Analog and Digital Communications

What is the analog and digital communications? Draw the basic block diagram of transmitter and

Answer: Analog and digital communication

Analog communication :

Analog communication is that types of communication in which the transmitted message or information signal is analog in nature. This means that in analog communication the modulating signal i.e. base‐band signal is an analog signal. This analog message signal may be obtained from sources such as speech, video shooting etc.

Digital communication :

In digital communication, the message signal to be transmitted is digital in nature. This means that digital communication involves the transmission of information in digital form. The immunity to channel noise and external interference is better than analog communication.

Block diagram of transmitter and receiver :

block diagram of transmitter and receiver

  1. Radio transmitter : A radio transmitter consists of several elements that work together to generate radio waves that contain useful information such as audio, video, or digital data.

(i)  Power supply : It provides the necessary electrical power to operate the transmitter.

(ii) Oscillator : It creates alternating current at the frequency on which the transmitter will transmit. The oscillator usually generates a sine wave, which is referred to as a carrier wave.

(iii) Modulator : It adds useful information to the carrier wave. There are two main ways to add this information. The first, called amplitude modulation or AM, makes slight increases or decreases to the amplitude of the carrier wave. The second, called frequency modulation or FM, makes slight increases or decreases the frequency of the carrier wave.

(iv) Amplifier : It amplifies the modulated carrier wave to increase its power. The more powerful the amplifier, the more powerful the broadcast.

(v) Antenna : It converts the amplified signal to radio waves.

  1. Radio receiver : A radio receiver is the opposite of a radio transmitter. It uses an antenna to capture radio waves, processes those waves to extract only those waves that are vibrating at the desired frequency, extracts the audio signals that were added to those waves, amplifies the signals.
  •  Antenna : It captures the radio waves. Typically, the antenna is simply a length of wire. When this wire is exposed to radio waves, the waves induce a very small alternating current in the antenna.
  •  RF amplifier : A sensitive amplifier that amplifies the very weak radio frequency (RF) signal from the antenna so that the signal can be processed by the tuner.
  •  Tuner : A circuit that can extract signals of a particular frequency from a mix of signals of different frequencies. On its own, the antenna captures radio waves of all frequencies and sends them to the RF amplifier, which dutifully amplifies them all.
  •  Detector : It is responsible for separating the audio information from the carrier wave. For AM signals, this can be done with a diode that just rectifies the alternating current signal.
  •  Audio amplifier : This componentʹs task is to amplify the weak signal that comes from the detector so that it can be heard. This can be done using a simple transistor amplifier circuit.

Photovoltaic Cell

What is photovoltaic cell (PV Cell)? What are the Characteristics of

PV Cell?


Photovoltaic Cell

The Photovoltaic cells (PV cell) is also made of semiconducting material such as silicon. Basically, when the photon (light) strikes to the cell, a certain amount of light is absorbed within the semiconducting material and produce electricity. This means that the energy of the absorbed light is transferred to the semiconductor. The energy knocks electrons loose, allowing them to flow freely. The name photovoltaic (PV) implies photo meaning ‘light’ and voltaic meaning ‘electricity’.

So, a photovoltaic cell is a semiconductor diode that converts visible light into direct current. Some photovoltaic cell can also convert infrared (IR) or ultraviolet (UV) radiation into DC electricity. Photovoltaic cells are an integral part of solar-electric energy systems, which are becoming increasingly important as alternative sources of utility power.

Photovoltaic cell

The photovoltaic cell characteristics depends on three basic variables; intensity of solar radiation, temperature and area of the cell. The intensity of solar radiation has no significant effect on the open circuit voltage; vice versa the intensity of the short-circuit current varies in proportion to the varying intensity of the irradiation, increasing as this increases.

