Lecture-1

CHAPTER 1- Circuit Concepts:

Electrical Quantities :

- Charge and Electric Force: A charge is responsible for an electric field and charges exert forces on each other. Like charges repel, whereas unlike charges attract. Such an electric force can be controlled and utilized for some useful purpose. Coulomb’s law gives an expression to evaluate the electric force in newtons (N) exerted on one point charge by the other

where Q1 and Q2 are the point charges (C); R is the separation in meters (m) between them; ε0is the permittivity of the free-space medium with units of C2/N · m or, more commonly, faradsper meter (F/m); and ¯a21 and ¯a12 are unit vectors along the line joining Q1 and Q2.

- Conductors and Insulators: Materials through which charge flows readily are called conductors. Examples include most metals, such as silver, gold, copper, and aluminum.- Insulators are materials that do not allow charge to move easily. Examples include glass, plastic, ceramics, and rubber. Electric current cannot be made to flow through an insulator, since a charge has great difficulty moving through it.

- Current and Magnetic Force: The rate of movement of net positive charge per unit of time through a cross section of a conductor is known as current,

-Ampere’s law of force is concerned with magnetic forces associated with two loops of wire carrying currents by virtue of the motion of charges in the loops

.

Illustration of Ampere’s law (of force).

Lecture-2 Atomic Energy level:

The range of energies possessed by valence electrons is called valence band. The range of energies possessed by free electrons is called conduction band. Valence band and conduction band are separated by an energy gap in which no electrons

normally exist this gap is called forbidden gap.

Conductors: In terms of energy bands, conductors are those substances in which there is no forbidden gap. Valence and conduction band overlap

Insulators:-In terms of energy bands, insulators are those substances in which the forbidden gap is very large.

Semiconductors:-Semiconductors are those substances whose conductivity lies in between that of a conductor and Insulator.

Example: Silicon, germanium, Gallium, arsenide etc

Pauli Exclusion Principle: no two electrons may occupy the same quantum state.

Lecture-3

Chapter 2:-Transport Phenomena in Semiconductors:

Drift current

Diffusion current The directional movement of charge carriers due to their concentration gradient produces

a component of current known as Diffusion current.

Cureent Density:- J=I/A=n.q.u.E

Conduction in hole and semiconductor:

Lecture- 4 Classification of semiconductors

Semiconductors are classified into two types.

a) Intrinsic semiconductors.A semiconductor in an extremely pure form is known as Intrinsic semiconductor.

b) Extrinsic semiconductors.This process of adding impurity to a semiconductor is called Doping and the impure semiconductor is called extrinsic semiconductor.

N-type(donor) P-type(acceptor)

Phosphorus, Arsenic Boron, Aluminium

Lecture - 5 Charge Densities in Semiconductor:

nn=Nd

pn=ni^2 /Nd

Electrical Properties of Ge and Si1.Conductivity-Semiconductors are semi-good electrical conductors because

although their valence band is completely filled, the energy gap between the valance band and the conduction band is not too large. Hence some electrons can bridge it to become charge carriers. The difference between a semiconductors and an insulator is the magnitude of the energy gap. For semiconductors Eg < 2eV and for Insulators Eg > 2eV.

2.Energy gap:-

3.Mobility- The electron mobility characterizes how quickly an electron can move through a metal or semiconductor, when pulled by an electric field. In

semiconductors, there is an analogous quantity for holes, called hole mobility. The term carrier mobility refers in general to both electron

and hole mobility in semiconductors.

Conductivity of extrinsic SemiconductorTotal= hole conductivity+ Electron conductivity

Lecture- 6

HALL EFFECT-If a piece of metal or semiconductor carrying a current I is placed in a transverse magnetic field B then an electric field E is induced in the direction perpendicular to both I and B. This phenomenon is known as Hall effect.

Hall voltage VH is given by

The Hall coefficient is defined as

Applications of Hall effectHall effect is used to determine,

carrier concentration, conductivity and mobility. The sign of the current carrying charge. Charge density. It is used as magnetic field meter.

Lecture- 7 Einstein's Relationship:- at fixed temprature the ration of diffusion constant to

mobility is constatnt

Volt equivalent temprature(Vt) Vt=kT

Continuity Equation:It is based on fact that charge can neither be created nor be destroyed. A continuity equation in physics is an equation that describes the transport of a conserved quantity. Since mass, energy, momentum, electric charge and other natural quantities are conserved under their respective appropriate conditions, a variety of physical phenomena may be described using continuity equations.

