
exceeds the limit for which the circuit was designed. However, there are certain cases when it is
not necessary to bias a stage to half the supply voltage. Examples would be an RF amplifier
design where the input signal is in microvolts or millivolts. If the stage had a gain of 200 then the
output (assuming a 2mV peak input) would only need to swing up and down 400mV about the Q-
point. Hence a stage with a supply voltage of 12 volts could have its Q-point set at 10 volts or
even 2 volts without problems. Another example would be a microphone stage where similar low
level input signals are involved.

Output Characteristic Curve for a BC107A)

20


Function Field Effect Transistor
The common transistor is called a junction transistor,
and it was the key device which led to the solid state
electronics revolution. In application, the junction
transistor has the disadvantage of a low input
impedance because the base of the transistor is the
signal input and the base-emitter diode is forward
biased. Another device achieved transistor action with
the input diode junction reversed biased, and this
device is called a quot;field effect transistorquot; or a quot;junction
field effect transistorquot;, JFET. With the reverse biased
input junction, it has a very high input impedance.
Having a high input impedance minimizes the
interference with or quot;loadingquot; of the signal source when
a measurement is made.
For an n-channel FET, the device is constructed from a bar
of n-type material, with the shaded areas composed of a p-
type material as a Gate. Between the Source and the Drain,
the n-type material acts as a resistor. The current flow
consists of the majority carriers (electrons for n-type
material).

Characteristic curves
Common source amplifier

Since the Gate junction is reverse biased and because there is no minority carrier contribution to
the flow through the device, the input impedance is extremely high.

The control element for the JFET comes from depletion of charge carriers from the n-channel.
When the Gate is made more negative, it depletes the majority carriers from a larger depletion
zone around the gate. This reduces the current flow for a given value of Source-to-Drain voltage.
Modulating the Gate voltage modulates the current flow through the device.

he insulated-gate field-effect transistor (IGFET), also known as the metal oxide field effect
transistor (MOSFET), is a derivative of the field effect transistor (FET). Today, most transistors
are of the MOSFET type as components of digital integrated circuits. Though discrete BJT's are
more numerous than discrete MOSFET's. The MOSFET transistor count within an integrated
circuit may approach the hundreds of a million. The dimensions of individual MOSFET devices
are under a micron, decreasing every 18 months. Much larger MOSFET's are capable of
switching nearly 100 amperes of current at low voltages; some handle nearly 1000 V at lower
currents. These devices occupy a good fraction of a square centimeter of silicon. MOSFET's find
much wider application than JFET's. However, MOSFET power devices are not as widely used
as bipolar junction transistors at this time.

The MOSFET has source, gate, and drain terminals like the FET. However, the gate lead does
not make a direct connection to the silicon compared with the case for the FET. The MOSFET
gate is a metallic or polysilicon layer atop a silicon dioxide insulator. The gate bears a
resemblance to a metal oxide semiconductor (MOS) capacitor in Figure below. When charged,
the plates of the capacitor take on the charge polarity of the respective battery terminals. The

21

lower plate is P-type silicon from which electrons are repelled by the negative (-) battery terminal
toward the oxide, and attracted by the positive (+) top plate.. This excess of electrons near the
oxide creates an inverted (excess of electrons) channel under the oxide. This channel is also
accompanied by a depletion region isolating the channel from the bulk silicon substrate.

N-channel MOS capacitor: (a) no charge, (b) charged.

In Figure below (a) the MOS capacitor is placed between a pair of N-type diffusions in a P-type
substrate. With no charge on the capacitor, no bias on the gate, the N-type diffusions, the source
and drain, remain electrically isolated.

N-channel MOSFET (enhancement type): (a) 0 V gate bias, (b) positive gate bias.

A positive bias applied to the gate, charges the capacitor (the gate). The gate atop the oxide
takes on a positive charge from the gate bias battery. The P-type substrate below the gate takes
on a negative charge. An inversion region with an excess of electrons forms below the gate
oxide. This region now connects the source and drain N-type regions, forming a continuous N-
region from source to drain. Thus, the MOSFET, like the FET is a unipolar device. One type of
charge carrier is responsible for conduction. This example is an N-channel MOSFET. Conduction
of a large current from source to drain is possible with a voltage applied between these
connections. A practical circuit would have a load in series with the drain battery in Figure above
(b).

The MOSFET described above in Figure above is known as an enhancement mode MOSFET.
The non-conducting, off, channel is turned on by enhancing the channel below the gate by
application of a bias. This is the most common kind of device. The other kind of MOSFET will not
be described here. See the Insulated-gate field-effect transistor chapter for the depletion mode
device.

The MOSFET, like the FET, is a voltage controlled device. A voltage input to the gate controls the
flow of current from source to drain. The gate does not draw a continuous current. Though, the
gate draws a surge of current to charge the gate capacitance.

22

The cross-section of an N-channel discrete MOSFET is shown in Figure below (a). Discrete
+
devices are usually optimized for high power switching. The N indicates that the source and
drain are heavily N-type doped. This minimizes resistive losses in the high current path from
-
source to drain. The N indicates light doping. The P-region under the gate, between source and
drain can be inverted by application of a positive bias voltage. The doping profile is a cross-
section, which may be laid out in a serpentine pattern on the silicon die. This greatly increases
the area, and consequently, the current handling ability.

N-channel MOSFET (enhancement type): (a) Cross-section, (b) schematic symbol.