PV Cell Characteristics

  • Regardless of size, a typical PV cell produces about 0.5-0.6 DC volt under open-circuit, with no load condition.
  • The current and power output of a PV cell depends upon size (surface area) and efficiency and intensity of light source striking the surface of the cell.
  • For example, under peak sunlight condition, a typical commercial PV cell with a surface area 0f 25 inch2 and V = 0.5-0.6 VDC will produce about 2 watts peak power.
  • If the sunlight intensity were 40% of peak, this PV cell would produce about 0.8 watts.

Zener Diode

What is Zener diode in physics? How does a zener diode work? Why zener diode is used as a voltage regulator?


Zener diode

A scientist Zener invented the properties of Zener diode that’s why it is named after him. It is nothing but a simple diode connecting in reverse bias. So the Zener diode is a PN junction diode which flow current in the forward direction similarly to the conventional PN junction diode but in the reverse biased it also permits flow of current (dramatic increase in current) when the applied voltage reached at above a certain value; This value of voltage is known as the Zener knee voltage or Zener breakdown voltage, or peak inverse voltage.

The electron quantum tunneling under high electric field strength is responsible for the reverse breakdown effect and this effect is known as the Zener effect.

Zener Diode

Application of Zener Diode

The constant reverse voltage of a Zener diode renders it a very useful component in regulating the output voltage against variations in the Input voltage or load resistance from an unregulated power supply. The current through the Zener diode will change in order to keep the voltage within the threshold limits of Zener action and the maximum power that it can dissipate.

With the help of these types of properties we can use Zener diode as:

Voltage regulator, peak clippers, switching operations, reference elements,  Over voltage protector, as voltage reference, surge suppressors for device protection, and in meter protection etc.

Photo Diode

What is Photo Diode and How does the photo diode work? What are the applications of Photo Diode?


Photo Diode

A photo diode is a semiconductor device with a p-n junction and an intrinsic layer between p and n layers that converts light into current. Photo-diode produces photo-current by generating electron-hole pairs, due to the absorption of light in the intrinsic or depletion region. The photo-current thus generated is proportional to the absorbed light intensity. Photo-diodes usually have a slower response time as their surface area increases. Photo diodes can be used in two ways — in a photo-voltaic or photo conductive role. To use a photodiode in its photo-conductive mode, the photo-diode is reverse-biased; the photo diode will then allow a current to flow when it is illuminated.

Principle operation of Photo-diode

A photodiode is a PN junction, When a photon of sufficient energy (<1.1ev) strikes the diode, it creates an electron-hole pair. The intensity of photon absorption depends on the energy of photons – the lower the energy of photons, the deeper the absorption is. This mechanism is also known as the inner photoelectric effect. If the absorption occurs in the depletion region of the p-n junction, these hole pairs are swept from the junction – due to the built-in electric field of the depletion region. Thus holes move toward the anode, and electrons toward the cathode, and a photocurrent is produced. The total current through the photo-diode is the sum of the dark current (Dark current is a generated current in the absence of light) and the photo-current, so the dark current must be minimized to maximize the sensitivity of the device.

Figure: Biasing arrangement and Symbol of Photo diode

Operation modes of Photo-diode

Photo-diodes can be operated in different modes like Photovoltaic mode, Photoconductive mode, Avalanche diode mode.

Operation modes of Photo diode

  1. Photo-conductive mode – In this mode device used in reverse biased. The depletion layer increased with the help of reverse voltage, which in turn reduces the response time and capacitance of the junction. The Photo-conductive mode is very fast, and exhibits electronic noise
  2. Avalanche diode mode – This mode also used in a high reverse bias condition, which allow multiplication of an avalanche breakdown to each photo-generated electron-hole pair; which gradually increases the responsivity of the device.
  3. Photo-voltaic mode – In this mode voltage is generated by the illuminated photodiode and it is also known as zero bias mode. It provides a small dynamic and non-linear dependence of the voltage produced.

Applications of Photo diode

  • Surveying instrument
  • Safety equipment
  • Cameras
  • Bar code scanners
  • Medical devices
  • Optical communication devices
  • Position sensors
  • Automotive devices

Two important characteristics of a photodetector are its quantum efficiency and its responsivity. These parameters depend on the material band gap, the operating wavelength, and the doping and thickness of P-N regions of the device.


