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Basics about Circuits

Basic Electric Circuit Concepts

System of Units:

We use the SI (System International) units. The system uses meters (m),

kilograms (kg), seconds (s), ampere (A), Resistance (? ) & Volts (V)

as the fundamental units.

We use the following prefixes:

pica (p): 10-12

nano (n): 10-9

micro (): 10-6

milli (m): 10-3

tera (T): 1012

giga (G) : 109

mega (M): 106

kilo (k): 103

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Basic Quantities: Current

The unit of current is the ampere (A). We note that

1 ampere = 1 coulomb/second

We normally refer to current as being either direct (dc) or

alternating (ac).

i(t)i(t)

t t

dc currentac current

0 0.5 1 1 . 5 2 2 .5 3 3.5-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

ac current3


Basic Quantities: Current

In solving for current in a circuit, we must assume a direction, solve

for the current, then reconcile our answer. This is illustrated below.

Circuit 1 Circuit 2

(a) (b)

I1 = 4 A I2= - 3 A

In the diagram above, current I1 is actually 4 A as assumed. The

actual positive direction of current I2 (equal to -3 A) in the opposite

direction of the arrow for I2.

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Basic Quantities: Voltage

The next quantity of interest is voltage. Voltage is also called an

electromotive force (emf). It is also called potential energy.

Suppose one coulomb of charge is located at point b and one joule

of energy is required to move the charge to point a. Then we say

that Vab = 1 volt = 1 joule/coulomb = 1 newton.meter/coulomb.

Vab = 1 volt states that the potential of point a (voltage at point a)

is l volt (positive) with respect to point b.

The sign associated with a voltage is also called its polarity.

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Basic Quantities: Voltage

As in the case for current, we must assume a positive direction (polarity) for

the voltage. Consider the three diagrams below.

.+

-

.

v = 4 v

a

b

vab = 4 v v = 4 v

(a) (b) (c)

Each of the above gives the same information.6


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Basic Quantities:Voltage

We need to keep in mind that we assume a polarity for the

voltage. When we solve the circuit for the voltage, we may find

that the actual polarity is not the polarity we assumed.

+

-

v = -6 v

The negative sign for 6 v indicates that if

the red lead(probe) of a voltmeter is

placed on + terminal and the black

lead(probe) on the terminal the meter

will readdownscale or6v.

Adigital meter would read 6 v.7


Circuit Elements:

We classify circuit elements as passive and active.

Passive elements cannot generate energy. Common examples

of passive elements are resistors, capacitors and inductors. We

will see later than capacitors and inductors can store energy

but cannot generate energy.

Active elements can generate energy. Common examples of

active elements are power supplies, batteries, operational

amplifiers.

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circuits

Fundamentals

Introduction to Electronics

(Semiconductor Devices)

Why Semiconductors?


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Ge has 32 orbiting electrons andneutrons and protons in Nucleus

Bohr Atomic Model of Ge

Nucleus

1st shell electrons

2nd shell electrons

3rd shell electrons

4th and Valence shell electrons

Bohr Atomic Model of Si

Si has 14 orbiting electrons andneutrons and protons in Nucleus

Nucleus

1st shell electrons

2nd shell electrons

3rd shell and Valence shell

electrons


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E

E

The characteristics of a semiconductor

material can be altered significantly bythe addition of certain impurity atoms

into the relatively pure semiconductor

material.

Terms: Doping, impurity, intrinsic

semiconductor

A semiconductor material that has been subjected to the

doping process is called as an Extrinsic

Materials


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Intrinsic Semiconductor Materials

Free electrons in an intrinsic semiconductorare only due to natural causes (photon lightand thermal heating). A free electron willhave its complementary hole. Electrons andholes due to only natural causes are alsocalled as intrinsic carriers.

Extrinsic Semiconductor

Base Material

(Si and Ge)

Impurity Pentavalentor Trivalent

Base Material+Pentavalent impurity n-typesemiconductor

Base Material+Trivalent impurity p-typesemiconductor


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Trivalent Impurity (p-type impurity)

Boron (B), Gallium (Ga) and Indium:

have 3 electrons in valence cellAcceptor impurity

Pentavalent Impurity(n-type impurity)

Antimony (Sb), Arsenic (As) and

Phosporous (P) : have 5 electron in valence

shell Donar Impurity

P NJunction

Doping a crystal with both types of impurities forms a

P-N junction diode.

Some electrons will cross the junction and fill holes.A pair of ions is created each time this happens.

Negativeion

Positiveion

As this ion charge builds up, it prevents furthercharge migration across the junction.


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P N

Each electron that migrates across the

junction and fills a hole effectivelyeliminates both as current carriers.

This results in a region at the junction that isdepleted of carriers and acts as an insulator.

Depletion layer

Energy

Abrupt junction

P-side

Valence band

Conduction band

N-side

In an abrupt junction, the p side bandsare at a slightly higher energy level.


