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1 DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING QUESTION BANK Subject Code: EC1203 Subject Name: ELECTRONIC CIRCUITS I Year/Sem: II/III UNIT-1 TRANSISTOR BIAS STABILITY 1. How transistors do amplification? Passing the input current signal from a region of low resistance to a region of high resistance. This concept of transfer of resistance has given the name TRANSfer-resISTOR (Transistor) 2. Why CE configuration is widely used in amplifier circuit? Voltage gain and current gain greater than unity Ratio of output resistance to input resistance small 10Ω to 100Ω. Coupling is ideal between various transistor stages. 3. Write the transistor operating regions S.no Region Emitter-Base junction Collector-Base junction 1 Cut-off Reverse bias Reverse bias 2 Active Forward bias Reverse bias 3 Saturation Forward bias Forward bias 4. Define biasing (or) discuss the importance of biasing? In order to operate transistor in the desired region we have to apply external dc voltage of correct polarity and magnitude to the two junctions of the transistor. This is nothing but the biasing of the transistor. 5. What is the need for biasing? To operate the transistor in the active region Biasing is done for faithful amplification of the input signal 6. Write the Requirements of a biasing circuit? The emitter base Unregistered junction must be forward Version biased and collector-base junction must be reverse biased.opearating point should be fixed at the center of the active region. 1

2 The circuit design should provide a degree of temperature stability The operating point should bemade independent of the transistor parameters. 7. Define operating point or quiescent point When we bias a transistor we establish Collector current (Ic)and Collector to Emitter Unregistered voltage (Vce) conditions Version for the transistor. These conditions are known as operating condition or dc operating point or quiescent point. 8.Define load line The line drawn between saturation point and cutoff point of the output characteristics of the transistor is called load line. The load line drawn between Ic and V CE under no signal condition is called DC load line. 9.Why do you fix the operating point in the middle of the dc load line? We fix the operating point in the middle of the dc load line the output signal is sinusoidal Unregistered waveform without any distortion, Version thus the point is the best operating point. 10. Define bias stability Designing the biasing circuit to stabilize the Q-point is known as bias stability. 11. Operating point depends what are the parameters? Temperature o I C0 o V BE o β dc Transistor current gain(h fe /β) 12. What are the techniques is used to maintain the Q-point in the centre of the load line? 2

3 Stabilization technique Compensation technique. 13.Why Stabilization and Compensation techniques are needed? To maintain the operating point stable by keeping Ic and VCE constant so that the transistor will always work in active region 14.Define Stabilization The process of making operating point stable, irrespective of the variations in temperature and temperature parameter is known as Stabilization. 15. Why is it necessary to stabilize the operating point of transistor? (OR) What is meant by stabilization techniques? Stabilization techniques Created refer to by the use PDFTiger of resistive biasing circuits which allow I B to vary so as to keep I C relatively constant with variations in I C0,β and V BE. To maintain the operating point stable. So that the transistor will always work in Unregistered active region. Version 16. What is meant by compensation techniques?write its types. It refers to the use of temperature-sensitive devices such as diodes, transistors, thermisotrs, etc., which provide compensating voltages and currents to maintain the operating point stable. 1. Diode compensation 2. Thermistor compensation 3. Sensistor compensation 17. What are the advantages and disadvantages of fixed bias circuit? Advantages: Simple circuit By changing the value of RB, operating point can be fixed anywhere in the active region It provide maximum flexibility. Disadvantages: 3

4 Thermal stability is not provided by this circuit Since Ic is varies due to β the operating point is not fixed. 18. What are the advantages and disadvantages of Collector to base bias circuit? Advantages: Simple RB can be easily calculated. Disadvantages: Does not provide good stabilization Gain is reduced due to negative feedback 19. What are the advantages and disadvantages of self bias or voltage divider circuit? Advantages: It provides Created good stabilization by PDFTiger High stability factor compared to other biasing circuits. Disadvantages: It provides poor stability if ratio RB / RE is high Gain is reduced due to negative feedback if RE is not bypassed 20What is the advantage of using emitter resistance RE in the context of biasing? RE is used to increase the stability by providing negative feedback. 21. Define stability factor? And what its ideal value? It is a rate of change of collector current with respect to the I C0,β and V BE. It indicates degree of change in operating point due to variation in temperature. Ideal value is Define the various stability factors? i) Stability factor S is defined as the rate of change of collector current with respect to the 4

5 reverse saturation current I C0, maintaining β and V BE are constants. ii) Stability factor S is defined as the rate of change of collector current with respect to Created the β, by PDFTiger maintaining I C0 and V BE are constants. iii)stability factor S is defined as the rate of change of collector current with respect to the V BE,maintaining β and I C0 are constants. 23. Write short notes on zero current drift in FET In JFET, the drain current varies with changes in the temperature due to two factors. One factor increases drain current and other factor decreases drain current with increase Created in temperature. by PDFTiger Therefore, it is possible to design biasing circuit which compensates these tow factors so that there is no change of drain current with temperature. Such biasing is called biasing for zero current drift. 24. FET is a unipolar device-justify Current conduction by only one type of carriers ie, majority carriers. 25. Distinguish between FET and BJT Si.no Parameter BJT FET 1 Control element Current controlled device Voltage controlled device 2 Device type Bipolar device Unipolar device 3. Input resistance Less High 4. Sensitivity High Less 26. Define thermal runaway? The increase in the collector current increases the power dissipated at the collector junction. Unregistered This turn further increases Version the temperature of the junction and hence increases the collector current. The process is cumulative. 5