It is defined as the ratio of the number of electron-hole pairs (ehp) generated to the total number of incident photons and is given by

η= Number of ehp’s generated/total no. of incident photons

η= Ip/q/Pop/h𝛎          or

η= Ip x h𝛎/q x Pop

where Ip is the photo-generated current, Pop is the incident optical power, h𝛎 is  the photon energy and q is the free carrier charge.


It is defined as the ratio of photo generated current to the incident optical is given by the following formula:


Responsivity is related to the quantum efficiency by


This parameter is quite useful, since it specifies the photocurrent generated per unit optical power.


Problem 1: The responsivity of a photodiode is 0.85 A/W and the i/p saturation is 3.5 mW. What is the photocurrent if the incident light power is (a) 1mW?

Solution: Given that photodiode responsivity  Ʀ = 0.85 A/W

Pop = 1 mW

Ʀ = IP / Pop

IP =   Ʀ =Pop

Ip = 0.85 x 1

IP = 0.85 mA

Problem 2: What is the responsivity of an InGaAs photodiode if its quantum efficiency is equal to 70%? The energy gap of InGaAs is 0.75 eV.

Solution: Given that energy gap of InGaAs is 0.75 eV; quantum efficiency = 70%.

According to Einstein energy Eg = hc/l;

l = 6.6 x 10-34 (J-s) x 3x 108 (m/s) / 0.75 (ev) x 1.6 x 10-19

l = 1664X 10-9 m

So, the reponsivity in terms of efficiency is defined by

Ʀ = (h/1248) l

Ʀ = (0.70/1248) 1664 x 10-9

Ʀ =0.933 A/W

Hall Effect: Hall coefficient, Hall potential, Hall Voltage

What is Hall Effect? Derive an expression for Hall coefficient, Hall potential, Hall Voltage and Hall angle. What are the importance of the Hall effect?


Hall Effect

The Hall Effect discovered by Edwin Hall in 1879. He state that “when we flow a current along the length of the conducting  ribbon (conductor or semiconductor) and placed it into the transverse magnetic field, then an electric potential developed across the ribbon. The direction of electric potential is perpendicular to applied current I and magnetic field B. The electric potential is known as Hall potential and generated field is known as Hall Field and this effect is known as “Hall Effect”.

Hall Effect

If we take a conducting ribbon flowing current in +ve X-direction and the magnetic field is applied in the +ve Z-direction. A force is experienced on charge carrier in the downward direction of conducting ribbon means –ve Y- direction. Through this force the +ve charges arranged at upper edegs and –ve charges arranged at bottom edges of the ribbon. The separation of charge carrier developed the electric field inside the ribbon in the direction of Y.

If we consider a ribbon has a width W and thickness is T. There are two forces acts on electron due to electric field and magnetic field


Electric field force on electron = -eEH                                                                                  (1)

Magnetic field force on electron = -eVxBZ                                                                                 (2)


In equilibrium condition,   -eEH = -eVxBZ

EH = VxBZ                                                                                                                (3)

Now we define current density (Current density is a measure of the density of an electric current.) in the direction of X i.e.

Jx = -neVx      ( n = no. of charge carrier / unit Volume)

Vx = – Jx / ne                                                              (4)

Fron equation (4) and (3)

EH = -BZ Jx / ne = -(1/ne) BZ Jx                                  (5)

RH = -(1/ne)                                             (6)

RH  is known as Hall Coefficient.


RH = EH / BZ Jx                                                                              (7)

If we take length L and breadth b of the conducting ribbon and hall field EH and developed potential is VH

Then EH = VH / L                                                     (8)

RH = VH / L BZ Jx

VH = RH L BZ Jx                                                                             (9)

Again current density is equal to the current per unit area

i.e Jx  = Ix/ LB


VH = RH BZ Ix/ B                          (10)

VH  is known as Hall Potential.

