SESSION:34

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1)       Given only two XOR gates one must function as buffer and another as inverter? 

Tie one of XOR gates input to 1 it will act as inverter. Tie one of XOR gates input to 0 it will act as buffer.

2)       Differentiate between Mealy and Moore state machine? 

Mealy and Moore models are the basic models of state machines. A state machine which uses only Entry Actions, so that its output depends on the state, is called a Moore model. A state machine which uses only Input Actions, so that the output depends on the state and also on inputs, is called a Mealy model. The models selected will influence a design but there are no general indications as to which model is better. Choice of a model depends on the application, execution means (for instance, hardware systems are usually best realized as Moore models) and personal preferences of a designer or programmer. Mealy machine has outputs that depend on the state and input (thus, the FSM has the output written on edges) Moore machine has outputs that depend on state only (thus, the FSM has the output written in the state itself).

Advantages and Disadvantages

In Mealy as the output variable is a function both input and state, changes of state of the state variables will be delayed with respect to changes of signal level in the input variables, there are possibilities of glitches appearing in the output variables. Moore overcomes glitches as output dependent on only states and not the input signal level. All of the concepts can be applied to Moore-model state machines because any Moore state machine can be implemented as
a Mealy state machine, although the converse is not true. Moore machine, the outputs are properties of states themselves, which means that you get the output after the machine
reaches a particular state, or to get some output your machine has to be taken to a state which provides you the output. The outputs are held until you go to some other state Mealy machine, Mealy machines give you outputs instantly, that is immediately upon receiving input, but the output is not held after that clock cycle.

3)       Tell some of applications of buffer? 

They are used to introduce small delays.

·         They are used to eliminate cross talk caused due to inter electrode capacitance due to close routing.

·         They are used to support high fan-out.

4)       Give two ways of converting a two input NAND gate to an inverter?

·         Short the 2 inputs of the NAND gate and apply the single input to it.

·         Connect the output to one of the input and the other to the input signal.

5)       What is a p-n junction diode? State and explain some of its applications.

A p-n junction is a metallurgical junction formed at the boundary between a p-type and n-type semiconductor created in a single crystal of semiconductor by doping. It acts as a diode as is a two-terminal electronic component with asymmetric transfer characteristics. The following are some of its applications:

Rectifiers: A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. The types of rectifiers are

Half wave rectifier: Uses one p-n junction

Full wave rectifier: Uses two p-n junction.

Signal diodes: These diodes are used in circuits that mix signals together (mixers), detect the presence of a signal (detector), and act as a switch “to open or close a circuit”. 

Tunnel diodes: Very high frequency applications using the tunnel diode are possible because the tunneling action occurs so rapidly that there is no transit time effect and therefore no signal distortion. Tunnel diodes are also used extensively in high-speed switching circuits because of the speed of the tunneling action.

Zener diodes: It is very highly doped acts as a voltage regulator and operates under reverse bias in the breakdown region.

6)       What is Band Gap Theory?

The Band Gap Theory gives us a useful way to visualize the difference between conductors, insulators and semiconductors. 

A plot is made between the available energy levels for electrons in the materials. The available energy states form discrete energy levels instead of a continuous spectra. 

Crucial to the conduction process is whether or not there are electrons in the conduction band. 

In insulators the electrons in the valence band are separated by a large gap from the conduction band.

In conductors like metals the valence band overlaps the conduction band.

In semiconductors there is a small enough gap between the valence and conduction bands that thermal or other excitations can bridge the gap.

An important parameter is the Fermi level energy. The position of the Fermi level with the relation to the conduction band is a crucial factor in determining electrical properties.

7)       What is the construction of a Solar Cell? How is EMF generated?

The construction is explained as follows:

·         A Silicon or Germanium p-n junction diode packed in a can with a glass window on the top.

·         A p-Silicon wafer of about 300 micrometer is taken over which a thin layer (about 0.3 micrometer) is grown on one side.

·         The other side is coated with a metal. This acts as a “Back Contact”.
On top of the n-Silicon layer, a metal finger electrode is deposited. This acts as a “Front Contact”.

·         The EMF is generated as:

·         Photons having energy greater than threshold energy (i.e. the energy gap) generate electron-hole pairs.

·         The electron-hole pairs get separated due to the electric field in the depletion region.

·         Electrons reaching the n-region get collected by the front contact while the holes reaching the p-region are collected by the back contact.

Thus, due to the charge accumulation, an EMF is generated.

8)       What do you understand by LEDs? How do they work?

LEDs refer to the term Light Emitting Diode. LEDs are heavily doped p-n junctions which work under forward bias and emit spontaneous radiation. When the p-n junction is forward biased:

·         The excess minority chargers accumulate at the junction boundary on either side.

·         When they recombine, energy is released in the form of photons, having energy equal to or lesser than the band gap energy.

·         The intensity of light emitted is directly proportional to the forward current.

·         The reverse breakdown voltage is low (around 5V).

9)       What are Laser Diodes?

Laser Diodes are compact transistor-like packages with two or more electrical leads. Inside the package is a small semiconductor chip, containing the laser cavity. The chip contains a stack of layers with different doping and compositions that provide optical and electrical confinement.
Lasing occurs when stimulated emission causes the amplification of photons confined to the lasing mode. Photons bounce back and forth between the front and back mirror.

Hence, a diverging beam emerges from the laser diode package.