Potential Variation within a Graded Semiconductor:

V21=Vt* ln(p1/p2);

Lecture- 8

Ch-3 Junction –Diode Characteristics: Open –Circuited p-n Junction

Holes begin to migrate (diffuse) across the junction from the anode to the cathode.

Free electrons begin to migrate (diffuse) across the junction from the cathode to the anode.

Using the Lorentz force equation, we find that the forcevector + F on a hole (with charge Q e + = − ) located at position r is:F+ = Q+ (r )

Lecture- 9 Forward biasing: connect +ve terminal of the battery to p-type and –ve terminal to n-type

Reverse biasing :connect –ve terminal of the battery to p-type and +ve terminal to n-type. only reverse saturation current flow

Lecture- 10 Volt-Ampere Characteristic:

Lecture- 11 Diode Resistance:

DCorStatic Resistance:

Rf=Vf/If

AC or dynamic Resistance

Rf=(V2-V1)/(I2-I1)

Diode current equationI = I0( e V/ηV

T –1)

Diode Model: Ideal Diode characteristics

Piece-Wise Linear Model

Lecture- 12 Diode Capacitance:

Transition Capacitance(CT)- Reverse Bias Because positive ions exist on one side of the junction and negative ions on the

other, the transition capacitance, CT is analogous to a parallel plate capacitor Ct=E*A/W

A= Junction areaε= Permittivity of the semiconductor

The transition capacitance represents the change incharge stored in the depletion region with respect to a change in junction voltage.

Diffusion Capacitance(Cd):- When a p-n junction is forward biased, capacitance, which is much larger than transition capacitance, is evident. This is diffusion capacitance . For a forward bias region, the depletion region and hence potential barrier reduces. Now from n-side enter p-side and holes from p-side enter n-side. These charge carriers diffuse away from the junction and progressively recombine. The density of charge carriers is higher near the junction when forward biased. Thus charge is stored on both sides of the junction and decays exponentially with distance i.e. Diffusion Capacitance , which is observed that the amount of stored charge varies with the applied voltage as for a capacitor.

, Cd=TI/nVt

Lecture- 13 Diode as Switch:

Forward Bias- ON Reverse Bisa - OFF

Junction Diode Switching Times:- Forward Recovery time: Reverse to Froward Reverse Recovery Time: Forward to Reverse

Diode Break Down

Zener breakdownIn Zener breakdown the electrostatic attraction between the negative electrons and a large positive voltage is so great that it pulls electrons out of their covalent bonds and away from their parent atoms. ie Electrons are transferred from the valence to the conduction band. In this situation the current can still be limited by the limited number of free electrons produced by the applied voltage so it is possible to cause Zener breakdown without damaging the semiconductor.

Avalanche breakdownAvalanche breakdown occurs when the applied voltage is so large that electrons that are pulled from their covalent bonds are accelerated to great velocities. These electrons collide with the silicon atoms and knock off more electrons. These electrons are then also accelerated and subsequently collide with other atoms. Each collision produces more electrons which leads to more collisions etc. The current in the semiconductor rapidly increases and the material can quickly be destroyed.

Lecture- 14

Avalanche Effect:- Higer, Carrier Multiplication, not Sharp,in Lightly Doped

Zener Effect:- Lower, High field, Very sharp, Heavily Doped

Tunnel Diode: Tunnel diode is the p-n junction device that exhibits negative resistance. That means when the voltage is increased the current through it decreases.

Zener Diode:- Zener diodes are the diodes which are designed to operate in the breakdown region. They are also called as Breakdown diode or Avalanche diodes.

Lecture- 15

Photo Diode: A photodiode is a type of photo detector capable of converting light into either current or voltage, depending upon the mode of operation. The common, traditional solar cell used to generate electric solar power is a large area photodiode.

Characteristic:

Lecture- 16

Light Emitting Diode(LED):

Principle Of Operation:-

Lecture- 17

Clipper Circuit:

In electronics, a clipper is a device designed to prevent the output of a circuit from exceeding a predetermined voltage level without distorting the remaining part of the applied waveform.