6 The excess heat produced at the collector base junction may even burn and destroy the transistor. This situation is called thermal runaway. 27.What is the condition for thermal stability? The rate at which heat is released at the collector junction must not exceed the rate at which heat can be dissipated under steady state condition or to prevent thermal runaway. 28. Why thermal runaway is not there in FETs? FET is temperature dependent. In FET, as temperature increases drain resistance also increases, reducing the drain current.so thermal runaway does not occur in FET. 29.Write the application of JFET used as voltage variable resistor The VVR can be used to vary the voltage gain of a multistage amplifier,as signal level is increased.this action is called AGC. PART B 1. Explain the need for biasing, Stability factor and Fixed bias circuit (16) Write why biasing circuit is needed Define Stability factor and derive the stability factor w.r.t Vbe, Ic0 and β Explain the fixed bias circuit and derive expression for IC,VCE and Stability factor 2. Explain in detail different types of biasing circuits (16) Explain the operation of four methods of biasing and derive Ic and VCE for (i)fixed bias (ii)voltage divider bias (iii) Collector to base bias (iv) Emitter bias 3. Explain the advantage of self bias (voltage divider bias) over other types of biasing.(16) 6

7 Explain the voltage divider bias and derive the expression of Ic and VCE List advantage of this over other biasing 4. Explain the various types of bias compensation techniques. (16) Explain Unregistered why compensation needed Version Explain about (i) Diode compensation (ii) thermistor compensation (iii) sensistor compensation 5. i) Explain biasing of FET (8) write about self bias and voltage divider bias of FET derive ID and Vds ii) Explain biasing of MOSFET (8) write about drain to gate bias circuit and voltage divider bias of FET 6. (i) Draw and explain voltage divider bias using FET and derive for its stability factor. (16) Ans: Draw and explain voltage divider bias using FET Define Stability factor Derive stability factor for it 7

8 UNIT-II MIDBAND ANALYSIS OF SMALL SIGNAL AMPLIFIERS. 1. Define amplifier? A device which accepts an input signal and produces an output signal proportional to the input is called an amplifier. 2.Define small signal equivalent circuit? The analysis of a non-linear device is complex. The term small signal amplifiers refers to the use of signal that takes up a relatively small percentage of an amplifier s operational range. With small input singles the transistor can be replaced with small signal linear model. This model is also called small signal equivalent model. 3.Compare CE,CB and CC amplifiers S.no Parameter Created CE by CB PDFTiger CC 1 Current gain High High Low 2 Voltage gain High unity High 3 Power gain High High High 4 Phase shift No phase shift No phase shift 5 Input Moderate High Low impedance 6 Output impedance Moderate Low High 4.What are h-parameters? h11 = hi = input resistance h12 = hr = reverse transfer voltage ratio h21 = hf = forward transfer current ratio h22 = ho = Output conductance 8

9 5. Draw the h-parameter equivalent circuit or hybrid model for transistor. 6. Draw the h-parameter equivalent circuit for CE configuration 7.Define benefits of h-parameter. Real numbers at audio frequencies Easy Unregistered to measure Version Can be obtained from the transistor static characteristic curves Convenient to use in circuit analysis and design. 9

10 Most of the transistor manufacturers specify the h-parameter. 8.Define current gain The ratio of output current to input current is called current gain,a I, of the amplifier. A I = I 2 / I 1. 9.Define voltage gain The ratio of output voltage to input voltage is called voltage gain,a V,of the amplifier. A V = V 2 / V 1 10.Define power gain The ratio of signal power delivered to the load to the signal power at the input of the amplifier is the power gain,a P, of the amplifier. A P = A V A I 11. Write the equation small signal analysis parameters of a transistor amplifier h11 = hi = input resistance h12 = hr = reverse transfer voltage ratio h21 = hf = forward transfer current ratio h22 = ho = Output conductance 12. State Miller s Theorem. Miller s theorem states that, if Z is the impedance connected between two nodes,node 1 and node 2, it can be replaced by two separate impedances Z1 and Z2; where Z1 is connected between node 1 and ground and Z2 is connectecd between node2 and ground 13.What are the techniques used to improve input impedance. Using Darlington connection 10

