Wednesday, 18 September 2013

Dr B Somanathan Nair

Dr. B Somanathan Nair, one of the top engineering text authors born on September 1, 1950 in Kerala, India. In 1971, he obtained his Bachelor of Science (B. Sc) from University of Kerala. He had done his Master of Engineering (M.E) from Indian Institute of Science, Bengaluru in 1983 and PhD from Rajasthan Vidyapeedh, Udaipur, Rajasthan in the year 2010. Since 1972, he has been working as a teacher in various Engineering colleges and Universities in India. At the University of Idaho (Dept. of Electrical Engg), USA, he worked as a professor for the period of 1 year. He is a member of IEEE (Institute of Electrical and Electronics Engineers), USA and a life member of ISTE (Indian Society For Technical Education), India. He is a member of the Advisory Board, M G University, Kerala. Presently he is working as the Principal of Shahul Hameed Engineering College (S H M), kadakkal, Kollam, Kerala and Professor of Karyavattom University, Kerala.

He added his names in the history of every institution where he studied as a brilliant student and top scorer. During his high schooling, he had H. H. THE MAHARAJA’S MERIT SCHOLARSHIP and with UNIVERSITY MERIT SCHOLARSHIP, he completed his Pre Degree course. During his Graduation, he was awarded with NATIONAL MERIT SCHOLARSHIP. In 1972, he started his career as a lecturer in NSS College, Palakkad. In 1973, he entered into Government Service as Lecturer in Government Engineering College, Thiruvananthapuram. Dr. B.S Nair succeeded being the Head of the Department for 10 years. He worked as Principal/Joint Director of various Engineering Colleges for another 8 years. For one and half years, he held the post as Controller of Examinations.

Apart from teaching he proved himself as a guide for students. His guidance includes research associated with M.Tech dissertations since 1988, Guidance of seminars of B.Tech and M.Tech students.

Administrative Positions held by Dr. B.S Nair :

Dr. B.S Nair succeeded being the Head of the Department for 10 years. He worked as Principal/Joint Director of various Engineering Colleges for another 8 years. For one and half years, he held the post as Controller of Examinations. He was the University Syndicate Member for 4 years and University Senate Member for 4 years. He was also the University Academic Council Member for 4 years. He worked as a Coordinator, GOI Impact Project.

Academic Positions held by Dr. B.S Nair:

He has 39 years of teaching experience in various universities around the world. Currently he is working as the Chairman, Board of Studies in Electronics (PG AND UG), MG University, Kottayam and the CO-Chairman, M.Tech Core Committee, MG University, Kottayam. He is  a member of , Board Of Studies In Electronics (UG), MG University. He held a position as a member in University Of Kerala, in the section "Board of Studies in Optoelectronics".

Research done by Dr. B.S Nair

Reviewer, International Conference on Mobile Telecommunication - 2010, St. Peters berg, Russia.
Reviewer, International Conference on Mobile Telecommunication - 2009, St. Peters berg, Russia.
Reviewer, International Conference on Simulation and Modelling MS 09, Thiruvananthapuram, India.
Co-Investigator, Research Project on “Applications of Thin-Film Coir Fibbers' in Electronic Industry”, a current research Work in collaboration with the Central Coir Board.
Research Collaborator, Central plantation Crops Research institute, Kayamkulam, in the “Development of Electronic Pest Detection methods” since 1985.

Guidance Of Research Projects 

His research topics associated with M.Tech dissertations includes the design and development of

Fiber-optic liquid sensor.
Y-guide fiber-optic vibration sensor.
Optical stethoscope.
Surface Profilometer.
Sensor for detection of adulteration in petrol.
Sensor for smoke and pollution detection.
Optical OFDM-performance analysis.
Automatic viscosity meter (using PIC).
Sensor for the detection of stress in heavy beams.

Topics Guided  of Seminars of B. TECH AND M. TECH Students Since 1971 Includes

Speech-signal processing
Holography
VLSI technology
Advances in lasers
Fiber-optic technology
Research Project on “Applications of Thin-Film Coir Fibers in Electronic Industry”.

His guidance for various research projects includes:

Automatic blood-group analyzer.
Fiber-optic liquid sensor.
Y-guide fiber-optic vibration sensor.

Publication of articles

He published Articles in Popular Books and News Papers.
Two Articles published in “Sarva Vignana Kosam”, (Malayalam Encyclopaedia).
A Very Popular Article on Antenna published in the Malayala Manorama Daily.

Text Books Published by Dr. B.S Nair

Network Analysis and Synthesis.
Electronics Lab and Workshop Manual, IK International.
Information Theory and Coding.
Signals and Systems.
Linear Integrated Circuits Analysis Design & Applications.
Applied Electromagnetic Theory: Analyses, Problems And Applications.
Basic Electronics (Includes Solved Problems & MCQs) .
Basic Communication and Information Engineering Book Description.
Solid State Devices .
Microwave Engineering.
Digital Signal Processing: Theory, Analysis And Digital- filter Design.
Electronics Devices And Applications.

Online order of books :

we can order text books written by Dr. B. Somanathan Nair online from the website 'www.bsnair.com'. Also we can purchase his books from all academic book stores.


CONTACT ADDRESS :

Dr. B. Somanathan Nair
Indeevaram,
VRA C-6,
 Vanchiyoor,
Thiruvananthapuram
email id: profbsnair@gmail.com
phone no: +919446030878

Monday, 9 September 2013

Semiconductors - Classification, Types, Band Theory

We know that the basic building blocks of all electronic circuits are devices having controlled flow of electrons. Such devices are mostly vacuum tubes (or valves) before the discovery of transistors in 1948. Vacuum tubes includes vacuum diode with two electrodes, an anode and a cathode; triode with three electrodes - i.e., cathode, plate(anode) and a grid; tetrode - with four electrodes and pentode with five electrodes. In a vacuum tube, the inter-electrode space requires vacuum, it is to avoid lose of energy due to collision with air molecules. The heated cathode in the vacuum tube supplies the electrons  and the varying voltage between the electrodes provides the flow of electrons in a controlled manner. Here the electrons flows in one direction, ice, from cathode to anode; hence they are called as valves. But vacuum tubes have many disadvantages: vacuum tubes are bulky, and requires high voltage for it to operate (~100 V), high power consumption and also it has limited life and the reliability is low. The number and the direction of flow of charge carriers are controlled by some semiconductors and  their junctions. The number of mobile charges in a semiconductor can be changed by application of light, heat or small applied voltage. In the semiconductor device, the supply and flow of charge carriers are within the solid state and large evacuated space or external heating is not required. They have many advantages over the vacuum tubes like smaller size, consumption of low power, low power of operation and has high reliability and long life. In computer monitors and television, the  CRT (Cathode Ray Tube) was used earlier, which works on the principle of vacuum tubes, now it is being replaced by LCD (Liquid Crystal Display) with supporting solid state electronics.


