Analog And Digital Electronics For Cbcs Degree Students

Syllabus From 2017-18 Academic Year

Paper–VII-(A) Elective (Electronics) Semester –VI Elective Paper –VII-(A): Analog and Digital Electronics No. of Hours per week: 04 Total Lectures:60 Unit-I (14 Hours) 1. FET-Construction, Working, characteristics and uses; MOSFET-enhancement MOSFET, depletion MOSFET, construction and working , drain characteristics of MOSFET, applications of MOSFET 2. Photo electric devices: Structure and operation, characteristics, spectral response Unit-II (10Hours) 3. Operational Amplifiers: Characteristics of ideal and practical Op-Amp (IC 741), Basic differential amplifiers, Op-Amp supply voltage, IC identification, Internal blocks of Op-Amp, its parameter off set voltages and currents. Unit-III (10 Hours) 4. Applications of Op-Amp: Op-Amp as voltage amplifier, Inverting amplifier, Non-inverting amplifier, voltage follower, summing amplifier, difference amplifier, comparator, integrator, differentiator. Unit-IV(14 Hours) 5. Data processing circuits: Multiplexers, De-multiplexers, encoders, decoders, 6. Data processing circuits: Characteristics for Digital ICs -RTL, DTL, TTL, ECL CMOS (NAND & NOR Gates). Unit-V (12 Hours) 7. Sequential digital circuits: Flip-flops, RS, Clocked SR, JK, D, T, Master-Slave, 8. Flip- flop, Conversion of Flip flops. ****************** Elective Paper-VII Practical: Analog and Digital Electronics 2hrs/Week Minimum of 6 experiments to be done and recorded 1) Characteristics of FET 2) Characteristics of MOSFET 3) Characteristics of LDR 4) Characteristics of Op-amp.(IC741) 5) Op-Amp as amplifier/inverting amplifier 6) Op-Amp as integrator/differentiator 7) Op-Amp as summing amplifier/difference amplifier 8) IC 555 as astable multivibrator 9) IC 555 as monostable amplifier 10) Master slave flip-flop 11) JK flip-flop

Model paper B.Sc. DEGREE EXAMINATIONS (Regular) (Examination at the end of Third Year)- Semester-VI Elective Paper VII-A:: Analog & Digital Electronics Maximum: 75 marks Time: Three hours Section – A Answer ALL questions (5x10=50 marks) (Essay Type Questions) 1(a).Write construction and working of FET and its characteristic curves and uses Or 1(b).Explain the structures & working of photo electric devices and & name such devices. 2(a).Differentiate the characteristics of an idea & practical op- amplifier write Internal blocks of op – amplifier. Or 2(b).How op amplifier works as voltage amplifier with blocks diagrams and write its applications. 3(a).Write briefly about non-inverting amplifier summing amplifier comparator voltage followers. Or 3(b).Design a CMOS circuit using NAND such that Y=AB + CD. write characteristics of Digital IC’S. 4(a) what are data processing circuits?. Differentiate multiplexers & de multiplexers. Or 4(b) What are the Characteristics of Digital ICs-Describe RTL and DTL ICs 5(a) Explain the concept of master – slate-flip-flop with necessary diagram & Difference between RS & clocked SR. Or 5(b).Design a circuit to convert JK Flip – Flop to D Flip –Flop what is difference between them. Section – B (5 × 5 = 25 marks) Answer any FIVE questions. 6. Differentiate between enhance MOSFET & Depleti on MOSFET. 7. Explain characteristic is of photo electric devices. 8. What are different types of operational amplifiers? 9. Draw the circuit diagram of differentiator and integrator 10..Explain encodes &decodes with suitable diagrams 11. Draw circuit diagram of T flip flop. 12 .Design NAND & NOR gates using 2:1 multiplexes. 13. Briefly explain the characterictics of different logic families. 14. Write the Applications of MOSFET