Calculation of Hall angle and Mobility of charge carrier:

The charge carrier mobility is equal to the drift velocity per unit electric field

i.e. µ = Vx/ E

Since for equation (3) EH = VxBZ  

then EH = µEBZ


Since EH = RH BZ Jx

Compare both equation µEBZ  = RH BZ Jx

µ= RH Jx/ E                                                                                        (11)

Mobility µ= sRH                              (s = Jx/ E current density per unit electric field)

Since EH = µEBZ

µ= EH  / EBZ

qH = EH  / E is known as Hall angle.


The importance of the Hall effect is supported by the need to determine accurately

  • carrier density.
  • electrical resistivity.
  • and the mobility of carriers in semiconductors.
  • Hall Effect proved that electrons are the majority carriers in all the metals and n-type semiconductors.
  • In p-type semiconductors, holes are the majority carriers.


Problem 1: Find Hall coefficient for 5 x 1028 atom / m3 in copper block.

Solution: RH =-1/ne

RH = -1/5 x 1028 x 1.6 x 10-19

RH = -0.125 x 10-9 m3/C

Problem 2: Calculate mobility and charge carrier density when the resistivity of doped Si sample is 9 x 10-3 Ω–m and the hall coefficient is 3.6 x 10-4 m3/C.

Solution: Given that  r =9 x 10-3 Ω–m; RH = 3.6 x  10-4 m3/C

So conductivity s = 1 / r = 1/ 9 x 10-3 /Ω–m

s = 111.1 / Ω–m

r = ne;

n = r/e = 1/ RHe

n = 1/ 1.6 x 10-4 x 1.6 x 10-19

n = 1.7363 x 1022 / m3

Mobility µ = RH s

µ = 111.1 x 3.6 x  10-4

µ = 0.04 m2/V-s

Band Theory of Solids

What is the Band Theory of Solids?


Band Theory of Solids

In case of solids material all the atoms are close to each other, so the energy levels of outermost orbit electrons are affected by the neighboring atoms. When the isolated atom or two single atoms are bring close to each other then the outermost orbit electrons of two atoms are interact with each other. i.e, the electrons in the outermost orbit of one atom experience an attractive force from the nearest or neighboring atomic nucleus.  Due to this the energies of the electrons will not be in same level, the energy levels of electrons are changed to a value which is higher or lower than that of the original energy level of the electron. The electrons in same orbit exhibit different energy levels. The grouping of these different energy levels is called energy band.

Band overlap will not occur in all substances, no matter how many atoms are close to each other. In some substances, a substantial gap remains between the highest band containing electrons is known as Valence band and the next band, which is empty is known as conduction band. As a result, valence electrons are “bound” to their constituent atoms and cannot become mobile within the substance without a significant amount of imparted energy. So, the band gap is the minimum amount of energy required for an electron to break free of its bound state. When the band gap energy is met, the electron is excited into a free state, and can therefore participate in conduction. Similarly the electronic band structure of solids, the band gap generally refers to the energy difference between the top of the valence band and the bottom of the conduction band.

Band Thoery of Solids

In terms of Band Gap energy solids are in 3 categories: Metal, Insulator and Semiconductor. Band Theory of Solids

The electrical properties of a given material depend on the electronic populations of the different allowed energy bands. Electrical conduction is the result of electron motion within each band. When an electric field is applied to the material, electrons start to move in the direction opposed to the direction of the electric field. An empty energy band (in which there is no free electron) does not of course participate in the formation of an electric current. It is also the case for a fully occupied band.


According to the band theory of solids this implies that there is a no energy gap between the energies of the valence electrons i.e valance band and the energy at which the electrons can move freely through the material i.e the conduction band. For a conductor, conduction bands and valence bands are not separated and there is therefore no energy gap. The conduction band is then partially occupied (even at low temperatures), resulting in a “high” electrical conductivity.