Classification of Metals, Conductors and Semiconductors on the basis of conductivity:

Based on the relative values of electrical conductivity (σ) or resistivity (ρ=1/σ), the solids are broadly classified as:

(i) Metals: Metals possess very low resistivity or high conductivity.
    р ~ 10-2-10-8 Ω m

(ii) Semiconductors: Semiconductors have resistivity or conductivity intermediate to metals and insulators. 
    р ~ 10-5-10-8 Ω m 
    σ ~ 105-10-6  S m-1

(iii) Insulators: Insulators possess high resistivity or low conductivity.
     р ~ 1011-1019 Ω m
     σ ~ 10-11-10-19 S m-1

The values of р and σ given above are indicative of magnitude and may go outside the ranges as well. Relative values of the resistivity are not the only criteria for the distinguishing of metals, semiconductors and insulators from each other. Most of the currently available semiconductor devices are based on elemental semiconductors Si or Ge and compound inorganic semiconductors. After 1990, a few semiconductor devices using organic semiconductors and semiconducting polymers have been developed signaling the birth of a futuristic technology of polymer-electronics and molecular-electronics.

On the Basis of Energy Bands:

Based on Bohr atomic model, "In an isolated atom, the energy of any of its electrons is decided by the orbit in which it revolves". But when solids are formed, the atoms come together in such a way that outer orbits of electrons would become very close or even overlap. Hence the motion of electrons in a solid is very different from that in an isolated atom. Each electron inside the crystal has got a unique position and no two electrons are having exactly same pattern of surrounding charges.

Each electron inside the crystal has got a unique position and no two electrons are having exactly same pattern of surrounding charges. These different energy levels with continuous energy variation form can be called as energy bands. The energy level of valence electrons is included in the energy band called as valence band. The energy band above the valence band can be called as the conduction band. All the valence electrons will reside in the valence band due to the absence of external energy. The electrons from the valence band can easily move into the conduction band, if the lowest level in the conduction band happens to be lower than the highest level of the valence band. Usually the conduction band is empty. But, electrons from the valence band moves freely into the conduction band when both the bands overlaps. This occurs in case of metallic conductors.

Electrons in the valence band all remain bound and no free electrons are available in the conduction band, when there is some gap between the conduction band and the valence band. This makes a material as an insulator. Some of the electrons in the valence band may gain external energy and cross the gap between the conduction band and the valence band. Then these types’ electrons will move to the conduction band. These electrons will create vacant energy levels in the valence band, where other valence electrons can move. Thus this process makes the possibility of conduction due to the electrons present in the conduction band and the vacancies present in the valence band.

Now consider the case of Si or Ge crystal containing N atoms. We know that for Si, the outermost orbit is third orbit (n=3) and for Ge, it is the fourth orbit (n=4). The number of electrons present in the outermost orbit is 4 (2s and 2p electrons). Thus, the total number of electrons present in the outer orbit of the crystal is 4N. The maximum number of electrons present in the outer orbit is 8 (2s + 6p electrons).  So, for the 4N valence electrons, there exist 8N available energy states. Depending up on the distance between the atoms in the crystal, these 8N discrete energy levels can either form a continuous band or they may be grouped in different bands.

The energy band of these 8N states is split apart into two which are separated by an energy gap Eg. The valence band is the lower band, which is completely occupied by the 4N valence electrons at temperature of absolute zero. The other band consisting of 4N energy states and is completely empty at absolute zero is called the conduction band.  

Band Theory of Solids :

Consider that N atoms are present in the Si or Ge crystal. In different orbits, electrons of each atom will have discrete energies. If all atoms are isolated, the electron energy will be same i.e. separated from each other by a large distance. The atoms are close to each other (2 to 3 Å) in a crystal and therefore the electrons interact with each other and also with the neighboring atomic cores. In the outermost orbit, the interaction (overlap) will be more felt by the electrons, while the inner orbit or the core electron energies may remain unaffected. So we need to consider the changes in the energies of the electrons present in the outermost orbit only for understanding the electron energies in Si or Ge crystal. The outermost orbit for Si is the third orbit (n=3) and for Ge, it is the fourth orbit (n=4). The total number of electrons present in the outermost orbit is 4 (2s and 2p electrons). Hence the total number of outer electrons present in the crystal is 4N. The maximum possible number of outer electrons present in the orbit is 8 (2s+6p electrons). So, out of the 4N electrons, 2N electrons are in the 2N s-states (orbital quantum number l =0) and the 2N electrons are present in the available 6N p-states. This is the case of well separated or isolated atoms.

If these atoms start coming nearer to each other to form a solid, the energies of these electrons in the outermost orbit may change (both increase and decrease) due to the interaction between the electrons of different atoms. The 6N states for l=1, originally having identical energies in the isolated atoms, spread out to form an energy band. Similarly, the 2N states for l=0, having identical energies in the isolated atoms, split into a second band, separated from the first one by an energy gap. Figure 1.1shows The Band Theory of Solids.


Figure 1.1: Band Theory of Solids

At smaller spacing, there comes a region in which the bands merge with each other. The lowest energy state that is a split from the upper atomic level appears to drop beneath the upper state that comes from the lower atomic level. No energy gap exists in this region, where the lower and upper energy states get mixed. Figure 1.2 shows the conductivity of Conductors, semiconductors and insulators with the increase of temperature.

Figure 1.2 : Conductivity of Conductors, semiconductors and insulators with the increase of temperature.