A bipolar junction transistor (BJT) is a current controlled device i.e., output characteristics of the device are controlled by base current and not by base voltage. However, in a field effect transistor (FET), the output characteristics are controlled by input voltage (i.e., electric field) and not by input current. This is probably the biggest difference between BJT and FET. There are two basic types of field effect transistors: (i) Junction field effect transistor (JFET) (ii) Metal oxide semiconductor field effect transistor (MOSFET) Junction Field Effect Transistor: A junction field effect transistor is a three terminal semiconductor device in which current conduction is by one type of carrier i.e., electrons or holes. Construction: A JFET consists of a p-type or n-type silicon bar containing two pn junctions at the sides as shown in Fig. The bar forms the conducting channel for the charge carriers. If the bar is of n-type, it is called n-channel JFET as shown in Fig. and if the bar is of p-type, it is called a p-channel JFET as shown in Fig. The two pn junctions forming diodes are connected internally and a common terminal called gate is taken out. Other terminals are source and drain taken out from the bar as shown. Thus a JFET has essentially three terminals viz., gate (G), source (S) and drain (D). Principle. The JFET operates on the principle that width and hence resistance of the conducting channel can be varied by changing the reverse voltage VGS. Working. The working of JFET is as follows : (i) When a voltage VDS is applied between drain and source terminals and voltage on the gate is zero, the two p-n junctions at the sides of the bar establish depletion layers. The electrons will flow from source to drain through a channel between the depletion layers. The size of these layers determines the width of the channel and hence the current conduction through the bar. (ii) When a reverse voltage VGS is applied between the gate and source, the width of the depletion layers is increased. This reduces the width of conducting channel, thereby increasing the resistance of n-type bar. Consequently, the current from source to drain is decreased. On the other hand, if the reverse voltage on the gate is decreased, the width of the depletion layers also decreases. This increases the width of the conducting channel and hence source to drain current. Therefore, the current from source to drain can be controlled by the application of potential (i.e. electric field) on the gate. For this reason, the device is called field effect transistor.

Note 1: a p-channel JFET operates in the same manner as an n -channel JFET except that channel current carriers will be the holes instead of electrons and the polarities of VGS and VDS are reversed. Note 2: If the reverse voltage VGS on the gate is continuously increased, a state is reached when the two depletion layers touch each other and the channel is cut off. Under such conditions, the channel becomes a non conductor. Schematic Symbol of JFET

Drain Characteristic curve: (i) Since IDSS is measured under shorted gate conditions, it is the maximum drain current that you can get with normal operation of JFET. (ii) There is a maximum drain voltage [VDS (max)] that can be applied to a JFET. If the drain voltage exceeds VDS (max), JFET would breakdown as shown in Fig. (iii) The region between VP and VDS (max) (breakdown voltage) is called constant-current region or active region. As long as VDS is kept within this range, ID will remain constant for a constant value of VGS. In other words, in the active region, JFET behaves as a constant–current device. For proper working of JFET, it must be operated in the active region. Pinch off Voltage (VP). It is the minimum drain-source voltage at which the drain current essentially becomes constant. Note: For values of VDS greater than VP, the drain current is almost constant. It is because when VDS equals VP, the channel is effectively closed and does not allow further increase in drain current. It may be noted that for proper function of JFET, it is always operated for VDS > VP. However, VDS should not exceed VDS (max) otherwise JFET may breakdown. Gate-source cut off voltage VGS(off). It is the gate-source voltage where the channel is completely cut off and the drain current becomes zero. Transfer characteristics: As the gate source voltage increases from zero to cutoff voltage, Drain current decreases from maximum value to zero. Difference Between JFET and Bipolar Transistor The JFET differs from an ordinary or bipolar transistor in the following ways :

JFET

BJT

1. Current conduction is only due to one Current conduction by both types of type of carrier either electron or hole. carriers i.e., electrons and holes.

2. Voltage driven device.

Current driven device.

3. Low noise level.

High noise level.

4. High input resistance.

Low input resistance.

5. Gain is characterized transconductance. 6. Better thermal stability.

by Gain is characterized by voltage gain. Less thermal stability.

Expression for Drain Current (ID)

Salient Features of JFET The following are some salient features of JFET : (i) A JFET is a three-terminal voltage-controlled semiconductor device i.e. input voltage controls the output characteristics of JFET. (ii) The JFET is always operated with gate-source pn junction reverse biased. (iii) In a JFET, the gate current is zero i.e. IG = 0A. (iv) Since there is no gate current, ID = IS. (v) The JFET must be operated between VGS and VGS (off). For this range of gate-to-source voltages, ID will vary from a maximum of IDSS to a minimum of almost zero. (vi) The drain current ID is controlled by changing the channel width. (vii) Since JFET has no gate current, there is no β rating of the device. We can find drain current ID. Advantages of JFET (i) It has a very high input resistance of the order of 100 MΩ. This permits high degree of isolation between the input and output circuits. (ii) FET is less noisy than BJT, because no junctions are present like BJT. So, the partition noise is absent i.e., carriers are noise free. (iii) FET is relatively less affected by radiation. (iv) It has better thermal stablility. (v) A JFET has a smaller size, longer life and high efficiency. Applications: 1. High Input Impedance Amplifier 2. Low-Noise Amplifier 3. Differential Amplifier 4. Constant Current Source 5. Analog Switch or Gate 6. Voltage Controlled Resistor. MOSFET Introduction: The main drawback of JFET is that its gate must be reverse biased for proper operation of the device i.e. it can only have negative gate operation for n-channel and

