Two Marks Questions with Answers

Two Marks Questions with Answers

 

Q.1 What is the function of a series pass transistor in a voltage regulator ?

Dec.-03,15

Ans. : It regulates the load current as per the requirement and keeps the output voltage constant.

 

Q.2 How the current boosting is achieved in a 723 IC ?

Ans. : The maximum load current of IC 723 is 150 mA.

If the current is to be boosted then external transistor Q₁ is connected to the output terminal as shown in the Fig. 5.1.

 

Q.3 Differentiate between linear and switching regulators.

(Refer section 5.11)       

 

Q.4 What are the drawbacks of simple current limiting and how is it overcome ?

May-2007

Ans. : The disadvantage of constant current limiting is relatively large power dissipation in the series pass transistor when load terminals are shorted. Thus a large power rating transistor is required which increases the cost. The circuit which is used in practice which eliminates the disadvantage of simple current limiting is called foldback current limiting circuit.

 

Q.5 Draw the circuit of LM 317 voltage regulator, give the expression for V_o.

Ans. : Refer section 5.11. The expression for the output voltage is,

Vo = 1-25 [1+ R₂ / R₁]

 

Q.6 What is the need for current limiting in regulated power supplies ?

Ans. : The series regulators using transistors have no short circuit protection. Thus if load is shorted then, 1) Large load current flows 2) The series pass transistor may get destroyed 3) The diodes used in the circuit may get destroyed.

To avoid this, current limiting is necessary in regulated power supplies.

 

Q.7 What are the advantages of a switching regulator ?

Ans. : The advantages of the switching regulator are,

1. High efficiency.

2. Low ripple content.

3. Low cost, size and weight due to small values of L and C.

4. Large tolerance to line voltage regulation. 5. Simple to design.

6. The transformer is not required hence circuit becomes less bulky and cheaprer.

7. The transistor power dissipation is very low as it is operated as switch.

 

Q.8 Which are the important parts of regulated power supply ?

Ans. : The important parts of a regulated power supply are transformer, rectifier, filter and regulator. The basic regulator consists of voltage reference, error amplifier, feedback network and active shunt or series element which is generally transistor.

Q.9 List four major components of the switching regulator.

Ans. : Four major components of the switching regulator are :

i) Voltage source ii) Switching transistor iii) Pulse generator iv) Filter

 

Q.10 What is the need for using switching regulators ?

(Refer section 5.12)

 

Q.11 Which are three different waveforms generated by ICL 8038 ?

Ans. : The three different waveforms generated by ICL 8038 are sinusoidal, triangular and square.

 

Q.12 What is the function of voltage regulator?

Ans. : A voltage regulator circuit is used after the filter in the d.c. power supply and it maintains the output d.c. voltage constant under the variable load as well as variable input conditions.

 

Q.13 List the various voltage regulator characteristics.

Ans. : 1. Load regulation 2. Line regulation 3. Output resistance 4. Ripple rejection

5. Load current 6. Temperature dependence of parameters

 

Q.14 Define load regulation.

Ans. : The load regulation is the change in the regulated output voltage when the load current is changed from minimum (no load) to maximum (full load). Mathematically given by, 

% LR = V_NL - V_FL  / V_FL × 100

 

Q.15 Define line regulation.

Ans. : The line regulation is defined as the change in the regulated output load voltage for a specified range of the line voltage. It specifies the effect of changes in the source voltage on the regulator performance. Mathematically given by,

% SR = SR / V_nom  × 100

 

Q.16 Which are the two types of regulators ?

Ans. : 1. Series regulator in which the active element like transistor is connected in series with the load.

2. Shunt regulator in which the active element like transistor is connected in shunt with the load.

 

Q.17 State the advantages of 1C voltage regulator.

(Refer section 5.7)

 

Q.18 Which are the packages in which IC 723 is available ?

Ans. : IC 723 is available in packages like J or N Dual in line package or V Plug in package.

 

Q.19 List the important features of IC 723.

(Refer section 587.1)

Q.20 Draw the functional block diagram of IC 723.

(Refer section 5.8.2)

 

Q.21 Name the various protection circuits used for the voltage regulators.

Ans. :

1. Constant current limiting

2. Foldback current limiting

3. Over voltage protection

4. Thermal protection

 

Q.22 Explain the principle of switching regulator.

OR

What is SNIPS ?

Ans. : In a switching regulator the series pass transistor is used as a switch. It is operated in either cut-off region or saturation region. In cut-off region the current is very small while in saturation region the voltage across the transistor is very small. In any case the power dissipation across the transistor is very small and maximum power is delivered to the load. But due to cut-off region the power is transmitted to the load in the form of discrete pulses. The pulse width modulation is the basic technique used in such regulators. By varying the duty cycle of the pulse waveform used to control the on/off of the transistor, the average value of the voltage given to the load can be controlled proportionally.

 

Q.23 State the applications of SMPS.

Ans. :

i) Adjustable high voltage constant current sources.

ii) Battery powered systems,

iii) Telecommunication circuits 

iv) Personal computers

v) Printers

vi) Video games

vii) Motor and industrial control systems

viii) Automotive applications.

 

Q.24 What is power amplifier ?

Ans. : The amplifiers which are used to deliver large power to the load in the range of few watts and which handle large signals to produce large power are called power amplifiers or large signal amplifiers.

 

Q.25 How power amplifiers are classified ?

Ans. :

1. Class A where output exists for 360° for 360p of input.

2. Class B where output exists for 180° for 360° of input.

3. Class C where output exists for less than 18(P for 3609 of input.

4. Class AB where output exists for more than 180° but less than 360° for 3609 of input.

 

Q.26 List the various power amplifier ICs.

Ans. : The various power amplifier ICs are LM 380, LM 384 and IC 810.