The energy bands again split apart and are separated by an energy gap Eg when the distance between the atoms decreases. The total number of available energy states 8N has been re-apportioned between the lower and upper energy bands each of 4N states. Here there are exactly many states in the lower band (4N) as there are available valence electrons from the atoms (4N). So this band, the valence band is completely filled while the upper band, the conduction band is completely empty.


Figure 2 : Difference between energy bands of metals, insulators and semiconductors.

The Energy band gap is the gap between the top of the valence band and the bottom of the conduction band. Depending upon the material, it may be large, small or zero. We can classify materials into conductors, insulators and semiconductors based on the energy gap. Figure 2 shows the difference between energy bands of metals, insulators and semiconductors.

Conductors:

In case of conductors, the valence band and the conduction band overlaps. Here there is overlap electrons from valence band can easily move into the conduction band. This makes a large number of electrons available for electrical conduction. If the valence band is partially empty, the electrons from its lower level can move into the higher level. This makes conduction possible. So the resistance of such materials is low or the conductivity is high.  

Insulators:

In this case, a large energy band gap Eg (Eg >3 eV) exists between the two bands. Here no electrical conduction is possible, due to the absence of electrons in the conduction band. In case of insulators, the energy gap is very large that electrons cannot be excited from the valence band to the conduction band by thermal excitation.

Semiconductors:

Here in this case, a finite but small band gap (Eg <3 eV) exists. At room temperature, some electrons from valence band can acquire enough energy to cross the energy gap and enter the conduction band due to the small band gap. These electrons (in small numbers) can move in the conduction band. Thus the resistance of semiconductors is not high as that of insulators. Now we are discussing about the types of semiconductors

1. Intrinsic Semiconductors:

The lattice structures of Si and Ge are commonly called as the diamond-like structures. In this type of structure, each atom is surrounded by four nearest neighbors. We know that the valence electrons of Si and Ge atoms are four. Every Si or Ge tends to share one of its four valence electrons with each  of its four nearest neighbor atoms, and also to take share one electron from each such neighbor in its crystalline structure. These shared electron pairs are capable to form a covalent bond or also referred to as valence bond. These two shared electrons are to shuttle back-and-forth between the associated atoms holding together them strongly. 

Figure 3.1: Three dimensional diamond-like crystal structure for C,Si or Ge.


Figures 3.2 shows an idealized picture in which no bonds are broken or all bonds are intact. At low temperature condition only these situation arises. By the gradual increase of temperature, more thermal energy becomes available to these electrons and some of these electrons can break-away, i.e. becoming free electrons contributing to conduction. In the bond, the thermal energy effectively ionizes only a few atoms in the crystalline lattice and creates a vacancy. The atoms from which the free electrons, with charge (–q) is released by leaving a vacancy has got an effective charge of (+q). This effective positive electronic charge constituting a vacancy is referred to as a hole. The hole can be considered as an apparent free particle with effective positive charge.

In an intrinsic semiconductor, the number of free electrons (ne) equals the number of holes (nh). That is: 

ne = nh = ni, 

Where, ni is called as the intrinsic carrier concentration.
Figure 4.1 shows the  schematic two-dimensional representation of si or Ge structure showing covalent bonds at low temperatures.


Figure 4.1 : Schematic two-dimensional representation of si or Ge structure showing covalent bonds at low temperatures

The semiconductor materials posses an unique property, that is apart from the electrons, the holes can also move. Figure 4.1 shows the schematic two-dimensional representation of si or Ge structure showing covalent bonds at low temperatures.

Let us consider that there is a hole in figure 4.2. An electron from the covalent bond may jump to the vacant site  (hole). Thus after a jump, the hole is at the new site and an electron is present in the old site .   Thus we can say that apparently, the hole has moved from one site to other site. The free electrons moves completely independently as conduction electron and gives rise to an electron current, Ie under an applied electric field and is not involved in the process of hole motion.  It should be remembered that the motion of hole is only a convenient way of describing the actual motion of bounded electrons, whenever there exists an empty bond anywhere in the crystal. These holes move towards negative potential and giving the hole current Ih under the action of an electric field. Thus the total current I is the sum of the electron current Ie and the hole current Ih. 

I = Ie + Ih

Apart from the process of generation of conduction electrons and holes, a simultaneous process of recombination occurs, in which the electrons recombines with holes. The rate of generation is equal to the rate of recombination of charge carriers at equilibrium state. When an electron collides with a hole, the recombination process occurs. Figure 4.2 shows the schematic model of generation of hole due to thermal energy at moderate temperatures


Figure 4.2: schematic model of generation of hole due to thermal energy at moderate temperatures
When T=0K, an intrinsic semiconductor behaves like an insulator. At higher temperatures (T>0K), due to the thermal energy, some electrons excites from the valence band to the conduction band. These thermally excited electrons (T>0K), partially occupies in the conduction band. Hence in the energy band diagram of an intrinsic semiconductor, some electrons can be shown in the conduction band. These electrons have come from the valence band by leaving equal number of holes there. Figure 4.3 shows an Intrinsic semiconductor at T=0 K behaves like insulator and at T>0 K, four thermally generated electron hole pair. Figure 4.3 shows an Intrinsic semiconductor at T=0 K behaves like insulator and at T>0 K, four thermally generated electron hole pair.


Figure 4.3 : An Intrinsic semiconductor at T=0 K behaves like insulator and at T>0 K, four thermally generated electron hole pair
Here there may arise a question that why Carbon (C) is an insulator while Si and Ge are intrinsic semiconductor although Si, Ge and C have same lattice structures?
 
We know that the four bonding atoms of C, Si or Ge lies respectively in the second, third and fourth orbit. Hence the ionization energy (Eg), the energy required to take out an electron from these atoms will be least for Ge, followed by silicon and highest for C. Hence the number of free electrons for conduction in Si and Ge are significant and in case of C, which is negligibly small. 

2. Extrinsic Semiconductors:

We know that the conductivity of an intrinsic semiconductor mainly depends on its temperature, but at room temperature, its conductivity is very low. Thus it is not possible to develop important electronic devices by using these types of semiconductors. The conductivity of these materials can be improved by making the use of impurities.