11 Using Bootstrap techniques 14.Define Darlington connection Cascaded connection of two emitter followers is called Darlington connection. 15.What is Darlington pair? The two transistors are connected in such a way that the emitter of one transistor is connected to the base of the other transistor. 16.How does input impedance increase due to Darlington pair? In Darlington circuit two transistors are cascaded in CC configuration as a result of this input impedance is increased. 17.Why the Darlington connection is not possible for more number of stages? (NOV-2006) i) Due to very large amplification factor even two stages of Darlington connection has large output current Unregistered and output stage may Version have to be a power stage. As power amplifiers are not used in the amplifier circuits it is not possible to use more than two transistors in the Darlington connection. ii) leakage current in the first stage is amplified in the second stage. So if more number of stages are connected, overall leakage current will be very high. 18.Why bootstrapping technique is called so? The name arises from the fact that, if one end of the resistor R 3 changes in voltage, the other end of R 3 moves through the same potential difference; it is as if R 3 is pulling itself up by its bootstraps. 19.list the three basic configuration of FET common source 11

12 common drain Common gate. 20.Draw the small signal model for FET g m transconductace V gs voltage between gate and source r d output resistance G-Gate, D- Drain, S - Source 21. What is multistage amplifiers? An amplifier which having two or more stages by cascading is called multistage amplifiers. The meaning of cascading is to connect a number of amplifier stages to each other with the output of the previous stage to the input of next stage. 22. What is the need for cascading When the amplification of a single stage amplifier is not sufficient. Provide impedance matching 23.Methods of coupling multistage amplifiers RC coupling Transformer coupling Direct coupling 12

13 24.What is differential amplifier? An amplifier which amplifies the difference between the two input signals is called differential amplifier. 25.write Features of differential amplifier. High differential voltage gain Low common mode gain High CMRR High input impedance Large bandwidth Low output impedance 26.Define differential gain The A d is the gain with which differential amplifier amplifies the difference between two Created input signals. by Hence PDFTiger it is called differential gain of the amplifier V 0 =A d (V 1 -V 2 ) differential Unregistered gain A d = V 0 / V d Version 27.Define common mode gain? The gain with which it amplifies the average level of the two input signals to produce the output is called common mode gain A C. V 0 =A C V C common mode gain A C = V 0 / V c common mode signal V C = (V 1 +V 2 )/2 28.List the configuration of differential amplifiers. Dual input, balanced output differential amplifier Dual input, unbalanced output differential amplifier Single input, balanced output differential amplifier Single input, unbalanced output differential amplifier 29.what is meant by CMRR? 13

14 The ability of a differential amplifier to reject a common mode signal is expressed by a ratio called common mode rejection ratio denoted as CMRR. It is defined as the ratio of the differential voltage gain A d to common mode voltage gain A c CMRR=ρ= A d / A C 30.Mention the 2 modes of operation of differential amplifiers 1. Differential mode operation 2.Common mode operation 31.Define bisection theorem The two half networks can be connected using any number of wires but the wires are not crossing.then for such a bisected network at the imaginary line of Created symmetry, with by all PDFTiger the connecting wires open,the input impedance at input and output is while with all the connecting wires shorted,the input impedance at input and output is This is called Unregistered bisection theorem or Bartlett s Version bisection theorem. 32.What is the effect of R E on CMRR? To improve the CMRR, the common mode gain A c must be reduced. The common mode gain A c approaches zero as R E tends to infinity. This is because R E introduces a negative feedback in the common mode operation which reduces the common mode gain A c. So increasing the value of R E, makes A C to decrease. Thereby CMMR increases to a large value. 33.What are the methods to improve CMRR Constant current bias circuit Current mirror circuit. Active load 14

15 PART B ( 16 Marks) 1. Explain the AC and DC Analysis of Common Emitter amplifier. (16) Draw the CE amplifier Diagram DC Analysis derive the expression for IC and VCE and explain the operation with equations. Draw DC Unregistered equivalent circuit Version AC Analysis - Draw AC equivalent circuit derive the expression for Current gain, voltage gain, input impedance and output impedance. 2. Explain the AC and DC Analysis Common Base Amplifier. (16) Draw the CB amplifier Diagram DC Analysis derive the expression for IC and VCE and explain the operation with equations. Draw DC equivalent circuit AC Analysis - Draw AC equivalent circuit derive the expression Created for Current by PDFTiger gain, voltage gain, input impedance and output impedance. 3. Explain the AC and DC Analysis Common collector Amplifier. (16) Draw the CE amplifier Diagram DC Analysis derive the expression for I E and VCE and explain the operation with equations. Draw DC equivalent circuit AC Analysis - Draw AC equivalent circuit derive the expression for Current gain, voltage gain, input impedance and output impedance. 4. Draw the circuit of a common source FET amplifier & explain its operation? (16) Draw the CS FET amplifier Diagram DC Analysis derive the expression for ID and VDS and explain the operation Unregistered with equations. Version Draw DC equivalent circuit AC Analysis - Draw AC equivalent circuit 15

