Board Exams

Board Exams

Board Exam: HSC Electronics Paper Tips

PUBLISH DATE 15th March 2019

Electronics Paper I

Sudhir Bhosale (M.Sc)

Dear students,

        Warm greeting and best wishes to all the students appearing for HSC Board Examination. I am very happy that I got the opportunity to guide you through this medium. To sustain in this modern world of science, we need to be active for acquiring knowledge in an update state. Proper guidance and hard work together will ensure success while answering electronics paper keep in mind that the question paper is of 50 marks but the time allotted to you is 3 hours. So, the examiner is expecting each and every detail of the answer along with neatly labelled diagrams. Chapter wise marking scheme is a follow.

Name of chapter

%marks

1.  Instruments

15

2.   Power supplier

20

3.   Transducers

10

4.  Operational amplifiers

25

5.  Modern electronic communication

20

6.  Study of IC

10

 

Let us see chapter wise important topics

Chapter 1

It consists of instruments like CRO, function generator and digital multimeter (DMM). CRT is the heart of CRO. main parts of cathode ray tube are (a) electron gun assembly (b) deflection plate assembly (c) fluorescent screen and (d) glass envelope and base of the tube.

Electrostatics focusing system of CRT consist of pre accelerating anode, focusing anode and accelerating anode.

Deflection systems in CRT (a) electrostatics deflection (b) magnetic deflection

The deflection sensitivity (s) of a CRT is defined as the deflection on the screen (in meter) per volt of deflection voltage. S = D/Ed unit m/v

Block diagram of CRO and explanations

The blocks present in a CRO are as follows.

  1. CRT         
  2. vertical amplifier
  3. delay line
  4. trigger circuit
  5. Time base generator
  6. horizontal amplifier
  7. blanking circuit and
  8. power supply.

Applications of CRO

(a) AC and DC voltage measurement

(b) frequency measurement (i) using internal time base (ii) using lissajaous figures

(c) phase measurement

 

Function generator

The function generator provides sine, square and triangular waves with a frequency range of 0.01 Hz to 100 kHz. It consists of the frequency control network, upper and lower current source, integrator, voltage comparator multivibrator, wave shaping circuit and output amplifiers.

Digital Multimeter (DMM)

Digital multimeter is basically a digital voltmeter by including additional circuitry it can be used to measure AC &DC voltage, AC &DC current, resistance etc.

Chapter 2: Power Supplier

                 A power supply is used to provide the required amount of power at a specified voltage from a primary source. Rectifier is a device which converts alternating quantity into unidirectional i.e. direct quantity. 

Types of rectifiers

(a) half wave

 (b) centre- tapped

 (c) full wave bridge.

Comparison of rectifiers based on following points.

  1. (i) No of diodes
  2. (ii) peak output voltage
  3. (iii) dc o/p voltage
  4. (iv) PIV 
  5. (v) ripple frequency
  6. (vi) efficiency
  7. (vii) ripple factor

Problems based on rectifiers

Fitters: the circuit used to remove the ac components from the output (rectified) voltage is called a filter circuit. Types of filters are

(i) capacitor input filter

(ii) Inductor filter

(iii) RC filter

(iv) LC filter

Power supply characteristics:

The quality of a power supply depends on its load voltage, load current, voltage regulation and other factors:

  1. Load regulations         % LR = VNL-VFL/VNL*100
  2. Line or source regulation        % SR = SR/VNOM* 100
  3. Output impedance       rout(CL)= rout/1+A β
  4. Ripple Rejection
  5. Overvoltage / overload protection

TYPES OF REGULATORS.

Zener regulator – problem-based on more zener regulator IS = Vz-Vz/Rz,  IS = Iz+IL

VL=Vz and IL= VL/ RL

Transistorized regulator and current limiting in series regulator  Vout =ACL(Vz+VBE);  ACL=1/ β

ISL=VBE/R4  and power dissipation PD = (VIN + VBE) ISL

Three terminal IC regulator includes a series pass transistor that can handle 1.5 A of load current, the thermal shutdown and current limiting, comparator with internal reference voltage and feedback voltage obtained from a voltage divider. LM317 can provide 1.5 A load current over an adjustable output range of 1.25 to 37 v.