Series Clippers, where the diode is in series with the load resistance,

Shunt Clippers, where the diode in shunted across the load resistance.

Lecture- 18

Positively Biased Diode Clipper Negatively Biased Diode Clipper

Cliiping At Two independent Level

Lecture- 19

Sampling Gate:

A circuit that produces an output only when first activated by a preliminary pulse.

Lecture- 20

RECTIFIERS:- Rectifiers are the circuit which converts ac to dc

Rectifiers are grouped into two categories depending on the period of conductions.

1. Half-wave rectifier2. Full- wave rectifier

Half-wave rectifier

Full-wave rectifier

1. Centre tapped full-wave rectifier2. Bridge rectifier

1. Centre tapped full-wave rectifier

Lecture- 21 Bridge rectifier

Lecture- 22

Types of Filters1. Capacitor Filter (C-Filter)2. Inductor Filter3. Choke Input Filter (LC-filter)4. Capacitor Input Filter (Π-filter)

Capacitor Filter:

Lecture- 23

CHPTER-5: Transistor Characteristics

A transistor is a sandwich of one type of semiconductor (P-type or n-type) between two layers of other types.

Transistors are classified into two types;

1. pnp transistor 2. npn transisitor

IE = IPE +INE.

IC = IPC +ICO

Lecture- 24

TRANSISTOR CONFIGURATION

This gives rise to three different combinations.

1. Common base configuration (CB)2. Common emitter configuration (CE)3. Common collector configuration (CC)

1. Common base configuration (CB)

current amplification factor (α)

α =

IO

I E

Lecture- 25

CE configuration

Current amplification factor (β)

β = IC / IB

Ic = βI B+(1+β )I CBO

Lecture- 26

3. CC configuration

1. Current amplification factor (γ)

γ =

I E

I B

IE = γ (IB + ICBO)

γ= 11−α

Lecture- 27Transistor as an amplifier

A transistor raises the strength of a weak input signal and thus acts as an amplifier. The weak signal to be amplified is applied between emitter and base and the output is taken across the load resistor RC connected in the collector circuit.

Lecture- 28

Phototransistor symbol

The circuit symbol also has the convention arrow and directions on the emitter connection. It points inwards on a PNP phototransistor circuit symbol and outwards on an NPN phototransistor symbol.

Phototransistor circuit configurationsThe common emitter phototransistor circuit configuration is possibly the most widely used, like its more conventional straight transistor circuit. The collector is taken to the supply voltage via a collector load resistor, and the output is taken from the collector connection on the phototransistor. The circuit generates an output that moves from a high voltage state to a low voltage state when light is detected.

Common emitter phototransistor circuit Common collector phototransistor circuit

Lecture- 29CH-6 The Transistor At Low frequencies

Graphical Analysis of CE configuration

iC

vBEVss iB

vCE

RL

VCC

RBB

Collector voltage vCE, V

Collector current iC, mA

Consider a CE amplifier as shown below

VCC and VBB provide the biasing for the transistor RL is the load resistor RS is the source resistance

The input and output characteristics are shown below

Lecture- 30THE TRANSISTOR HYBRID MODEL:

Consider the amplifier circuit as shown in the fig. A common emitter configuration is considered. The transistor can be considered as a linear device in the active region for small signal amplitude. Then, the two-port network theory can be applied to the transistor. According

IB3

IB2

to this theory, the four variables of the transistor can be related by the following set of equations. For defining h-parameters, the quantities i b (input current) and vce (output voltage) are taken as independent variables and the remaining two quantities are represented in terms of independent variables

Vobe= f1(ib , v ce )

ic = f2(ib ,vce )

Lecture- 31

All the terms shown in the parenthesis which are the co-efficients of the equations also called transistor parameter. The above equations are rewritten as

vbe = hie ib + hre vce --------------------------------(1)

ic = hfe ib + hoe vce---------------------------------- (2)

Lecture- 32

Conversion Formulas for the Parameters of Three Transistor Configurations

Lecture- 33

Apply current divider rule to the output circuit

Ai=output currentinput current

Ai=iLib

−−−−(1) Av=A i

RL

Ri

iL=−h fe ib

1hoe

RL+1hoe

iL=−h fe

1+hoe RL

from−eqn(1 )

Ai=−h fe

1+hoe RL

RO= 1−h fehre

hie+hoe

Lecture- 34

AC equivalent circuit

Lecture- 35HIGH INPUT RESISTANCE AMPLIFIERS.