16 derive the expression for Current gain, voltage gain and output impedance. 5. Briefly explain the operation of a Darlington emitter follower and also derive an expression for its performance measures? (16) Draw the CE amplifier Diagram AC Analysis Unregistered - Draw AC equivalent Version circuit derive the expression for Current gain, voltage gain, input impedance and output impedance. Write how it has high Zin compared to other transistor amplifier 6. What is a differential amplifier? Explain its working in common mode operation. Obtain its AC equivalent circuit & drive the expression for voltage gain? (16) Draw the Differential amplifier diagram Explain about Common mode of operation Draw ac equivalent Created circuit by PDFTiger derive the expression for Current gain, voltage gain, input impedance and output impedance. 7. Draw the circuit diagram of a differential amplifier. Explain the operation in difference mode and common mode? (16) Draw the Differential amplifier diagram Explain about Common mode of operation Explain about difference mode of operation 8. What are the methods to improve the CMRR of differential amplifier? Detail any two of them. (16) CMRR is improved by using active loading ie constant current sourceand modified current source 9 i) Explain with circuit diagram the boot strapped Darlington emitter follower. (16) Draw the boot strapped Darlington emitter follower amplifier diagram Expalin about boostrapping 16

17 10.Draw the small signal hybrid model of CE amplifier and derive the expression for its A I,A V,R I and R O. (16) Draw the CE amplifier Diagram AC Analysis - Draw AC equivalent circuit Replace transistor with hybrid model derive the Unregistered expression for Current gain Version, voltage gain, input impedance and output impedance 17

18 UNIT III FREQUENCY RESPONSE OF AMPLIFIERS 2 MARKS 1.what is frequency response? Draw its general shape. The curve drawn between voltage gain and signal frequency of an amplifier is known as frequency response. 2. What is the significance of Octaves and decades? Octaves corresponds to a doubling or halving of the frequency. Ten times change in frequency is called as decades 3. What is the Bandwidth of an amplifier? It is defined as the difference between upper cutoff frequency f2 and lower cutoff frequency f1. Bandwidth = f2 f1 4. What do you understand by 3 db bandwidth of amplifier? It is the range of frequency over which the gain of an amplifier is equal to times or 70.7% of its maximum gain 18

19 5. Define Lower cut off frequency It is defined as lower value of frequency f1 where the gain is times the maximum gain. 6. Define Upper cut off frequency It is defined as higher value of frequency f2 where the gain is times the maximum gain. 7. What is low frequency response? At low frequencies,the amplifier behaves like a high pass filter,because at very low frequency, capacitive reactance Xc, and capacitor is treated as open circuit. 8. What is high frequency response? At high frequencies Created,the amplifier by PDFTiger behaves like a low pass filter,because at very high frequency, capacitive reactance Xc, and capacitor is treated as short circuit. 9.What is midband frequency response? In mid frequencies, Xc= 0 and therefore the gain is constant or flat over wide range of frequency. 10.Give the formula for voltage gain with mid band frequency region. Av= Vo/Vi 19

20 11. Give the significance of the coupling and bypass capacitor on bandwidth of amplifiers? At high and medium frequencies the coupling capacitors behave as short circuits.at low frequency the circuit gain is reduced.as the signal frequencies decreases the capacitor reactance increase and circuit gain continues to fall, reducing the output voltage. 12. Give the effect of internal transistor capacitances At high frequencies the internal transistor capacitances known as junction capacitances, it reduces the circuit gain. For BJT te internal capacitances are Cbe,Cce, Cbc 13.Why h-parameter is not suitable for high frequency? The values of h-parameters are not constant at high frequencies. At high frequencies h-parameters become complex in nature. 14. Draw the hybrid π model of BJT amplifier. 20

21 15. How the frequency response of an amplifier be improved? The Techniques are i) Dominant pole Compensation ii) Pole zero Compensation 16. Draw the small signal Equivalent circuit of common Source amplifier? 17. Draw the High Frequency Equivalent circuit of common Source FET amplifier? 21

22 18. Define β cutoff frequency (fβ ) It is the frequency at which transistor s short circuit CE current gain drops by 3 db or times of its value at low frequency. 19. Define α cutoff frequency(fα) It is the frequency at which transistor s short circuit CB current gain drops by 3 db or times of its value at low frequency. 20.Define Unity gain frequency(ft) It is the frequency at which transistor s short circuit CB current gain becomes unity. 21.Define Gain bandwidth product It is defined as the product of midband gain and bandwidth. 22

23 22.Give the equation for overall upper and lower cutoff frequencies of multistage amplifier Lower cutoff frequency = f cl = f cl / sqrt(2 1/n -1) Upper cutoff frequency = f cu = f cu * sqrt(2 1/n -1) 23.Define Rise time It is defined as the difference between the time required for Vo to reach 10% to 90% of final value. tr = t2-t1 24. Define Sag time It is defined as large time constant at which amplifier output tend to decrease The lower 3db frequency in terms of sag or tilt is given by 25. Give the relationship between rise time and bandwidth B.W =.35/tr 23