 

Q.27 State the applications of IC LM 380.

Ans. : The applications of IC LM 380 are,

1. Audio amplifier

2. High gain audio amplifier

3. Phono amplifier

4. Intercom system

5. In sirens or alarms

 

Q.28 What is the need for voltage regulator ?

Ans. : In an unregulated power supply the d.c. output varies as the input voltage changes and the load changes. This is undesirable. Hence to get smooth constant d.c. voltage, which is independent of changes in the supply voltage and the load, the voltage regulator is necessary.

 

Q.29 Why do switching regulators have better efficiency than the series regulators ?

Dec.-03, 04, May-12, 16

Ans. : In switching regulators, the transistor is operated in cut-off region or saturation region. In cut-off region, there is no current and hence power dissipation is almost zero. In the saturation region there is negligible voltage drop across it hence the power dissipation is almost zero. In any case, the power dissipation in the transistor is very small and the entire power is transmitted to the load. Hence the efficiency of the switching regulators is very high than the conventional series regulators.

 

Q.30 Define ripple rejection with respect to voltage regulators.

Ans. : The ripple rejection with respect to voltage regulators is defined as the ratio of ripple contents in the output to the ripple contents in the input. Mathematically it is expressed as,

RR = V_R(out) / V_R(in)

 

Q.31 Draw the internal block diagram of three terminal IC voltage regulator.

(Refer section 5.10.1)

 

Q.32 In a linear voltage regulator, the input     voltage is 20 V and output voltage is 15 V. For a load current        of 1 ampere,        calculate the  power      dissipated in the series pass element.

Ans. : V_in = 20 V, V_o = 15 V, I_L =1 A

P_D =  (V_o - V_in ) × I_L = (20 - 15) × 1 = 5 W

 

Q.33 Using LM 380 draw the circuit for audio power amplifier.

Dec.-l0

(Refer section 5.15)

 

Q.34 Why current boosting is done in IC 723 ?

Ans. : The IC 723 general purpose regulator can deliver maximum output current of 150 mA. For supplying the load which demands the current more than 150 mA, the current boosting is done in IC 723.

 

Q.35 State the applications of IC 723.

Ans. : IC 723 is used to obtain following regulators :

i) Basic low voltage regulator

ii) Low voltage high current regulator

iii) Positive high voltage regulator

iv) Positive high voltage high current regulator

v) Negative voltage regulator

 

Q.36 List the features or advantages of LM 380 power amplifier.

Ans. :

i) Internally fixed gain of 34 dB.

ii) Wide supply voltage range.

iii) Output is short circuit proff with internal thermal limiting

iv) Aunique input stage allows input to be ground referenced or ac coupled.

v) High input impedance of 150 kΩ.

vi) Low total harmonic distortion,

vii) High peal current capacbility of 1.3 A maximum.

 

Q.37 What are the limitations of a three terminal regulators ?

Ans. :

i) For fixed voltage regulators the output voltage is not very precise.

ii) Only selected values of output voltages are available.

iii) Current boosting with the help of series pass transistor is necessary if load demands more current than that of the basic capability of regulator. 

 

Q.38 What are the disadvantages of linear voltage regulators ?

Ans. :

1) The required input step down transformer is bulky and expensive.

2) Due to low line frequency (50 Hz), large values of filter capacitors are required.

3) The efficiency is very low.

4) Input must be greater than the output voltage.

5) As large is the difference between input and output voltage, more is the power dissipation in the series pass transistor.

 

Q.39 State the need for protection diodes in voltage regulators based on LM 317 regulator ?

Ans. : To avoid the discharging of the capacitors through low current points into the regulator, the protecting diodes are used in LM317.

 

Q.40 What is meant by thermal shutdown applied to voltage regulators ?

Ans. : Due to over heating, the series pass element of regulator may get damaged. To avoid this, thermal shutdown is provided. In this protection scheme, the junction temperature of the series pass element is sensed. By sensing this, its power dissipation is reduced by using certain circuit till its temperature drops to a lower safe value.

 

Q.41 Draw the internal block diagram of a function generator IC.

Ans. : 

 

Q.42 List the important features of IC 723.

(Refer section 5.8)

 

Q.43 Give some examples of monolithic IC voltage regulators.

Ans. : IC voltage regulators are,

1. 78 XX series, fixed output, positive voltage regulator.

2. 79 XX series, fixed output, negative voltage regulator.

3. 723 general purpose regulator.

4. 317 adjustable positive voltage regulator.

5. 337 adjustable negative voltage regulator.

 

Q.44 What are the applications of fixed voltage regulator ?

Ans. : The applications of fixed voltage regulator are,

1. To provide constant d.c. voltage to various components in complex electronic circuits.

2. To provide constant 5 V supply to digital circuits.

3. To provide dual supply to operational amplifiers used in variety of electronic circuits.

4. In regulated single and dual power supplies. 

 

Q.45 Give one comparison for switching regulator and variable voltage regulator.

AU : May-17

Ans. : The efficiency of switching regulator is very high and heat generation is low compared to linear variable voltage regulator.

 

Q.46 How are frequency of triangular waveform, obtained using ICL 8038 function generator?

Ans .: 

The Fig. 5.16.3 shows the pin configuration for ICL 8038 function generator, which is available in 14 pin DIP.

 

Q.47 What is an isolation amplifier ?               

Ans. : An isolation amplifier is an amplifier that offers an ohmic or electrical isolation between its input and output terminals. These are often used when there is a very large common-mode voltage difference between input and output sides of the device.

Q.48 List the characteristics of optocoupler.

Ans. : The characteristics of optocoupler are,

i) Current transfer ratio

ii) Isolation voltage

iii) Response time

iv) Common         mode rejection

v) Bandwidth      

vi) V_CE(max)        

vii) I_L(max)