EMITTER FOLLOWER Draw the circuit of emitter follower. Find ac performance quantities. What are the

limitations of emitter follower circuit??

Take R1=100K R2 = 10K RE = 1K hie=1K hfe=100

Limitations of Emitter Follower

1. Input resistance Ri (102 KΩ) is very high for an emitter follower circuit. But when

biasing network is considered (RE), input resistance reduces. R

i|= RB || Ri . Thus the input

resistance is limited by RB , for any value of Ri . Therefore the biasing network defeats the purpose of using emitter follower.

2. Current gain of emitter follower is very high [Ai = 1+ hfe]. But due to biasing network,

the overall current gain A

i| is reduced.

A

i|=

A i RB

RB + Ri

Lecture- 36MILLER’S THEOREM:- Miller’s theorem states that when a resistance ( or capacitance ) is connected across input and output terminals, the same can be replaced by two independent

resistances ( or capacitances ) connected one across the input terminals and the other across output terminals. These are called Miller equivalent resistances ( or capacitances).

Dual of Miller’s Theorem

The purpose of the following analysis is to remove the inter dependence of input and output circuits. So that either input circuits or output circuits can be solved independently

Lecture- 37HYBRID- EQUIVALENT CIRCUIT

To obtain Hybrid- Equivalent circuit-Consider a PNP transistor as shown above. The emitter current IE is divided in to base current IB and a component IE of the collector current. This division of current takes place in the entire base layer at infinite number of points. For mathematical convenience, it is assumed that the division of current takes place at an imaginary terminal B.

Lecture- 38

CE Amplifier with an Emitter Resistance,

The CE amplifier is often constructed with an emitter resistor as shown in figure.This resistor provides a form of negative feedback that can be used to stabilize both the DC operating point and the AC gain. It can be shown that the voltage transfer function across the transistor is

If , the gain becomes independent of the hybrid parameters:

Because it is unaffected by variations in the hybrid parameters, this result is valid even for a large-amplitude signal.

Lecture- 39

BOOT STRAPPED EMITTER FOLLOWER

Lecture- 40CH-7 TRANSISTOR BIASING AND THERMAL STABILIZATION

Biasing is referred to providing passive components such as resistances or capacitances or supply voltages etc. to provide proper operating characteristics of the device (i.e. transistor)

The purpose of dc biasing is to obtain dc collector current at a certain dc collector voltage. These value of current and voltage are expressed by operating point or quiescent point or point. To obtain proper Q point, biasing circuits are required.

stability factor:

Base bias circuit

An improvement in stability is obtained if resistor as shown in figure is returned to the collector junction rather than to the battery terminal.

The physical reason that this circuit is improvement is that if tends to increase, the decreases. Hence also decreases; and as a consequence of this lowered bias current, the collector current is not allowed to increase as much as it would have been in fixed bias or normal circuit.

Lecture- 41

SELF BIAS OR EMITTER BIAS CIRCUIT

If the load resistance is very small, then collector to base biasing is of no use for lowering S.A circuit that can be used even if zero dc resistance in series with the collector terminal is the self bias configuration as shown.

The physical reason for still low S is with tends to increase say has risen because of elevated temperature, the current in decreases. As a result, increase in voltage drop across the base current is decreased. Hence will increase less.

Lecture- 42

STABILITY FACTOR AGAINST VARIATIONS IN AND

The variation of with is given by the stability factor S’ where

and both and are held constant The variation of with respect to is defined by S” given as

For self bias circuit,

Substituting above equation in

It is clear that minimizing S also minimizes S”. It means that ratio must be small. In order to keep S’ small, a large or is required. Hence, it is desirable to use large and a compromise i required for selecting .Total change in collector current over a specified temperature range can be given as

Lecture- 43Bias Compensation

Compensation techniques refer to the use of temperature sensitive devices such as diodes, transistors, thermistors etc. which provide compensating voltages and currents to maintain the

operating point constant. In comparison, stabilization technique use only resistive biasing circuits.

Due to bias circuit, feedback is there and it reduce drastically the amplification of the signal. If this loss in signal gain is intolerable in particular application, it is often possible to use compensating techniques to reduce the drift of the operating point.