24 PARTB( 16 Marks) 1. Derive the expression for the CE short circuit current gain of transistor at high frequency (16) Draw the Unregistered Approximate hybrid π model Version of CE amplifier Short the o/p circuit side Derive the expression for current gain 2. What is the effect of Bypass capacitor on low frequency response? (16) Draw the Approximate hybrid π model of CE amplifier Derive low frequency voltage gain(avlf) and mid frequency voltage gain(avlf) Find Ratio of (AvLF) to (AvMF) Equate the ratio to Find CE 3. Draw the high frequency hybrid π model for a transistor in the CE configuration and explain the significance of each component. Define alpha cut off frequency. (16) Draw the hybrid π model of CE amplifier Explain about importance of all capacitor Define α-cut off frequency Derive fα 4. Draw the high frequency equivalent circuit of FET amplifier and derive all the parameters related to its frequency response. (16) Draw the Approximate hybrid π model of FET amplifier Derive voltage gain(av),input admittance, input capacitance and output admittance 5 (i) Define the frequency response of multistage amplifier and derive its upper and lower cut-off frequencies. (8) Derive the expression for f1 and f2 24

25 (ii) Explain in detail about miller effect?(8) Draw the two port network with miller capacitance Derive the input and output miller capacitance 6. Derive the upper and lower cut off frequencies of RC coupled BJT amplifier. Unregistered (16) Version Draw BJT CE amplifier with capacitive coupling in the input and output side Derive lower critical frequency using input RC circuit, Output RC circuit and Bypass RC circuit It will give the low frequency response Consider internal parasitic capacitances using miller s theorem to find the high frequency response 25

26 UNIT-IV LARGE SIGNAL AMPLIFIERS 1.What is power amplifier? It is an amplifier which is capable of producing a large amount of power to the load such as loud speaker,motor etc It is also called Unregistered as Large signal amplifiers.the Version term large signal arises because it use large part of load line for operation. 2.List the features of large signal amplifiers input signal level or amplitude of a power amplifier is large h parameter not used here low output impedance 3.Classification of Power amplifier class A amplifier class B amplifier class Created C amplifier by PDFTiger class AB amplifier class D amplifier class Unregistered S amplifier Version 4.State class A amplifier The power amplifier is said to be class A amplifier if the Q-point and the input signal are selected such that the output signal is obtained for a full input cycle(360 0 Conduction). 5.State class B amplifier The power amplifier is said to be class B amplifier if the Q-point and the input signal are selected, such that the output signal is obtained only for one half cycle for a full input cycle(180 0 Conduction). 6.State class C amplifier The power amplifiers is said to be class C amplifier, if the Q-point and the input signal are Unregistered selected such the output signal Version is obtained for less than a half cycle, for a full input cycle (less than180 0 Conduction). 26

27 7.State class AB amplifier In a class AB amplifier, the output current flows for more than half cycle but less than full cycle(more than but less than ). 8.What is class D amplifier? The amplifier consists of two complementary symmetry transistors driving a load Unregistered RL. This means one transistor Version in PNP and other is NPN.Thus entire power input is available to the load. Class D amplifiers are designed to operate with digital or pulse type of signals.efficiency is almost 100% 9.Define conversion efficiency of a power amplifier. The ratio of AC power delivered to the load to the DC power input is called conversion efficiency of a power amplifier. 10.Write the types of Class A amplifier 1. Direct coupled Class A amplifier 2.Transformer coupled Class A amplifier 11.List the advantages and disadvantages of directly coupled class A amplifier? Advantages: Circuit simple Output transformer is not necessary Circuit cheaper Less number of components required as load is directly coupled Disadvantages: Power dissipation is more. Impedance matching is not possible The efficiency is poor 27

28 12.List the advantages and disadvantages of transformer coupled class A amplifier? Advantage: Efficiency higher than directly coupled Impedance matching is possible Disadvantage: The Unregistered circuit complicated to design. Version The circuit bulkier, heavier and costlier Frequency response of the circuit is poor. 13. List the advantages and disadvantages of push pull class B amplifier Advantage: Efficiency higher than class A If No input signal, then power dissipation is zero Even harmonics get cancelled. This reduces harmonic distortion Ripple Created present in supply by voltage PDFTiger also get eliminated Impedance matching is possible Disadvantage: Two Unregistered center tap transformers are Version necessary The circuit bulkier, heavier and costlier Frequency response is poor. 14.List the advantages and disadvantages of complementary symmetry B amplifier Advantage: Due to absence of transformer,its weight, size and cost are less impedance matching is possible Frequency response is improved Disadvantage: The circuit needs two separate voltage supplies The output is distorted to cross over distortion. 15.Compare of push pull and complementary symmetry amplifier S.No push pull complementary symmetry 1. Both transistors are Transistors are 28