Vout=1.25(1 + R2/R1)

Chapter 3:  Transducer is defined as the device used to convert a non-electrical quantity into an electrical signal.

Active or self – generating transducers: they do not require external power. An analogue voltage or current is produced when stimulated by some physical form of energy e.g thermocouple, piezoelectric pick up etc.

Passive transducer: these transducers produce a variation is some electrical parameter such as resistance, capacitance etc. which can be measured as a voltage or current variation. These transducers require external power.

Following factors should be considered while selecting a transducer-

  1. Physical quantity to be measured.
  2. Transducer principle used to measure the quantity
  3. Accuracy required for the measurements.

Types of transducers:-

Thermistor :

Temperature transducer usually negative temperature coefficient of resistance and sintered mixture of metallic oxides such as cobalt, copper, nickel etc.

Piezoelectric transducer :

Pressure or displacement (active) transducer. When subjected to pressure or tension develop electric charges proportional to the magnitude of the mechanical pressure or tension. piezoelectric crystal are quartz, Rochelle salt, tourmaline etc.

LVTD:

The linear variable differential transformer consists of a single primary winding and two secondary windings connected in series opposition placed on either side of the primary. The output voltage (ESEC1- ESEC2) is a function of the core position. It is pressure or displacement transducer.

LDR:

Light transducer called as photoresistor or photoconductive cell. The conductivity of LDR depends upon light incident on it. Cadmium sulphide, selenium etc are photoconductive materials. CdS are commonly used.

Opto-coupler:

Light transducer. A LED and a photodetector are combined in a single package. Output voltage changes according to the input voltage. Provides electrical isolation between input and output. It is also called as optoisolator.

Capacitive transducer :

Pressure or displacement transducer C = KAE0/d Any change in d causes the proportional change in capacitance. This is the principle of capacitive transducer.

Loudspeaker:

The dynamic loudspeaker consists of the voice coil, voice coil former, centering spider, magnet, the cone, dust cap etc. if we apply an ac current to the coil, the cone reproduces sound as it moves the surrounding air back and forth.

Chapter 4:

Operational amplifier is a high gain dc amplifier it consists of three basic stages. First stage is an input stage consist of a balanced differential amplifier with constant current source. this state provides high CMRR and high input impedance. The second stage is an intermediate stage with unbalanced differential amplifier, emitter follower and dc level shifter. This stage increases the gain and CMRR also and brings down output voltage to zero in the absence of an input signal. The last stage is output state with a push-pull power amplifier.

Important parameters of op-Amp:

  1. Input bias current IB is defined as the average of burning currents of inverting & non-inverting input. IB= | IB+|+| IB-| / 2
  2. Input offset current Ios is defined as the difference between the biasing currents.

Ios=| IB+|-| IB-|

  1. Input offset voltage: this voltage must be applied to the particular input terminal to nullify output offset voltage i.e to obtain zero output voltage.
  2. CMRR: the ratio of differential voltage gain to common mode voltage gain. It indicates the ability to reject the common mode signal. CMRR= |Ad|/|Ac|.
  3. Slew rate: the maximum rate at which the output can change. SR= dVout/dt = Imax/Cc
  4. Open  loop gain (AOL)
  5. Closed loop gain (ACL)
  6. Frequency response

virtual ground: The open loop gain is infinite, therefore the difference voltage between two input terminals (Vd) is zero. Vd= V0 /A   As   A → ∞   Vd ≈ 0        Vinv = VNon-inv

As the input impedance is infinite, the current into the op-amp is zero. Thus the virtual ground is that point in a circuit that has zero voltage and draws no current.

 

Applications of op-amp:-

  1. Linear applications
  • Inverting amplifier
  • Non - Inverting amplifier
  • Buffer
  • Adder
  • Subtractor

For the explanation of the above application following points are important.

  1. Circuit diagrams
  2. Construction
  3. Virtual ground concept
  4. Derivation
  5. Equation for output voltage

 

  1. Non- Linear applications:
  • Integrator
  • Differentiator
  • Comparator
  • Schmitt trigger

Problem based on inverting amplifier, Non –inverting amplifier and adder.