Diode compensation for

Diode Compensation for :

Lecture- 44

Lecture- 45

THERMAL RUN AWAY

Fixing of a suitable operating point is not sufficient in transistors. It is also to be ensured that it remains fixed also. The transistor parameters are temperature dependent and parameter such as change from unit to unit are responsible for Q point to shift.

Flow of current in collector circuit produces heat at the collector junction. This increases the temperature. More minority carriers are generated in base collection region (since more bonds are broken). The leakage current increases. Since

The increases in will increase to increase, which in turn increases the . This further raises the temperature of the collector-base junction and whole cycle repeats again. Such cumulative increase in will ultimately shift the operating point into the saturation region. This is very dangerous. The excess heat at junction may even burn the transistor. This is known as thermal runaway.

Lecture- 46Field Effect Transistors:Junction FET,

The three terminals are called the source, drain, and gate

The voltage applied to the gate controls the current flowing in the source-drain channel.

No current flows through the gate electrode, thus the gate is essentially insulated from the source-drain channel

Because no current flows through the gate, the input impedance of the FET is extremely large (in the range of 1010to1015 Ω).

The large input impedance of the FET makes them an excellent choice for amplifier inputs

The two common families of FETs, the junction FET (JFET) and the metal oxide semiconductor FET (MOSFET) differ in the way the gate contact is made on the source-drain channel.

construction of JFET and its operation

Lecture- 47

PINCH-OFF VOLTAGE

how channel is varied in JFET by the gate voltage.

What is pinch off voltage:-When Sate-source voltage reaches the pinch off value, the channel width reduces to a constant minimum value. The flows through this constricted channel.

JFET VOLT-AMPERE CHARACTERISTICS

V-I characteristics of JFET

Drain current in the active region. Drain-Source channel resistance

Lecture- 48

FET Small-Signal Model

Transconductance and drain resistance ( ) :

Lecture- 49

The Metal Oxide FET - MOSFET

MOSFET is metal oxide semiconductor field effect transistor also known as insulated gate FET.

The n channel MOSFET consists of a lightly doped p-type substrate into which two highly doped regions. are diffused as shown.

Lecture- 50

Basic MOSFET Structure

Enhancement-mode MOSFETThe more common Enhancement-mode MOSFET is the reverse of the depletion-mode type. Here the conducting channel is lightly doped or even undoped making it non-conductive. This results in the device being normally "OFF" when the gate bias voltage is equal to zero.

Lecture- 51

LOW FREQUENCY CS AND CD AMPLIFIERS

Lecture- 52BIASING THE FET

Lecture- 53

THE FET AS A VOLTAGE VARIABLE RESISTOR

Lecture- 54CS AMPLIFIER AT HIGH FREQUENCIES

FET as a Voltage-Controlled Resistor

The region to the left of the pinch-off point is called the ohmic region.The JFET can be used as a variable resistor, where VGS controls the drain-source resistance (rd). As VGS becomes more negative, the resistance (rd) increases.

od 2

GS

P

r r = V 1 - V

FET High Frequency Model

Power Circuits and Systems:

Class A large Signal Amplifiers

An emitter follower (Q1) biased with a constant current I supplied by transistor Q2.

Lecture- 55

CLASS 'A' LARGE SIGNAL AMPLIFIERS

Lecture- 56

Transfer Characteristics

Transfer characteristic of the emitter follower. This linear characteristic is obtained by neglecting the change in vBE1 with iL. The maximum positive output is determined by the saturation of Q1. In the negative direction, the limit of the linear region is determined either by Q1 turning off or by Q2 saturating, depending on the values of I and RL.

HARMONIC DISTORTION IN POWER AMPLIFIERS

ic = ICQ + I0 + I1 Cos (ωt) + I2 Cos (2ωt)

By definition the percent of the second harmonic distortion is given as

D2 = I2/I1 * 100 % = ½( Ic max + Ic min) – ICQ /( Ic max - Ic min) * 100

or

D2 = V2/V1 * 100 % = ½( Vce max + Vce min) – VCEQ /( VCE max - VCE min) * 100

The second harmonic distortion is the percent of the second harmonic componenl present in the output current waveform with respect to the amount of the fundamenta component.