29 similar either NPN or PNP complementary type.one is NPN and another is PNP 2. Transformer is used Transformerless 3. Frequency response is Frequency response is poor improved 4. Dual power supply is not required Dual power supply is not required 5. The circuit is bulky, heavier and costly The circuit is not bulky and costly 16. What is the configuration used in complementary symmetry power amplifier? How does it help? The configuration Created used in complementary by PDFTiger symmetry power amplifier is common collector. This configuration has lowest output impedance due tow which impedance matching for the low impedance loads like loudspeaker is Unregistered easily possible. Version 17. What is meant by harmonic distortion or non linear distortion? At the output of power amplifiers, along with the fundamental frequency (f) component, additional frequency components are also present whose frequencies are integral multiples of fundamental frequency such as 2f, 3f.. These are called harmonics. The output gets distorted due to these components. This is called the harmonic distortion. 18. Write the formula for harmonic distortion %nth harmonic distortion = %Dn = ( An / A1 )*100% An Nth frequency component A1 Fundamental frequency component A1>>An 29

30 19. what is crossover distortion? how to eliminate it? For making transistor ON, it is necessary that V BE voltage must exceed 0.7V. Due to this, in class B amplifier while crossing over from one half cycle to other, as long as input is below 0.7V, none of the transistor is ON and output is Zero. Unregistered Due to this, there distortion Version in the output, which is called the cross over distortion. To over come this distortion, a small forward bias is kept applied to the transistors so that when input is zero, this additional forward bias can make the transistor ON immediately elimination cross over distortion. 20.Define thermal resistance? The temperature rise of a junction is proportional to the power dissipation. The constant of proportionality between the two is called thermal resistance. 21.What is heat sink? Give its advantages? A heat sink is a mechanical device which is connected to the case of a transistor, to dissipate Unregistered the Version heat developed by the transistor surrounding. Advantages: The temperature of the case gets lowered The power handling capacity of the transistors can approach the rated maximum value. 22.Write the features of MOSFET power amplifier Thermal runaway does not occur Large input impedance MOSFET transfer characteristics are more linear Used to switch large current ON and OFF. 23.List the application of Class C amplifier tuned circuit radio frequency circuit mixer radio receivers 30

31 24.Compare class A, class B, class C & class AB amplifier. Operatin g Cycle Power Efficienc y Maximu m output Distortio n Applicati on Class A Class AB Class B Class C 360 ₒ 180 ₒ to 360 ₒ 180 ₒ Less than180 ₒ 25% to 50% Between 25% and 78.5% 78.5% <Vcc Vcc Vcc 2Vcc Small, due to non lineardistortion Small to moderate due to cross over distortion Output power Small to moderate due to cross over distortion Output power Large Low power amplifiers Tuned RF power amplifier; final where amp; may amp; may amp stage in efficiency Unregistered is use use Version communications not important Darlington Darlington circuits configuration configuration s s and diodes and diodes for for biasing biasing 25. write the formulae for total harmonic distortion. D = ( D 2 2 +D D ) D2,D3and D4 Second order, higher order harmonics PART B 1. With neat circuit Unregistered diagram explain the working Version principle of complementary symmetry class-b amplifier (16) Draw the complementary symmetry class-b amplifier 31

32 Explain the operation of it 2. Explain and obtain the efficiency of transformer coupled class A power amplifier. (16) Draw the transformer coupled class A power amplifier Explain the operation of it Derive the efficiency of transformer- coupled class A power amplifier 3. Prove that the maximum efficiency of Push Pull class B amplifier is 78.5%. (16) Draw the Push Pull class B amplifier Explain the operation of it Derive the max. efficiency of Push Pull class B amplifier is 78.5% 4. i) Compare class A, class B and class C power amplifier based on their performance characteristics (8) Compare the class A, class B and class C power amplifier Compare the output, efficiency. ii) Explain the significance of heat sinks for thermal stability. (8) Draw the diagram of heat sink Define thermal stability. Explain how heatsink provides thermal stability 7. What is the difference between a voltage amplifier and a power amplifier? (16) Compare the diff between voltage amplifier and a power amplifier 8. Differentiate Class S from Class D amplifier and derive the efficiency of Class Unregistered D amplifier. (16) Version Compare the Class S from Class D amplifier with diagram Derive the efficiency of Class D amplifier 32

33 UNIT-V RECTIFIERS AND POWER SUPPLIES 2 MARKS 1. List the types of power supplies 1.Linear Mode power supply 2.Switched Mode power supply 2.What is Rectifier?Write its types A rectifier is a device which converts ac into pulsating dc using one or more diodes. Types: 1.Halfwave Rectifier(HWR) 2.Fullwave Rectifier(FWR) 3.Bridge Rectifier 3.Draw the circuit of Bridge Rectifier 4.Define Ripple factor Ripple factor γ = RMS value of AC component of output Average or DC component of output 33

34 5.Define Rectifier efficiency dc output power Efficiency = ac input power 6. Define Transformer Utilization Factor(TUF) The factor which indicates how much is the utilization of the transformer in the circuit. dc power delivered to the load TUF = ac power rating of the transformer 7.Define Peak Inverse Voltage It is the peak or maximum voltage across the diode under reverse bias condition. 8.Write the advantages & Disadvantages of Halfwave Rectifier? Advantages: 1.simple 2.Low cost Disadvantages: 1.Ripple factor is high(1.21) 2.Rectification efficiency is low(40.6%) 3.TUF is low(0.287),which shows that transformer is not fully utilized. 4.poor regulation 9.Write the advantages & Disadvantages of Fullwave Rectifier? 34