The conductivity of the semiconductor is increased manifold when small amount, say, a few parts per million (ppm) of a suitable impurity is added to the pure semiconductor material. These types of materials are commonly known as extrinsic semiconductors or impurity semiconductors. Doping is the process of deliberate addition of a desirable impurity and the impurity atoms are called as dopants. Such material is also referred to as doped semiconductor. The dopant should be selected such that it does not distort the original pure semiconductor lattice. Of the original semiconductor atom sites in the crystal, only a very few portions occupies by the dopants. In order to attain this, it should satisfy a necessary condition that the sizes of the dopant and the semiconductor atoms should be nearly the same.     

For the doping of the tetravalent Si or Ge atoms, two types of dopants are used:
1. Pentavalent dopants like Arsenic (As), Antimony (Sb), Phosphorous (P) etc having valency 5. 
2. Trivalent dopants like Indium (In), Boron (B), Aluminium (Al) etc having valency 3.
 Now we discusses the process of how the doping changes the number of charge carriers (and hence the conductivity) of semiconductors.  Since Si or Ge belongs to the fourth group of the periodic table, we choose the dopant elements from the fifth or third group elements, which are near by the fourth group, expecting and taking care that the size of the dopant atom is nearly the same as that of Si or Ge. The Pentavalent and trivalent dopants in Si or Ge give two entirely different types of semiconductors.

(i) n- type semiconductor:

Here we dope Si or Ge with a pentavalent element. In the crystal lattice of Si when an atom of +5 valency element occupies the position of an atom, four of its electron bonds with the four silicon neighbours while the fifth remains very weakly bound to its parent atom. This is due to the four electrons participating in bonding are seen as part of the effective core of the atom by the fifth electron. Hence the ionization energy needed to set this electron free is very small. So even at room temperature, it will be free to move in the lattice of the semiconductor. For example, the energy required to separate this electron from its atom for germanium is ~ 0.01 eV and 0.05 eV for silicon. This is in contrast to the energy required to jump the forbidden band at room temperature in the intrinsic semiconductor is about 0.72 eV for germanium and about 1.1 eV for silicon. Hence the Pentavalent dopant is donating one extra electron for conduction and is referred to as donor impurity. The number of electrons available for conduction by dopant atoms strongly depends upon the doping level and is independent of any increase in the ambient temperature. Also the number of free electrons generated by Si atoms (with an equal number of holes) increases weakly with temperature.

The total number of conduction electrons ne in a doped semiconductor is mainly due to the electrons contributed by donors and those generated intrinsically. The total number of holes nh is only due to the holes from the intrinsic source. Due to the increase in the number of electrons, the rate of recombination of holes would also increases. Hence the number of holes would get further reduced. Figure 5.1 shows the Pentavalent donor atom P doped for tetravalent Si giving n-type semiconductor.


Figure 5.1 : Pentavalent donor atom P doped for tetravalent Si giving n-type semiconductor.
Thus the number of conduction electrons can be made much larger than the number of holes with the help of proper level of doping. Thus in an extrinsic semiconductor doped with the pentavalent impurity, electrons are the majority carriers and holes are the minority carriers. Hence these semiconductors are also known as n-type semiconductors. For n-type semiconductors, it is clear that ne>> nh.

(ii) p-type semiconductors:

The p-type semiconductor materials can be obtained when Si or Ge is doped with a trivalent impurity like Al, B, In, etc. Due to the trivalent impurity dopants, the dopant has one valence electron less than Si or Ge. Hence this atom can form covalent bonds with neighbouring three Si atoms and does not have any electron to offer to the fourth Si atom. So the bond between the fourth neighbour and the trivalent atom has a vacancy or also known as a hole. The neighbouring Si atom in the lattice needs an electron in place of a hole; an electron in the outer orbit of an atom in the neighbourhood may jump to fill this vacancy, by leaving a vacancy or hole at its own site. Thus in p-type semiconductor, hole is available for conduction. When the trivalent foreign atom shares fourth electron with the neighbouring Si atom, it becomes effectively negatively charged. Thus the dopant atom of p-type material can be considered as core of one negative charge along with its associated hole. It is very clear that one acceptor atom gives one hole. These holes are in addition to the intrinsically generated holes. The source of conduction electrons is only intrinsic generation. Hence for these types of materials, holes are the majority carriers and electrons are minority carriers. Hence extrinsic semiconductors doped with trivalent impurity are called p-type semiconductors. The recombination process will reduce the number (ni) of intrinsically generated electrons to ne for p-type semiconductors. Thus for p-type semiconductors, nh>>ne. 
The crystal maintains an overall charge neutrality as the charge of additional charge carriers is just equal and opposite to that of the ionized cores in the lattice. Because of the abundance of majority current carriers in the extrinsic semiconductors, the minority carriers produced thermally have more chance of meeting majority carriers and thus getting destroyed. Hence the dopants become the majority carriers by adding a large number of current carriers of one type indirectly help to reduce the intrinsic concentration of minority carriers. Figure 5.2 shows the trivalent acceptor atom B doped in tetravalent Si lattice giving p-type semiconductor.


Figure 5.2 : Trivalent acceptor atom B doped in tetravalent Si lattice giving p-type semiconductor.

The energy band structure of the semiconductor is affected by doping. The additional energy states due to donor impurities (ED) and acceptor impurities (EA) also exist in case of extrinsic semiconductors. The donor energy level (ED) is slightly below the bottom EC of the conduction band in the energy band diagram of n-type Si semiconductor and electrons from this level move into the conduction band with very small supply of energy. Most of the donor atoms get ionized at room temperature, but very few (~10-12) atoms of Si get ionized. Thus the conduction band has most electrons coming from the donor impurities. In the similar way, for p-type semiconductor, the acceptor energy level (EA) is slightly above the top of EV of the valence band. An electron from the valence band can jump to the level EA and ionize the acceptor negatively, with very small supply of energy. In other words, we can also say that with very small supply of energy the hole from level EA sinks down into the valence band. When they gain external energy, electrons rise up and holes fall down. Most of the acceptor atoms get ionized leaving holes in the valence band at room temperature. Thus the density of holes in the valence band is predominantly due to impurity in the extrinsic semiconductor at room temperature. In thermal equilibrium, the electron and hole concentration in a semiconductor is given by,





Even though the above description is grossly approximate and hypothetical, it helps to understand the difference between metals, insulators and semiconductors (extrinsic and intrinsic) in a simple way. The difference in the resistivity of C, Si and Ge mainly depends upon the energy gap between their valence and conduction bands. The energy gaps for C (diamond), Si and Ge, are 5.4 eV, 1.1 eV and 0.7 eV respectively. We know that Sn is also an element of group IV, but it is a metal because the energy gap in its case is 0 eV. Figure 6 shows the energy bands of n-type and p-type semiconductors.