The MOSFET schematic symbol in Figure above (b) shows a “floating” gate, indicating no direct
connection to the silicon substrate. The broken line from source to drain indicates that this device
is off, not conducting, with zero bias on the gate. A normally “off” MOSFET is an enhancement
mode device. The channel must be enhanced by application of a bias to the gate for conduction.
The “pointing” end of the substrate arrow corresponds to P-type material, which points toward an
N-type channel, the “non-pointing” end. This is the symbol for an N-channel MOSFET. The arrow
points in the opposite direction for a P-channel device (not shown). MOSFET's are four terminal
devices: source, gate, drain, and substrate. The substrate is connected to the source in discrete
MOSFET's, making the packaged part a three terminal device. MOSFET's, that are part of an
integrated circuit, have the substrate common to all devices, unless purposely isolated. This
common connection may be bonded out of the die for connection to a ground or power supply
bias voltage.

N-channel “V-MOS” transistor: (a) Cross-section, (b) schematic symbol.

The V-MOS device in (Figure above) is an improved power MOSFET with the doping profile
arranged for lower on-state source to drain resistance. VMOS takes its name from the V-shaped
gate region, which increases the cross-section area of the source-drain path. This minimizes
losses and allows switching of higher levels of power. UMOS, a variation using a U-shaped
grove, is more reproducible in manufacture.

23

The Junction Field Effect Transistor
We saw previously that a bipolar junction transistor is constructed using two PN junctions in the
main current path between the Emitter and the Collector terminals. The Field Effect Transistor
has no junctions but instead has a narrow quot;Channelquot; of N-type or P-type silicon with electrical
connections at either end commonly called the DRAIN and the SOURCE respectively. Both P-
channel and N-channel FET's are available. Within this channel there is a third connection which
is called the GATE and this can also be a P or N-type material forming a PN junction and these
connections are compared below.

Bipolar Transistor Field Effect Transistor
Emitter - (E) Source - (S)
Base - (B) Gate - (G)
Collector - (C) Drain - (D)

The semiconductor quot;Channelquot; of the Junction Field Effect Transistor is a resistive path through which a
voltage Vds causes a current Id to flow. A voltage gradient is thus formed down the length of the channel with
this voltage becoming less positive as we go from the drain terminal to the source terminal. The PN junction
therefore has a high reverse bias at the drain terminal and a lower reverse bias at the source terminal. This
bias causes a quot;depletion layerquot; to be formed within the channel and whose width increases with the bias.
FET's control the current flow through them between the drain and source terminals

by controlling the voltage applied to the gate terminal. In an N-channel JFET this gate voltage is negative
while for a P-channel JFET the gate voltage is positive.

Bias arrangement for an N-channel JFET and corresponding circuit
symbols.

24

The cross sectional diagram above shows an N-type semiconductor channel with a P-type region
called the gate diffused into the N-type channel forming a reverse biased PN junction and its this
junction which forms the depletion layer around the gate area. This depletion layer restricts the
current flow through the channel by reducing its effective width and thus increasing the overall
resistance of the channel.

When the gate voltage Vg is equal to 0V and a small external voltage (Vds) is applied between the
drain and the source maximum current (Id) will flow through the channel slightly restricted by the
small depletion layer. If a negative voltage (Vgs) is now applied to the gate the size of the
depletion layer begins to increase reducing the overall effective area of the channel and thus
reducing the current flowing through it, a sort of quot;squeezingquot; effect. As the gate voltage (Vgs) is
made more negative, the width of the channel decreases until no more current flows between the
drain and the source and the FET is said to be quot;pinched-offquot;. In this pinch-off region the gate
voltage, Vgs controls the channel current and Vds has little or no effect. The result is that the FET
acts more like a voltage controlled resistor which has zero resistance when Vgs = 0 and maximum
quot;ONquot; resistance (Rds) when the gate voltage is very negative.

Output characteristic voltage-current curves of a typical junction
FET.

The voltage Vgs applied to the gate controls the current flowing between the drain and the source
terminals. Vgs refers to the voltage applied between the gate and the source while Vds refers to
the voltage applied between the drain and the source. Because a Field Effect Transistor is a
VOLTAGE controlled device, quot;NO current flows into the gate!quot; then the source current (Is) flowing
out of the device equals the drain current flowing into it and therefore (Id = Is).

The characteristics curves example shown above, shows the four different regions of operation
for a JFET and these are given as:

Ohmic Region - The depletion layer of the channel is very small and the JFET acts like a
•
variable resistor.
•
Cut-off Region - The gate voltage is sufficient to cause the JFET to act as an open circuit
•
as the channel resistance is at maximum.
•
Saturation or Active Region - The JFET becomes a good conductor and is controlled by
•
the gate-source voltage, (Vgs) while the drain-source voltage, (Vds) has little or no effect.
•
Breakdown Region - The voltage between the drain and source, (Vds) is high enough to
•
causes the JFET's resistive channel to break down and pass current.

25

What is CMOS?

Complementary metal-oxide semiconductor, or CMOS, typically refers to a battery-powered
memory chip in your computer that stores startup information. Your computer's basic input/output
system (BIOS) uses this information when starting your computer.

CMOS-related error messages could be caused by a faulty or discharged battery. The battery can
become discharged if your computer has been turned off for a very long time. To resolve CMOS-
related errors, check the information that came with your computer or contact your computer
manufacturer. Because your CMOS settings are specific to your computer's hardware, Microsoft
cannot provide specific instructions for changing them.

26

I.C ASSIGNMENT #1: INDUCTOR AND CAPACITOR

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