Chapter:5 Modern Electronic Communication

Basic elements of communication system are a transmitter, a communication channel or medium and a receiver. Types of electronic communication - one way communication is called ‘simplex’ e.g. Radio,  TV, FAX etc. Simultaneous two- way communication is called ‘Full Duplex’ e.g. Telephone. The communication  in which only one party transmits at a time is called ‘Half –duplex’ e.g Radio communication used in military, Fire, Police etc.

The need for modulation:

  1. Poor transmission
  2. Interference
  3. Height of antenna

The process in which some characteristics of the carrier such as amplitude, frequency or phase can be changed in accordance with the amplitude of modulating signal (audio) is called ‘Modulation’. In AM, the ratio of the peak value of the modulating signal to the peak value of the carrier signal is called ‘Modulation-index’

∴ m = Vm/VC

It is also given by m = (Vmax- Vmin) / (Vmax+ Vmin)

Sideband frequencies fc+fm and fc- fm, amplitude = Vm/2

Important terms in FM:

  1. Frequency deviation fd
  2. Centre frequency FC
  3. Modulation index m= FD/ FM
  4. Deviation ratio
  5. Carrier swing CS = 2fd etc.

The necessity of geosynchronous satellite:

A satellite which orbits the earth around the equator at a distance of 35,860 km is called geostationary satellite. Use of satellite as a relay station. The transmitter-receiver combination in the satellite is known as ‘transponder’. It consists of receiving and transmitting antenna, law noise amplifier, mixer, local oscillator and power amplifier. It has a wide bandwidth of 500 MHz and is divided into 12 separate channels, each 36MHZ wide.

Applications of satellite:

  1. In communication
  2. Surveillance
  3. Navigation (gps)
  4. Meterological observations etc.

The transmission of binary or digital information from one point to another is called ‘data communication’. Methods of data transmission

  1. Parallel
  2. Serial

A modem is a device containing both a modulator and a demodulator. It is used to convert binary signals into analogue signals capable of transmitting over the telephone line and to demodulate such signals and reconstruct the binary output. Block diagram of FSK modem and its explanation.

The network is an interconnection of two or more stations that wishes to communicate. Types of network

  1. WAN
  2. MAN
  3. LAN

Types of network topologies

  1. Star
  2. Ring
  3. Bus
  4. Tree

The main components of fiber –optics communication system are A/D converter, a light source transmitter, a fiber optics cable, a light detector with amplifier and wave shaper and D/A converter. Principle - total internal reflection.

Advantages:

  1. Wider bandwidth
  2. Small size
  3. Strength
  4. Lower loss
  5. Light weight
  6. Security
  7. Safety
  8. Interference immunity.

In fax concept of scanning, the block diagram of fax and its explanation is important. Similarly in the cellular radio system. The concept of the cellular radio system, the concept of a cell, block diagram and its explanation is important. The concept of radar, how to determine the distance of the remote object, types of RADAR system like pulsed radar and continuous wave radar, the principle of working, block diagram of pulsed radar and its working is important.

Chapter: 6 Study of IC

IC555 is an 8 pin timer IC. It consist of a potential divider two comparators, RS flip-flop and discharge transistor.

Application of IC555:

  1. Monostable multivibrator it is also called as one- short multivibrator. Pin 6 & 7 shorted together and the trigger pulse is applied to pin 2. External component RC controls the width of the output pulse W =1.1RC
  2. Actable multivibrator : it is also called as free – running multivibrator pin 6 & 2 are shorted. The external capacitor C charges through resistors RA and RB and discharges through RB.  TON = 0.693 (RA+ RB)C;  TOFF= 0.693 RB

D= RA+RB/ RA+2RB        F = 1.44 / (RA + 2RB)C

It produces a continuous train of pulse called clock pulses. Other applications as frequency shift keying, PAM, PPM

Typical parameters of IC741 is to be studied. Problems based a monostable and Astable are important.               

 

  Wish you all the best!!!

      

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