Monday, 12 August 2013

Digital Signal Processing


In figure below, x(t) is the input signal (analog signal) and y(t) is the analog output signal.


The input signal x(t) may be the signal from a transducer or a communication signal. The signal may be ECG or an EEG Signal. To the antialiasing filter, the input signal is applied. The antialiasing filter is a low pass filter, mainly used to remove the high frequency noise and to band limit the signal.

Since, the large part of the external noise is due to the 50 Hz frequency of the input wave, it is useful to include a 50 Hz notch filter that can remove the power-frequency component. If the required voltage range that is required by the input of analog to digital conversion unit is not meet, then an amplifier circuit can be used to amplify the signal. The input to the ADC can be provided by the sample and hold circuit. It is required if the input signal must remain relatively constant during the process of conversation of the analog signal to digital format. The input to the ADC is obtained from the output of the sample and hold circuits. Depending on the value of the analog signal at its input, the output of the ADC is an N-bit Binary number.

The ADC input signal may be either unipolar (0 to +10 V) or bipolar (-5 to +5 V). For obtaining the signal in this range, a preceding amplifier can be used. When the analog input signal is converted into its digital form, by using digital techniques the signal can be processed. The digital signal processor can be either a microprocessor (e.g.: Intel’s 80XX, Motorola’s 68XXX etc) programmed to perform the desired operations on input signal or may be a large programmable digital computer. For performing the specified set of operations on the input signal, a digital signal processing Hardware (e.g.: ADSP 2100, Motorola DSP 56000 etc) is used.

The input of the DAC is the digital signal from the Processor. The output of DAC is a continuous signal, but the signal is not a smooth one. The signal contains high frequency unwanted noise components. In order to eliminate the high frequency noise components, the reconstruction filter is used. The output of the DAC is applied to the reconstruction filter for this purpose. Since the reconstruction filter is used, the output of the reconstruction filter will be a continuous smooth signal.

ADVANTAGES OF DSP : 

The advantages of DSP over Analog Signal Processing are:

1.  High Accuracy: The accuracy of the analog filter is affected by the tolerance of the circuit components used for design the filter, but DSP has superior control of accuracy.

2.  Cheaper: The digital realization is much cheaper than the analog realization in many applications.

3.  Flexibility in Configuration: For reconfiguring an analog system, we can only do it by redesign of system hardware; where as a DSP System can be easily reconfigured only by changing the program.

4.  Ease of Data Storage: On magnetic media, without the loss of fidelity the digital signals can be stored and can be processed off-line in a remote laboratory.

5.  Time Sharing: The cost of the processing signal can be reduced in DSP by the sharing of a given processor among a number of signals.


LIMITATIONS OF DSP : 

1.   System Complexity:  In DSP, due to the use of devices such as D/A and A/D converters, the system complexity increases. The reconstruction filters will also contributes for system complexity.

2.   Power Consumption: The DSP chip consists of over 4 Lakh transistors, which will yields to dissipate high power (1 Watt), whereas the analog signal processing includes only passive circuit elements like resistors, capacitors and inductors, which will leads to only low power dissipation.

APPLICATIONS OF DSP :

DSP can be applicable in variety of fields such as

1.      Telecommunication
2.      Consumer Electronics
3.      Image Processing
4.      Instrumentation and Control
5.      Military Applications
6.      Speech Processing
7.      Seismology
8.      Medicine

Analog Signal Processing


A physical device that performs an operation on a signal is called as a system. Example: For reducing the noise corrupting a desired information bearing signal, a filter is used. So a filter can be considered as a system. Any operation which changes the characteristics of a signal is called as Signal Processing. The characteristics can be the amplitude, shape, frequency content and phase of a signal.

In science and engineering, most of the signals dealing with these branches are of analog in nature. The analog signals are functions of a continuous variable as space or time. Devices such as amplifiers, filters, frequency analyzers etc are required to process such type of signals. These devices respond to the continuous variation of the instantaneous amplitude of the input signal. These devices can change the characteristics of a signal. These devices are also useful in extracting some desired information from the signal. Thus in these cases, it is clear to say that the analog signal has been processed. An analog signal processing system can be defined as the system that processes an analog signal.



Wednesday, 10 April 2013

Simplex, Half Duplex, Full Duplex Communication Channel

A communications channel can be defined as a pathway over which information can be conveyed. A channel can be defined by a physical wire that connects communicating devices, or by a radio, laser, or other radiated energy source that has no obvious physical presence. The required data for transmission should be sent through a communications channel has a source from which the information originates, and a destination to which the information is delivered, the source is called as the transmitter and the destination is called as the receiver. In between the transmitter and the receiver, the transmission medium of the data, this is usually called as the channel of a communication system.  Although the required information for transmission originates from a single source, there may be more than one destination (more than one receiver), depending upon how many receive stations are linked to the channel and how much energy the transmitted signal possesses. If the channel length (transmission distance) is more and the transmission power is less, the receiver situated at a long distance cannot receive the data properly.

In a digital communications channel, the information can be represented by individual data bits that are encapsulated into multi bit message units. An example of a message unit that may be conveyed through a digital communications channel is a byte, which consists of eight bits. A collection of bytes can be grouped to form a frame or other higher-level message unit. These types of multiple levels of encapsulation facilitate the handling of messages in a complex data communications network.

If we consider any communications channel, it has a direction associated:

Simplex Channel: We know that the message source is the transmitter, and the destination is the receiver. A channel whose direction of transmission is unchanging is called as a simplex channel. In other words, a type of data transmission, which is taken place only in one direction (from one antenna to the other only), for example, a radio station is a simplex channel because it always transmits the signal to its listeners and never allows them to transmit back. A television set up can also be considered as the simplex type. The advantage of simplex mode of transmission is, since the data can be transmitted only in one direction, the entire band width can be used.