Lecture- 57

TRANSFORMER COUPLED AUDIO POWER AMPLIFIER

This is also sometimes referred to as single ended power amplifier. The term “single ended” (denoting only one transistor) is used to distinguish it from the push-pull amplifier using two transistors.

In case of a direct-coupled class A power amplifier shown, the quiescent current flows through the collector resistive load and causes large wastage of dc power in it. This dc power dissipated in the load resistor does not contribute to the useful ac output power.

Furthermore, it is generally inadvisable to pass the dc through the output device such as in a voice coil of a loudspeaker. For these reasons an arrangement using a suitable transformer for coupling the load to the amplifier is usually employed, as shown. This arrangement also permits impedance matching.In a power amplifier circuit shown Rt and R2 provide potential divider bias ing and emitter resistor RE is meant for bias stabilization. The emitter bypass capacitor CE is meant for RE to prevent ac voltage. The input capacitor Cin couples ac signal voltage to the base of the transistor but blocks any dc from the previous stage. A step-down transformer of suitable turn ratio is provided to couple the high impedance collector circuit to low impedance load.

Lecture- 58

PUSH-PULL AMPLIFIER:-

This "push-pull" amplifier is used where high power output and good fidelity are needed: receiver output

stages, public address amplifiers, and AM modulators. The circuit shown in is a class A transistor push-

pull amplifier, but class AB or class B operations can be used. Class operations were discussed in an

earlier topic. The phase splitter for this amplifier is the transformer T1, although one of the phase splitters

shown earlier in this topic could be used. R1 provides the proper bias for Q1 and Q2. The tapped

secondary of T1 develops the two input signals for the bases of Q1 and Q2. Half of the original input

signal will be amplified by Q-1, the other half by Q-2. T2 combines (couples) the amplified output signal

to the speaker and provides impedance matching.

Class A transistor push-pull amplifier.

Lecture- 59CLASS 'B'

A class 'B' amplifier uses complimentary transistors for each half of the waveform. A true class 'B' amplifier is NOT generally used for audio. In a class 'B' amplifier, there is

a small part of the waveform which will be distorted. You should remember that it takes approximately .6 volts (measured from base to emitter) to get a bipolar transistor to start conducting. In a pure class 'B' amplifier, the output transistors are not "biased" to an 'on' state of operation. This means that the the part of the waveform which falls within this .6 volt window will not be reproduced accurately.

Lecture- 60Class AB Operation

If the amplifying device is biased in such a way that current flows in the device for 51% - 99% of the input signal, the amplifier is operating class AB. A simple class AB amplifier is shown in figure

Class AB amplifiers have better efficiency and poorer fidelity than class A amplifiers. They are used when the output signal need not be a complete reproduction of the input signal, but both positive and negative portions of the input signal must be available at the output.

Class AB amplifiers are usually defined as amplifiers operating between class A and class B because class A amplifiers operate on 100% of input signal and class B amplifiers (discussed next) operate on 50% of the input signal. Any amplifier operating between these two limits is operating class AB.

Lecture- 61 REGULATED POWER SUPPLIES

A regulated power supply is an embedded circuit, or standalone unit, the function of which is to supply a stable voltage (or less often current), to a circuit or device that must be operated within certain power supply

limits. The output from the regulated power supply may be alternating or unidirectional, but is nearly always DC (Direct Current).

APPLICATIONS

D.C. variable bench supply (a bench power supply usually refers to a power supply capable of supplying avariety of output voltages useful for bench testing electronic circuits, possibly with continuous variation of the output voltage, or just some preset voltages; a laboratory (lab) power supply normally implies an accurate bench power supply, while a balanced or tracking power supply refers to twin supplies for use when a circuit requires both positive and negative supply rails),Mobile Phone power adaptors,Regulated power supplies in appliances

The output voltage is given by

A simple Series Voltage Regulator is shown. The transistor Q1 is the series element, which controls the amount of input voltage that gets to the output. The zener diode provides the reference voltage. The regulating operation can be described as:

1. If the output voltage decrease the increased base-emitter voltage causes the transistor Q1 to 2. conduct more, thereby raising the output voltage. I.e. maintaining the output constant. 3. If the output voltage increases, the decreased base-emitter voltage causes transistor Q1 to

conduct less, thereby reducing the output. I.e. maintaining the output constant.