35 Advantages: 1.dc output voltage is higher 2.ripple factor is less(0.483) 3. Efficiency(81.6%) is twice that of HWR 4. TUF is high(0.693), Disadvantages: 1.Expensive Unregistered because of center tapped transformer Version is needed. 1.PIV of diode is high(2vm) 10.Write the advantages & Disadvantages of Bridge Rectifier? Advantages: 1. center tapped transformer is not needed 2.suitable for high voltage application 3.current in both the primary and secondary of the transformer flows for the entire cycle Disadvantage: 1.It uses four diodes 11.What is the need of filters? Filters are circuits used to reduce the amount of ac components(ripples) present in the rectified dc output 12.Write the ripple factor for C,L,LC,CLC filters Ripple factor for (i)capacitive filter r = 1/(4* 3*f*C*Rl) (ii) inductive filter r = RL/(3* 2* 2*π*f * L) (iii) LC filter r= ( 2/ (48 * π 2 *f))*(1/lc) (iv) CLC or π section filter r = 2 * (Xc/RL)*(Xc1/XL1) 35

36 13.Compare Capacitor and LC filter S.No Capacitor filter LC filter 1. The first element of the The first element of the filter as looked from rectifier side is filter as looked from rectifier side is inductor capacitor 2. Regulation is poor Regulation is better 3. Frequency response is poor Frequency response is improved 4. Not suitable for variable loads 5. Bleeder resistance is not required Suitable for variable loads Bleeder resistance is not required 14. what is voltage multiplier?write its types The voltage multiplier are used to step up the output voltage level to two, three or more times the peak voltage of the input. Types: 1.Voltage doubler Vo = 2 Vin 2.Voltage tripler Vo = 3 Vin 3.Voltage quadrupler Vo = 4 Vin 15.What is meant by regulation? It is defined as variation of dc output voltage with change in dc load current %regulation = ((VnL - VL)/VL) * 100 % VnL No load output voltage VL - Voltage with normal Load 36

37 16. What is meant by voltage regulator? It is an electronic circuit that provides constant dc voltage irrespective of the changes in load current,input voltage and temperature. 17. Define line regulation and load regulation Line regulation or source regulation (SR): The line regulation Unregistered defined as the change Version in the regulated load voltage for a specified range of line voltage, SR=V HL -V LL V HL =load voltage with high line voltage V LL =load voltage with low line voltage Load regulation: The load regulation is the change in the regulated output voltage when the load current is charged from minimum (no load) to maximum (full load). LR=V NL -V FL V NL =load voltage with no load current V FL =load Unregistered voltage with full load current Version 18.What are the limitations of using Zener diode regulator? The maximum load current that can be supplied is limited to (I z max -I z min ) A large amount of power is wasted in the zener diode and the series resistance R, in comparison with the load power. The regulation factor and the output resistance are not very low 19.State the principle of SMPS Pulse width modulation(pwm) is the basic principle of the SMPS or switching regulator. The average Unregistered value of repetitive waveform Version is proportional to the area under the waveform so the switching regulators use the fact that of duty cycle of the pulse waveform is varied. 37

38 20. Draw the block diagram of SMPS 21.Write the advantages of SMPS As the heat dissipation is less in SMPS due to switching action. Its efficiency Created is high. by PDFTiger The unit is isolated from the mains without need for a large transformer. It also reduces the ripple at output. 22. Where is SMPS used? The SMPS is used where perfect DC voltage is required for the proper functioning of the circuit. It is used in computers, printers, inverters, stabliziers etc. 23. Write the ripple factor for a half wave and Full Wave Rerctifier. Ripple factor is (1.21) for half wave ripple factor is (0.483) full wave 24. What is voltage multiplier The voltage multiplier Created are used to by step up PDFTiger the output voltage level to two, three or more times the peak voltage of the input. Types: 1.Voltagedoubler Vo = 2 Vin 2.Voltagetripler Vo = 3 Vin 3.Voltagequadrupler Vo = 4 Vin 38

39 25. Find the ripple factor for a FWR with capacitor filter with the output wave form a shown in the figure. Assume Rl= 100Ω with capacitor C = 1000µF Ripple factor = = = = 10% PART B 1. Explain the operation of half wave rectifier and derive the expression for Vrms. Ripple factor, PIV, Efficiency and TUF(16) Draw the half wave rectifier cicuit Explain the operation Derive the Vrms. Ripple factor, PIV, Efficiency and TUF 2. Explain the operation of Full wave rectifier and derive the expression for Vrms. Ripple factor, PIV, Efficiency and TUF(16) Draw the full wave rectifier cicuit Unregistered Explain the operation Version Derive the Vrms. Ripple factor, PIV, Efficiency and TUF 39