Half Duplex Channel: A half-duplex channel can be considered as a single physical channel in which the direction may be reversed. Messages can flow in two directions in a half-duplex type, but never at the same time. In other words it can be said that at a single time, the transmission of data are done in only one direction. For example, in a telephone call, one party speaks while the other listens. After a pause (when one party stops his speech), the other party speaks and the first party listens. Speaking simultaneously will result in a garbled sound that cannot be understood. The main difficulty of half-duplex mode of transmission is since two channels are used, the band width of the channel should be decreased.

Full Duplex Channel: A full-duplex channel can be used for transmitting simultaneous message exchange in both directions. It consists of two simplex channels, a forward channel and a backward (reverse) channel, linking at the same points. The transmission rate of the reverse channel will be very slow if it is used only for flow control of the forward channel. The main problem of the full duplex mode of transmission is, since we are using two channels, the band width should be decreased.

Tuesday, 9 April 2013

NPN and PNP Transistor Structure

The BJT (Bipolar junction Transistor) can be constructed by using three doped semiconductor regions separated by two p-n junctions. These three regions are called as emitter, base, and collector. The emitter will emits the electrons, while the collector will collects the electrons emitted from the emitter. The emitter region and the collector regions are separated by the help of a less doped region called as the base. Based on the combination of p and n regions, there are two types of BJTs, either pnp (two p regions separated by one n region) and npn (two n regions separated by one p region).

The npn and pnp BJTs are represented below:


The easiest method to identify a transistor is just check the arrow mark represented in the transistor circuit. The arrow mark points to the n-region of the transistor. The regions near to the arrow mark show the p-region. In figure (a), the emitter section shows the arrow mark. So the emitter section is a n-region. The region near to emitter from figure is base. So from the configuration of transistor, it is clear that no two n regions occur near. Thus the Base region is P type. Also by the configuration of transistor it is clear that the side near to p-region should be n-type. Hence the transistor is a npn transistor. In the similar way from figure (b), it is clear that the arrow mark points to the base region. So the base region is n-type. The regions opposite to base should be p-regions. Hence the structure is a pnp transistor.

The C, E, and B symbols represent the collector, emitter, and base regions, respectively. The base region is lightly doped and it is very thin region. The emitter side is heavily doped and the collector region is moderately doped.


Monday, 8 April 2013

Basic NPN and PNP Transistor Operation

For correct operation if a transistor, the two pn junctions must be correctly biased with  the externally applied dc voltages.

Operation of the pnp transistor is same as that of a npn transistor, but the change is that the  roles of electrons and holes, bias polarities, and current directions are all reversed.

The figure shows the correct biasing of a BJT circuit.

The base-emitter (BE) junction should be forward biased and the base-collector (BC) junction is reverse biased for the proper working of the BJT..

Due to the forward bias condition, base to emitter narrows the BE depletion region. The reverse bias voltage applied from base to collector will leads to the widening of  the BC depletion region. The heavily doped n¬-type emitter region is packed with conduction-band (free) electrons and it will emit electrons when the emitter-base junction is forward biased. The free electrons from the emitter diffuse through the forward biased BE junction into the base region (p-type). The electrons become minority carriers (like in a forward biased diode) in the base region. Since the base region is lightly doped and very thin, it has a limited number of holes only. Only a small percentage of all the electrons flowing through the BE junction can combine with the available holes in the base region because of that light doping. A small base electron current is formed due to these relatively few recombined electrons flow out of the base lead as valence electrons. Most of the electrons flowing from the emitter to the base region will not recombine. These electrons will diffuse into the BC depletion region.

These electrons are pulled through the reverse-biased BC junction by the electric field set up. It is mainly due to the force of attraction between the negative and positive ions.

Electrons can move through the collector region, out through the collector lead, and into the positive terminal of the collector voltage source due to the externally applied voltage.

The collector electron current is thus formed. It is clear that the collector current (Iccr) is much larger (higher) than the base current.

Due to this reason, transistors exhibit current gain.

It is clear that the emitter current is higher than the base and collector currents. So the emitter current will constitute the base and the collector currents.

Hence,

IEcr = ICcr + IBcr

The Capital letters shows the dc values of current.

Saturday, 6 April 2013

Full Wave Rectifiers Theory and Circuit Operation

Full-wave rectifiers are used for various electronic applications. Full-wave rectifiers are the most commonly used devices for the dc power supplies.

A full-wave rectifier is as same as the half-wave rectifier circuit, but the difference is a full wave rectifier allows unidirectional current through the load during the entire sinusoidal cycle (as opposed to only half the cycle in the half-wave). In other words, for both the positive and negative half cycles of the input sinusoidal wave, the full wave rectifier conducts current through the load resistance. This will leads to a constant dc voltage through out the input wave (sinusoidal wave).


Average value of output of the full wave rectifier becomes twice that of the half wave rectifier output:

VA = 2Vpi/p

Full wave rectifiers are of two types:

i) Center-tapped Full-Wave Rectifier:

To the secondary of a center-tapped transformer, two diodes connected.
Between the center tap and each secondary half of Vin  is shows up.
Only one of the diodes is forward biased, at any point in time.
A continuous conduction  of current through load will flows due to this.


The peak inverse voltage (PIV) across then diode  D2 is:

PIV = (Vpi(sec)/2 – 0.7) – (-Vpi(sec)/2)
      = (Vpi(sec)/2 + Vpi(sec)/2 – 0.7)
      = Vpi(sec) – 0.7

Since we know that,

Vpi(out) = Vpi(sec)/2 – 0.7, we get:
Vpi(sec) = 2Vp(out) + 1.4

SO the PIV across each diode will be:

PIV = 2Vpi(out) + 0.7 V

ii) Bridge Full-Wave Rectifier:


The diodes D1 and D2 are forward biased during the positive half cycle of the input. 
The diodes D3 and D4 are the conducing diodes during the  negative half cycle of the input.
The output voltage can be written as:
The PIV is a lot smaller, we can use a full bridge rectifier than a center-tap:

PIV = Vpi(out) + 0.7 Volts

Friday, 5 April 2013

Half Wave Rectifiers Theory and Circuit Operation

Half Wave Rectifiers Theory and Circuit Operation:

In order to convert an AC voltage into DC voltage, rectifiers are used  In all power supplies that operate from an ac voltage source, rectifiers are used 

1) Basic Power Supply:

The power supply converts the standard 110 Vac (AC voltage) into a constant dc voltage. All electronic devices such as (TVs, VCRs, DVDs, etc) have at least one rectifier circuit. A rectifier circuit is used to convert an ac input voltage to a pulsating dc voltage. The filter circuit can eliminates the fluctuations in the rectified output voltage. The regulator circuit is used for maintaining a constant dc voltage for various inputs and load resistances. The circuit (or device) receiving power from the source s called as the load .
Figure: Block diagram of a rectifier ciruit and a dc power supply with a load
a) Half-Wave Rectifier :

The half-wave rectification process is illustrated below.