They have applications in the telecommunications industry and in testing instrumentation.

10)    What is a rectifier, what are its various types?

A rectifier is an electronic device that changes alternating current into direct current. This process is called rectification.

The three main types of rectifier are as follows:

Half-wave rectifier: It is the simplest type of rectifier, which is made with just one diode.

·         When the voltage of the alternating current is positive, the diode becomes forward-biased and current flows through it.

·         When the voltage is negative, the diode is reverse-biased and the current stops.

·   The result is a cropped copy of the alternating current waveform with only positive voltage. This pulsating direct current is adequate for some components, but others require a more steady current. 

Full-wave rectifier: This rectifier is essentially made of two half-wave rectifiers, and can be made with two diodes and an earthed centre tap on the transformer. The centre tap allows the circuit to be completed because current cannot flow through the other diode. 

·         When the voltage of the alternating current is positive, one of the diodes become forward biased whereas the other gets reverse biased. Hence, current flows through the forward biased diode.

·         When the voltage of the alternating current is negative, the previous reverse biased diode becomes forward biased whereas the other gets reverse biased. Hence, current flows through the forward biased diode.

·         Thus, current flows at least through one of the diodes at a time.
Therefore, the result is still a pulsating direct current but with double the frequency.

Bridge rectifier: A bridge rectifier makes use of four diodes in a bridge arrangement to achieve full-wave rectification. 

·         The main advantage of this bridge circuit is that it does not require a special centre tapped transformer, thereby reducing its size and cost.

·         The single secondary winding is connected to one side of the diode bridge network and the load to the other side.

·         The result is still a pulsating direct current but with double the frequency.

11)    What is a BJT? What are its advantages and disadvantages?

BJT refers to Bipolar Junction Transistor. It is a three-terminal electronic device constructed of doped semiconductor material. It is called ‘Bipolar’ because charge is carried by both, electrons and holes. Hence, operation of the device is due to bi poles (or opposite charges). Charge flow in a BJT is due flow of charges flow across a junction between two regions of different charge concentrations.

The following are the advantages of BJT:

·         There is high current density.

·         It operates in medium to high Voltage range. 

·         There is low forward voltage drop.

However, the following are some of the disadvantages:

·         It has a very complex base control. Hence, it may lead to confusion and hence, requires skilful handling.

·         The switching time is not very fast compared to the high alternating frequency of voltage and current source.

12)    How do touch screens work?

A touch screen is a surface, usually glass, which signals the microcontroller about the position of the surface being touched. It is a type of input device in which no other external pointing device is required.
Basically, there are two types of touch screens used in everyday life.

Resistive touch: These types of screens are constructed of multiple layers. There are two electrically resistive layers that have a thin gap between them. Whenever voltage is passed through a layer they get detected by the other layer. These types of screens are highly resistant to liquid and are cheap to manufacture. On the downside these screens do not produce enough contrast and may get damaged by sharp objects. They do not support multi touch.

Capacitive touch: These types of screens do not contain various layers instead they have a solid glass which is coated with some form of a conductor. It works on the principle of electric fields. The capacitive type of touch screens can support multiple touches in one go, something that is not possible in resistive screens.

13)    What is Power Electronics?

Power Electronics refers to a subject of research in electrical engineering which deals with design, control and computation of nonlinear, time varying energy processing electronic systems which have very high frequency alternating current.

14)    Explain the construction and pros of using plasma screens.

Plasma is ionized gas. Actually the gas is ionized only partly. This is done to a gas by adding energy into the system, that is, by heating the gas. This process also leads to the breaking of the molecular bonds of the gas. What makes plasma so different or maybe special from other forms is that the charges in the gas make the gas extremely responsive to electromagnetic fields.

In plasma TVs the thousands of pixels are made using gases and each pixel is inside electric plates which are controlled by the processing unit. Each pixel is independently lit, allowing us to view it from any angle.

Advantages of plasma screens are as follows:

·         Colors are more vibrant and the blacks are deeper.

·         Viewers sitting at extreme angles can see a decent picture.

·         They usually cost cheaper than LCD TVs but the quality of image may be compensated.

·         They can easily be mounted on the wall as they are flat and thin and clear up much-needed space in your home.

15)    What is the difference between LED and LCD?

A Light-Emitting-Diode (LED) display generates light energy as current is passed through the individual segments.

It refers to the backlight system used in many newer
LCD televisions and not the chips that produce the images. 

A liquid-crystal display (LCD) controls the reflection of available light. Traditional LCD TVs are normally back lit by fluorescent bulbs.

LCD TVs and LED TVs work by blocking light. The following explains how they work:

A solution of TN liquid crystals is inserted between two perpendicularly aligned panes of polarized glass. It is possible to manipulate the intensity of light as it passes through this crystalline matrix and out of the glass panel at the other end by twisting one of the panes w.r.t the other.

Depending on the voltage of the electrical charge running through them, liquid crystals will untwist so that the intensity of light that can pass through the second polarized pane is affected. 

Fundamentally, these displays can switch between light states (where the liquid crystals are fully twisted) and dark states (where the liquid crystals are fully untwisted), or somewhere along the gray scale in between. 

The following are the advantages of LEDs over LCDs:

·         LEDs are longer lasting than fluorescent bulbs.

·         LEDs are more energy efficient than fluorescent bulbs.

·         LEDs give greater contrast and better image quality than LCDs.