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Energy

P-side

Valence band

N-side

Conduction band

Energy bands after the depletion layer has forme

To an electron trying to diffuse across the junctiothe path it must travel looks like an energy hill. It

must receive the extra energy from an outside sour

Energy hill


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Diode Types

Rectifier Diodes

Light Emitting Diodes (LED)

Photodiodes

Zener Diodes, etc

Diode Applications

Against Reverse Voltage Protection

Rectifying {AC DC }

Display (indicators, Advertise hoardings)

Light Sensing (Solar panals etc)


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Light Emitting Diode (LED) circuit How to connect LED

A

K

V s

VR

R

V

ILED

Expected ILED = 20mA,

Assuming LED has V =1.5V.

Find the value of R, if the source

voltage 5 volts.

LED

S

I

VVR

=

7 Segment Display Consist of 7+1 LEDs Available in :

Commond Anode configuration

Commond Cathode configuration


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7 Segment Display Commond anode and Commond

Cathode configuration


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7 Segment Display

Commond anode and CommondCathode configuration

7 Segment Display Application: Logic Display


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Classification of TransistorsTRANSISTORS

Bipolar Junction

Transistors

(BJT)

Field Effect

Transistors

(FET)

NPN - B JT PNP - B JTJunction

FET (JFET)

Metal Oxyde

Semiconductor FET

(MOSFET)

N-Channel

JFET

P-Channel

JFET

DepletionMode

MOSFET

EnhancedMode

MOSFET

N-Channel

D-MOSFET

P-Channel

D-MOSFET

N-Channel

E-MOSFET

P-Channel

E-MOSFET

A bipolar (junction) transistor (BJT) is athree-terminal electronic device constructed

of doped semiconductor material and may

be used in amplifying or switchingapplications. Bipolar transistors are so

named because their operation involves

both electrons and holes.


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Classification of Bipolar Junction

Transistors

Base

Collector

Emitter

IC

IB

IE

VCE

VBE

Base

Collector

Emitter

IC

IB

IE

VCE

VBE

Bipolar Junction Transistor (BJT)

NPN Transistor PNP Transistor

ass ca on o po ar unc onTransistors

Base

Collector

Emitter

IC

IB

IE

V

CE

VBE

NPN Transistor

Current Gain : or hFE

Then

And

So

B

C

I

I =

BCII =

CBE

III +=

( )II BE += 1


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NPN

NPN is one of the two types of bipolar

transistors, consisting of a layer of P-dopedsemiconductor (the "base") between two N-doped layers.

A small current entering the base is

amplified to produce a large collector andemitter current.

That is, an NPN transistor is ON" when its

base is pulled high relative to the emitter.

PNP

The other type of BJT is the PNP, consisting

of a layer of N-doped semiconductorbetween two layers of P-doped material.

A small current leaving the base is

amplified in the collector output.

That is, a PNP transistor is "on" when itsbase is pulled low relative to the emitter.


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Applications of Transistors

1. Darlington transistor (often called aDarlington pair) is a compound structureconsisting of two bipolar transistors (eitherintegrated or separated devices) connectedin such a way that the current amplified by

the first transistor is amplified further by thesecondone

2. Transistor As a Switch

An electronic amplifier, in which the input " signal" is

usually a voltage or a current. In audio applications,

amplifiers drive the loudspeakers used in PA systemsto make the human voice louder or play recorded

music.

Amplifiers may be classified according to the input

(source) they are designed to amplify (such as a guitar

amplifier, to perform with an electric guitar), the

device they are intended to drive (such as a headphone

amplifier), the frequency range of the signals (Audio,

IF, RF, and VHF amplifiers, for example), whether

they invert the signal (inverting amplifiers and non-inverting amplifiers), or the type of device used in the

amplificat ion (valve or t ube am plifier s, F ET

amplifiers, etc.).


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If for a small change in input voltage, a proportional

large change in output voltage is obtained, then we say

that, voltage amplification has taken place

Biasing in Transistor


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In order to use transistor as an amplifier, it must be operated in its

active region. The biasing of the PNP and NPN transistor for theiractive region operation and the directions of the currents are


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The field-effect transistor(FET), sometimes called aunipolar transistor, uses either electrons (in N-

channel FET) or holes (in P-channel FET) forconduction.

In FETs, the drain-to-source current flows via aconducting channel that connects the sourceregionto the drainregion.

The conductivity is varied by the electric field that is

produced when a voltage is applied between the gateand source terminals; hence the current flowingbetween the drain and source is controlled by thevoltage applied between the gate and source.

N-CHANNELP-CHANNEL

The four terminals of the FET are named source, gate,

drain, andbody(substrate).


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Operation of N-channel with VGS =0

Due to the supply voltage VDS , current starts flowingthrough the channel.

Therefore current flow through the channel get voltageacross the channel.

This voltage will Reverse BIAS the G to S p-n junction


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