Figure: Operation of a half-wave rectifier circuit
–The diode is forward biased, when the sinusoidal input (Vin) goes positive thus the diode conducts current on the positive half cycle of the input voltage. This will makes the output voltage keeps as the shape of the input voltage.

– When the input voltage (Vin ) becomes negative (second half of cycle), the diode is reverse biased. (This is due to the condition that in order to make a diode as forward biased condition; a higher voltage should be applied at the positive side (P side) of the diode compared to the negative part of the diode. In other words if a higher voltage is applied at the P side of the diode, it conducts).

When the diode is in the reverse biased state, there is no current.

AT this time, the voltage across the load resistor RL is 0V.

SO the net result is a pulsating dc voltage with the same frequency as the input voltage.

The average value of the pulsating wave is

VAVG = Vp/p
Since the cut off voltage for a silicon diode is 0.7 Volts, we also have to take the 0.7 V from the barrier potential into account.

Thus we should get:
Vp(out) = Vp(in) – 0.7 V

a) Peak Inverse Voltage (PIV):  PIV equals the peak value of the input voltage. In other words PIV can be defined as the maximum reverse voltage that can be with stand by the diode. After the PIV voltage, the diode will get damaged or (breakdown condition occurs after PIV).

A diode must be able to withstand this amount of the applied repetitive reverse voltage.

PIV = Vp(in)

Thursday, 4 April 2013

Power Supply Filters and Regulators

Power Supply Filters and Regulators: 

We know that a pulsating dc wave is the output of the rectifier circuit. Our ultimate aim is to obtain a constant dc output. In order to obtain the constant dc output, we need to filter out the oscillations from the pulsating dc wave. with the help of  a diode capacitor combination, this can be achieved.

The charging and discharging of a capacitor-input filter is such that it fills in the “gaps” between each peak value. The variations of voltage can be reduced by this action. This variation in voltage can be defined as  ripple voltage.

We know that the performance (advantage) of a full-wave rectifier over a half-wave is much good. When the time between peaks is shorter, the capacitor can more effectively reduce the ripple.


The capacitor appears as a short circuit While charging.
This will leads to a large current flow through the diodes.
A surge resistor (Rsurge) is added, in order to avoid damaging the devices.
The surge resistance value (Rsurge)should be small in comparison to the load resistor (RL).

We can use IC voltage regulator, to effectively reduce the ripple occurring after filtering.

We know that a regulator consists of 3 terminals: input, output and reference (or adjust) terminal. It is better to add capacitors after (and before) the regulator circuit. Further filtering of the signal can be done by the help of a large capacitor between the input voltage and the input terminal.

A smaller capacitor is added after the regulator in order to improve transient response.


Examples of positive output regulators are the 78XX series .
Examples of negative output regulators are the 79XX series.

Type Number
(Series)
Output Voltage
(in Volts)
7805
5 V
7806
6 V
7808
8 V
7809
9 V
7812
12 V
7815
15 V
7818
18 V
7824
24 V

Voltage regulation can be measured by two means:
Line regulation : For a given change in input voltage, how much change occurs in the output voltage.
Line regulation = (DVout/DVin)*100%
Load regulation : The rate of output voltage change over a certain range of current values: minimum (no load, NL) to maximum current (full load, FL).
Load regulation = (VNL – VFL)/VFL 100%

Wednesday, 3 April 2013

Diode Limiting and Clamping Circuits

Diode limiting and Clamping Circuits
.
1) Limiters:

The Diodes can be used to clip off (To cut the waveform) portions of signal voltages (above or below certain specified levels). If we want to get a wave form which is in the form of clip off, we can generate that types of wave forms by the help of diodes. In electronic circuits, diodes are considered as the limiting circuit.


The diode will become forward biased as soon as VA (The voltage across the positive side of the diode) becomes larger than VBIAS+0.7. When diode is in forward biased condition, the voltage VA cannot become larger than VBIAS + 0.7 V (Since the cut off voltage for silicon diode is +0.7 V, in order to turn the diode ON we need to apply a positive voltage of +0.7 Volts at the positive part of the diode for the diode to get forward biased).

Thus, the voltage across the load resistance RL, will also be equal to VBIAS + 0.7.

When diode is in reverse biased condition, it appears as an open circuit (since the positive part of the diode is always getting a negative voltage in the reverse biased condition), so the output voltage is the voltage of RL alone will be obtained.

With the help of a voltage divider, the desired (required) voltage levels can be attained.


Here we replace the voltage source with a resistive voltage divider circuit.

VBIAS = R3/(R2 + R3) VSUPPLY ;  

VSUPPLY is the supply voltage.

Example wave form for diode limiter:


b) Diode Clampers :

Any device that adds a dc level to an ac voltage is called as diode clampers.

Diode clampers are also called as dc restorers.

The diode is forward biased, when the input voltage goes initially negative,

The capacitor charges to near peak voltage of input voltage (Vp(in) – 0.7).

After the negative peak voltage, the diode is reverse biased (because cathode is held near Vp(in) – 0.7 by charge on the capacitor).

Capacitor can discharge through the load resistance (RL) only.

The capacitor discharges very little in each period, since the load resistance has high resistance.

The time constant should be large enough (at least 10 times the period of the input voltage).

The capacitors can acts like a battery in series with the input voltage, since the capacitor retains charge.