positive gate operation for p-channel. This means that we can only decrease the width of the channel (i.e. decrease the *conductivity of the channel) from its zero-bias size. This type of operation is referred to as depletion-mode operation. Therefore, a JFET can only be operated in the depletion-mode. However, there is a field effect transistor (FET) that can be operated to enhance (or increase) the width of the channel (with consequent increase in conductivity of the channel) i.e. it can have enhancement-mode operation. Such a FET is called MOSFET. Metal Oxide Semiconductor FET (MOSFET) A field effect transistor (FET) that can be operated in both depletion mode and the enhancement-mode is called a MOSFET. Types of MOSFETs There are two basic types of MOSFETs, 1. Depletion-type MOSFET or D-MOSFET. The D-MOSFET can be operated in both the depletion-mode and the enhancementmode. For this reason, a D-MOSFET is called depletion /enhancement MOSFET. 2. Enhancement-type MOSFET or E only MOSFET. The EMOSFET can be operated only in enhancement-mode. Symbols for D-MOSFET There are two types of D-MOSFETs viz (i) n-channel D-MOSFET and (ii) pchannel D-MOSFET. Construction: The gate construction of D-MOSFET. A thin layer of metal oxide (usually silicon dioxide, SiO2) is deposited over a small portion of the channel. A metallic gate is deposited over the oxide layer. As SiO2 is an insulator, gate is insulated from the channel. Note that the arrangement forms a capacitor. One plate of this capacitor is the gate and the other plate is the channel with SiO2 as the dielectric. Since the gate is insulated from the channel, we can apply either negative or positive voltage to the gate. Therefore, D-MOSFET can be operated in both depletion-mode and enhancement-mode. However, JFET can be operated only in depletion-mode. The negative-gate operation is called depletion mode whereas positive-gate operation is known as enhancement mode. (i) Depletion mode. The electrons repel the free electrons in the n-channel, leaving a layer of positive ions in a part of the channel as shown in Fig. In other words, we have depleted (i.e. emptied) the n-channel of some of its free electrons. Therefore, by changing the negative voltage on the gate, we can vary the resistance of the n-channel and hence the current from source to drain. Note that with negative voltage to the gate, the action of D-MOSFET is similar to JFET. Then it is called depletion mode.

(ii) Enhancement mode. Fig. shows enhancement-mode operation of n-channel DMOSFET. Since the gate is positive, it induces negative charges in the n-channel as shown in Fig. These negative charges are the free electrons drawn into the channel. Then, the total number of free electrons in the channel is increased. Thus a positive gate voltage enhances or increases the conductivity of the channel. The greater the positive voltage on the gate, greater is the conduction from source to drain. Transfer characteristics: (i) The point on the curve where VGS = 0, ID = IDSS. (ii) As VGS goes negative, ID decreases below the value of IDSS till ID reaches zero when VGS =VGS (off) just as with JFET. (iii) When VGS is positive, ID increases above the value of IDSS. E only MOSFET The E-MOSFET has no physical channel from source to drain because the substrate extends completely to the SiO2 layer. It is only by the application of VGS (gate-to-source voltage) of proper magnitude and polarity that the device starts conducting. The minimum value of VGS of proper polarity that turns on the E-MOSFET is called Threshold voltage [VGS (th)]. Operation: When gate is made positive (i.e. VGS is positive) as shown in Fig., it attracts free electrons into the p region. The free electrons combine with the holes next to the SiO2 layer. If VGS is positive enough, all the holes touching the SiO2 layer are filled and free electrons begin to flow from the source to drain. The effect is the same as creating a thin layer of n-type material (i.e. inducing a thin n-channel) adjacent to the SiO2 layer. Thus the E-MOSFET is turned ON and drain current ID starts flowing form the source to the drain. Transfer characteristics: When VGS is less than VGS (th), there is no induced channel and the drain current ID is zero. When VGS is equal to VGS (th), the E-MOSFET is turned ON and the induced channel conducts drain current from the source to the drain. Beyond VGS (th), ID increases. If the value of VGS decreases, the channel becomes narrower and ID will decrease.