Electrons in metals are also arranged in bands, but in a metal the electron distribution is different – electrons are not localized on individual atoms or individual bonds. In a simple metal with one valence electron per atom, such as sodium, the valence band is not full, and so the highest occupied electron states lie some distance from the top of the valence band. Such materials are good electrical conductors, because there are empty energy states available just above the highest occupied states, so that electrons can easily gain energy from an applied electric field and jump into these empty energy states. Band Theory of Solids


In terms of the band theory of solids this implies that there is a large gap between the energies of valance band and conduction band. Glass is an insulating material which may be transparent to visible light for reasons closely correlated with its nature as an electrical insulator. The visible light photons do not have enough quantum energy to bridge the band gap and get the electrons up to an available energy level in the conduction band.


Materials that fall within the category of semiconductors have a narrow gap between the valence and conduction bands. Thus, the amount of energy required to motivate a valence electron into the conduction band where it becomes mobile is quite modest. For Pure semiconductors like silicon and germanium, the Fermi level is essentially halfway between the valence and conduction bands. Although no conduction occurs at 0 K, at higher temperatures a finite number of electrons can reach the conduction band and provide some current. In doped semiconductors, extra energy levels are added. The increase in conductivity with temperature can be modeled in terms of the Fermi function, which allows one to calculate the population of the conduction band

Key Features:

  • Metals have free electrons and partially filled valence bands, therefore they are highly conductive; at last conductors are materials with high conductivities (like silver: 106S/cm)
  • Insulators have filled valence bands and empty conduction bands, separated by a large band gap Eg (typically >4eV), they have high resistivity; Insulators are materials having an electrical conductivity order of 10^-8 s/cm (like diamond: 10^-14S/cm)
  • Semiconductors have similar band structure as insulators but with a much smaller band gap. Some electrons can jump to the empty conduction band by thermal or optical excitation  Eg=1.12 eV for Si, 0.67 eV for Ge and 1.43 eV for GaAs; semiconductors have a conductivity order of 10^-8 to 10^3 s/cm (for silicon it can range from 10^-5S/cm to 10^3S/cm). Band Theory of Solids

Problem 1: The mobility of free electron & holes in pure Ge are 0.38 & 0.18 m2/Vs. The corresponding values for pure Si are 0.13 & 0.05 m2/Vs. Determine the value of intrinsic resistivity for both Ge an Si. If ni for Ge = 2.5 x 1019 /m3 & for Si = 1.5 x 1016 /m3 at room temperature.

Solution: Given That : ni for Ge = 2.5 x 1019 /m3 ; ni for Si = 1.5 x 1016 /m3;

For Ge : mobility of ee)=0.38 m2/Vs ; mobility of Holes (µh)= 0.18 m2/Vs

The intrinsic conductivity of Ge semiconductor is:

Solid Material: Crystalline solids and Amorphous solids

What are the types of solids? Explain Crystalline and amorphous solids.


As we know that the matter is a substance that has inertia and occupies physical space and in the view of modern physics matter consists of various types of particles, each with mass and size. The matters can exist in several states like solid, liquid and gas. Less familiar states of matter include plasma, foam and Bose-Einstein condensate. These states occur under special conditions. All matter such as solids, liquids, and gases, is composed of atoms. Therefore, the atom is considered to be the basic building block of matter. Atoms are very small and typical sizes are around ~100 pm. A combination of atoms forms a molecule. Atoms and/or molecules can join together to form a compound.

Solid Material:

In the base of structure and some specific properties solids are categorized in 6 main types i.e Ionic Solids, Metallic Solids, Network Atomic Solids, Atomic Solids, Molecular Solids and Amorphous Solids. In a broad sense, solids may be categorized as crystalline solids or amorphous solids. Crystalline solids have regular ordered arrays of components held together by uniform intermolecular forces, whereas the components of amorphous solids are not arranged in regular arrays. The intermolecular forces in a solid can take a variety of forms. If we take a crystal of NaCl (common salt) is made up of ionic sodium and chlorine, which are held together by ionic bonds. In diamond or silicon, the atoms share electrons and form covalent bonds. In metals, electrons are shared in metallic bonding. Some solids, particularly most organic compounds, are held together with van-der Waals forces resulting from the polarization of the electronic charge cloud on each molecule. The dissimilarities between the types of solid result from the differences between their bonding. Solid Material

Now we discuss about Crystalline solids and Amorphous solids.