Tuesday, 2 April 2013

Voltage Multipliers Using Diodes and Capacitors

Voltage Multipliers Using Diodes and Capacitors:

To increase peak rectified voltages, voltage multipliers use clamping action without increasing input transformer’s rating.
The commonly used multiplication factors of 2, 3, and 4.
 Voltage multipliers are generally used in high-voltage, low-current applications.
i) Voltage doubler.
–There are mainly two types of voltage doublers:

1) Half-wave doubler:


During the positive half-cycle of the applied secondary voltage at the input side, diode D1 is forward-biased and D2 is reverse-biased.
The Capacitor C1 is charged to the peak (Maximum) of the secondary voltage (Vp) less diode drop
 During the negative half-cycle of the input voltage, diode D2 is forward-biased and D1 is reverse-biased.
At this time, C1 cannot discharge.
So, C1’s voltage adds to the applied input secondary voltage for charging C2 to approximately 2Vp.
Under zero-load conditions, the capacitor C2 remains charged.
If a load is added, then the capacitor C2 will discharge through the load on the next positive half-cycle. Only recharged in the following negative half-cycle.
Obtained wave form is a half-wave, capacitor-filtered voltage.
The Peak Inverse Voltage (PIV) across each diode is 2VP.

2) Full-wave doubler:


– When the applied input secondary voltage is positive, the diode D¬1 is forward biased and C1 charges to approximately Vp.
– During the negative half-cycle, the diode D2 is forward biased (FB) and C2 charges to approximately V¬p.
- Output voltage can be taken across the two capacitors which is in series connection.

3) Voltage tripler:

The circuit diagram for voltage tripler is exactly same as the half-wave doubler, but another diode-capacitor pair is added to the circuit.

Diode Data Sheet & its Electrical Characteristics


The data sheets for diodes Provides maximum ratings, electrical characteristics, mechanical data, graphs of parameters, etc. for electrical device which are currently in use.
We can see several parameters in a data sheet for diodes:

Maximum ratings of Diodes:

i) VRRM: (Peak repetitive reverse voltage).
–The Peak repetitive reverse voltage can be defined as the maximum reverse peak voltage that can be applied repetitively across the diode.

ii) VR: (DC blocking voltage)
DC blocking voltage can be defined as the maximum reverse dc voltage that can be applied across the diode.

iii) VRSM: (Nonrepetitive peak reverse voltage)
Non repetitive peak reverse voltage is the maximum reverse peak value of non repetitive voltage that can be applied across the diode.

iv) IO: (Average rectified forward current)
 IO is the maximum average value of a 60Hz rectified forward current.

v) IFSM: (Nonrepetitive peak surge current)
IFSM is the maximum peak value of non repetitive (of one cycle) forward surge current..

vi) TA:
– Ambient temperature can be denoted by TA.

vii) TJ:
– Operating junction temperature range can be defined by TJ.

viii) Tstg:
Tstg is the storage junction temperature range.

Electrical Characteristics of a diode :

vF: vF can be defined as the instantaneous voltage across the diode in forward-biased condition when forward current is 1 A at 25oC. That can be generally shown by the help of a graph.

VF(avg): VF(avg) is the maximum forward voltage drop averaged over a full cycle.

IR: IR is the maximum current occurred when the diode is in reverse-biased condition.

IR(avg): IR(avg) can be defined as the maximum reverse current averaged over one cycle (when reverse-biased with an ac voltage).

The Transistor as an Amplifier

The process of linearly increasing the amplitude of an electrical signal is called as Amplification. The signal can be defined as anything which carries some relevant information. By using the gain , a transistor can act as an amplifier. Amplifier is any device which strengthens the amplitude of a signal. The important applications of amplifier are in loud speakers, Bio medical applications etc. When a transistor gets biased in the active (linear) region, the Base-Emitter (BE) junction has a small (Low) resistance due to the forward biasing condition and due to reverse biasing condition; the Base-Collector (BC) junction has a high (Large) resistance.

The Speculations used for the design of transistor as an amplifier are:

i) Consider the DC and AC quantities:

1. Both ac and dc quantities are used in amplifier circuits.
2. For both ac and dc currents capital letters are used.
3. For dc quantities subscript will be capital letter (Uppercase).
4. For ac quantities Subscript will be lowercase (Small letters) .

ii) Transistor Amplification :

– Since the collector current is equal to the base current multiplied by the current gain (, a transistor can amplify the current.
The Base current (IB) is very small (comparatively very low) compared to IC and IE.
– So it is clear that, IC is almost equal to IE.

Now we can consider the following circuit for the transistor application


– From the circuit, it is clear that at the input side, an ac voltage Vin¬, is superimposed on the applied dc bias voltage VBB.

– Through the collector resistance, RC, the Dc bias voltage VCC is connected to the collector.

– The ac input voltage can produces an ac base current (at the Base), which will results in a much higher ac collector current than the base current.

– The ac collector current yields to an ac voltage across RC (The collector resistance), will produces an amplified, inverted, reproduction of the ac input voltage in the active region of the transistor.

To the ac wave, the forward biased base-emitter junction present at low resistance.
This internal ac emitter resistance can be denoted (represented) by r’e.
Ie (or) Ic = Vb/ r’e
Vc = Ic*RC , which is the ac collector voltage,
Since Ie ? Ic, the ac collector voltage can be Vc ? IeRC.
Vb = Vin – IbRB, where Vb can be considered as the ac input voltage of the transistor
The transistor ac output voltage can be considered as Vc

– The ratio of Vc to V¬b (Vc /V¬b) can be defined as the ac voltage gain Av, of the above  transistor circuit.
ie, Av = Vc/Vb       (1)
Now we can Substitute IeRC for Vc and Ie r’e for Vb  which will make (1) as
Av = Vc/Vb ? (IeRC)/(Ie r’e) = RC/ r’e     (2)

– Thus from (2) it is clear that the amplification depends on the ratio of RC and r’e.

– RC will be always much larger in value than r’e, In other words, the output voltage is larger than the input voltage.

Now we can consider one example for the transistor circuit:
Determine the voltage gain and the ac output voltage for the following circuit if r’e = 120 .


Solution:
We know that the voltage gain is
Av ? RC/r’e = 1 k /120  = 8.333
Hence the output voltage is
Vout = AvVb = (8.333)*(100 mV) = 0.8333 Vrms