Uses of MOSFET: 1. MOSFETs are used even in the case of motor control applications. 2. MOSFETs are used to perform switching actions in case of basic buck converters used in DC-DC power supplies. 3. MOSFET working leads to their usage as a switch. 4. MOSFET is used in chooper circuits. 5. Depletion type MOSFETs in source-follower configuration are used in linear voltage regulator circuits. Differentiate between enhance MOSFET & Depletion MOSFET. D-MOSFET : 1.Voltage from gate to source(Vgs) can be positive or negative. 2.Has two modes of operation: depletion and enhancement mode. 3.Biasing: Self bias, Gate bias, Voltage divider bias, Zero bias. 4. No threshold voltage. E-MOSFET : 1.Voltage from gate to source(Vgs) is always positive. 2.Has one mode of operation: enhancement mode. 3.Biasing: Gate bias, Voltage divider bias, Drain feedback bias. 4.E-MOSFETs have threshold voltage. Photo electric devices: photoelectric effect: The photoelectric effect refers to the emission, or ejection, of electrons from the surface of, generally, a metal in response to incident light. That is, the average energy carried by an ejected (photoelectric) electron should increase with the intensity of the incident light. Photoelectric Device An electron device in which an electromotive force (emf) or photoelectric current is generated as a result of the absorption of the energy of optical radiation that is incident on the device is called photo electric device. Or A device which gives an electrical signal in response to visible, infrared, or ultraviolet radiation called photo electric devices.  Phototubes are vacuum-tube devices that operate on the basis of photoemission.  photocells are solid-state devices that operate on the basis of the internal photoelectric effect of photo-emf generation. Photo tube: A typical phototube is a two-electrode vacuum-tube device that contains a photocathode and an anode, or electron collector. The electrodes are placed in an evacuated or gas-filled envelope made of glass or quartz. A luminous flux that is incident on the photocathode causes photoemission from the cathode’s surface; when the

phototube’s circuit is closed, a photoelectric current that is proportional to the luminous flux flows in the circuit. In gas-filled phototubes, the photoelectric current is amplified as a result of the ionization of the gas and the occurrence of a non-self-sustaining avalanche gas discharge. The most widely used phototubes have cesium anti monide or cesium oxide-silver photo cathodes.

Figure 1. Schematic diagrams of: (a) a phototube and (b) a photocell Photocell: A photocell is a semiconductor device with a homogeneous p-n junction diode. The absorption of optical radiation in photocells causes an increase in the number of free carriers in the semiconductor. The electric field at the junction or contact spatially separates the charge carriers; for example, in a p-n-type photocell, the electrons accumulate in the n-region and the holes, in the p-region. Consequently, a photo-emf is generated between the layers. When the external circuit of a photocell is closed through a load, an electric current begins to flow. Photocells are made of such materials as Se, GaAs, CdS, Ge, or Si. Photocells are often photo-diodes. Photocells are also used for the direct conversion of the energy of solar radiation to electric energy in solar batteries and photovoltaic converters. Light Emitting Diode(LED), Light depending resistor, solar cell are such devices. characteristics of photoelectric devices: 1. The luminous sensitivity is the ratio of the photoelectric current to the luminous flux producing the current at the rated anode voltage (for phototubes) or when the output terminals are short circuited (for photocells). 2. The spectral response gives the optical wavelength range in which a given photoelectric device is sensitive. For example, this wavelength range is 0.2–0.7 micrometers (μm) for a phototube and 0.5–2.0 μm for a germanium photocell. 3. The current-voltage characteristic shows the relationship between the photoelectric current and the voltage across a given photoelectric device with a constant luminous flux and makes it possible to determine the best operating conditions for the device. 4. The conversion efficiency (also applicable to solar cells) is the ratio of the electric power generated by a given device at a nominal load and the incident luminous power. Applications:  Visual signals where light goes more or less directly from the source to the human eye, to convey a message or meaning  The light from LEDs can be modulated very quickly so they are used extensively in optical fiber and free space optics communications.  This includes remote controls, such as for TVs, VCRs, and LED Computers, where infrared LEDs are often used.

 LED combined with a photodiode or phototransistor to provide a signal path with electrical isolation between two circuits.  This is especially useful in medical equipment where the signals from a lowvoltage sensor circuit.  LEDs are used as motion sensors, for example in optical computer mice. **** ALL THE BEST ****