Crystalline solids,

or crystals, have distinctive internal structures that in turn lead to distinctive flat surfaces, or faces. The faces intersect at angles that are characteristic of the substance. When exposed to x-rays, each structure also produces a distinctive pattern that can be used to identify the material. The characteristic angles do not depend on the size of the crystal; they reflect the regular repeating arrangement of the component atoms, molecules, or ions in space. Solid Material


Amorphous solids

have two characteristic properties. When cleaved or broken, they produce fragments with irregular, often curved surfaces; and they have poorly defined patterns when exposed to x-rays because their components are not arranged in a regular array. Almost any substance can solidify in amorphous form if the liquid phase is cooled rapidly enough. Some solids, however, are intrinsically amorphous, because either their components cannot fit together well enough to form a stable crystalline lattice or they contain impurities that disrupt the lattice; they reflect the irregular arrangement of the component atoms, molecules, or ions in space.

So that the key difference between the crystalline and amorphous structure is the ordering of the structure. Crystalline structure can be thought of as the highest level of order that can exist in a material, while an amorphous structure is irregular and lacks the repeating pattern of a crystal lattice.

Solid Material

Properties of Crystalline solids:

  • Crystals have highly ordered (regular periodic arrangements in all direction) three-dimensional arrangements of particles.
  • Crystalline solids have characteristic geometrical shape.
  • Planes of a crystal intersect at particular angles.
  • Crystals have sharp melting and boiling points.
  • Examples: NaCl, Diamond, CuSO4, Graphite, Sugar etc

Properties of Amorphous solids:

  • Solids that don’t have a definite geometrical shape are known as Amorphous Solids.
  • Amorphous solids don’t have sharp melting points.
  • Amorphous solids melt over a wide range of temperature.
  • In these solids particles are randomly arranged (irregular periodic arrangements) in three dimensions.
  • Amorphous solids are formed due to sudden cooling of liquid.
  • Examples: Plastic, rubber, Coal, Glass etc


Types of Solids:

The branch of physics that deals with solids is called solid-state physics.  According to the band gap concept the solid material specified in three categories such as Conductor, Insulator, and Semiconductor.


As we know that Conductors have very low resistivity, usually in the µΩ- metre. This low value allows them to easily pass an electrical current due to there being plenty of free electrons floating about within their basic atomic structure. Similarly the band gaps of conductors are also very low.


An insulator is also known as non metal and which is exact opposite to conductors with high resistivity. The band gap of Insulating material is very high. The Insulator are made of materials that have very few or no “free electrons” floating about within their basic atom structure because the electrons in the outer valence shell are strongly attracted by the positively charged inner nucleus. So if a potential voltage is applied to the material no current will flow as there are no electrons to move and which gives these materials their insulating properties. Examples of good insulators are Plastic, rubber, p.v.c. etc.


Semiconducting materials such as silicon, germanium have electrical properties somewhere in the middle, between those of a conductor and an insulator. They are not good conductors nor good insulators. Semiconductors have very few free electrons because their atoms are closely grouped together in a crystalline pattern called a “crystal lattice”. These types of semiconductor are also known as pure or intrinsic semiconductor. The band gap of Si and Ge semiconductors is 0.7ev and 1.11ev respectively. However, their ability to conduct electricity can be greatly improved by replacing or adding certain donor or acceptor atoms to this crystalline structure thereby, producing more free electrons than holes or vice versa. That is by adding a small percentage of another element to the base material, either silicon or germanium; and also called extrinsic (doped) semiconductors.