40 3. Explain the operation of half wave rectifier with capacitve, LC filter and derive the expression for Vrms. Ripple factor, PIV, Efficiency and TUF(16) Draw the half wave rectifier cicuit with capacitve and LC filter Explain the operation Derive the Vrms. Ripple factor, PIV, Efficiency and TUF 4. Explain the operation of Full wave rectifier with capacitve, LC filter and derive the expression for Vrms. Ripple factor, PIV, Efficiency and TUF(16) Draw the full wave rectifier cicuit with capacitve, LC filter Explain the operation Derive the Vrms. Ripple factor, PIV, Efficiency and TUF 5. Explain the voltage regulator with current limiting and over load protection. Draw the circuit of voltage regulator with current limiting and Unregistered over load protection. Version Explain the operation 6. Explain in detail about SMPS. Draw the circuit of SMPS Explain the operation 7. Derive the output voltage, I L of zener diode voltage regulator. Draw the circuit of zener diode voltage regulator Dereive the the expression for output voltage, I L 8. Explain the line and load regulation of zener diode voltage regulator Draw the circuit of zener diode voltage regulator with Unregistered line and load regulation separately. Version Draw graph. 40

41 B.E/B.Tech, Degree Examination, May/June Third Semester Created ECE by PDFTiger EC 1203 Electronic circuits -1 Answer All the Questions Part A 2 MarksQ&A 1.Draw the fixed bias and Self Bias Circuit. Fiexed bias Voltage divider bias 2. Define Various Stability Factor 41

42 (i) Stability factor S is defined as the rate of change of collector current with respect to the reverse saturation current I C0, maintaining β and V BE are constants. ii) Stability factor S is defined as the rate of change of collector current with respect to the β, maintaining I C0 and V BE are constants. iii)stability factor S is defined as the rate of change of collector current with respect to the V BE,maintaining β and I C0 are constants. 3.Define Various hybrid parameter h11 = hi = input resistance h12 = hr = reverse transfer voltage ratio h21 = hf = forward transfer current ratio h22 = ho = Output conductance 4.Define CMRR The ability of a differential amplifier to reject a common mode signal is expressed by a ratio called common mode rejection ratio denoted as CMRR. It is defined as the ratio of the differential voltage gain A d to common mode voltage gain A c CMRR=ρ= A d / A C 42

43 5. Define fl in a high frequency transistor It is defined as lower value of frequency f1 where the gain is times the maximum gain. 6.Define rise time and sag in an amplifier Rise time It is defined as the difference between the time required for Vo to reach 10% to 90% of final value. tr = t2-t1 sag: It is defined as large time constant at which amplifier output tend to decrease 7.what is conversion efficiency in a power amplifier? The ratio of AC power delivered to the load to the DC power input is called conversion efficiency of a power amplifier 8. What is cross over distortion in a power amplifier and how to eliminate it? For making transistor ON, it is necessary that V BE voltage must exceed 0.7V. Due to this, in class B amplifier while crossing over from one half cycle to other, as long as input is below 0.7V, none of the transistor is ON and output is Zero. Due to this, there is distortion in the output, which is called the cross over distortion. To over come this distortion, a small forward bias is kept applied to the transistors so that when input is zero, this additional forward bias can make the transistor ON immediately elimination cross over distortion. 9. Derive the ripple factor for a half wave and Full Wave Rerctifier. Ripple factor is (1.21) for half wave 43

44 Ripple factor is (0.483) full wave 10. Draw the block diagram of switched mode power supply. Part B 11. (i) Derive the various stability factors (12) Define Stability factor and derive the stability factor w.r.t Vbe, Ic0 and β Explain Unregistered the fixed bias circuit and derive Version expression for IC,VCE and Stability factor (ii) Draw the source self bias and voltage divider bias of FET(4) source self bias and voltage divider bias of FET 12.a.(i) Derive the expression for Current gain, voltage gain, input impedance and output admittance of small signal transistor amplifier using h-parameter. AC Analysis - Draw AC equivalent circuit derive the expression for Current gain, voltage gain, input impedance and output impedance. (ii) Draw the darlington emitter follower circuit(4). darlington emitter follower digram 44

45 12 b(i) Explain the operation of emitter coupled differential amplifier (12) Draw the Differential amplifier diagram Explain about Common mode of operation and differential mode operation B (ii) Discuss about Unregistered transfer characteristics Version of differential amplifier.(4) Draw transfer characteristics of differential amplifier Write about it 13. a. Discuss about the low frequency and high frequency response of Amplifier. Low frequency response Draw each rc network (containing bypass and coupling capacitor) Derive gain of that each network. Interms of frequency High frequency response Draw each rc network (containing internal capacitor) Derive gain of that each network. Interms of frequency 13. b. Draw the high frequency hybrid π model for a transistor in the CE configuration and explain the significance of each component. Define alpha cut off frequency. (16) Draw the hybrid π model of CE amplifier Explain about importance of all capacitor Define α-cut off frequency Derive fα 14. a. Explain the operation of transformer coupled audio power amplifier. (16) Draw the transformer coupled class A power amplifier Explain the operation of it Derive the Unregistered efficiency of transformer- Version coupled class A power amplifier b. Explain the operation of of Push Pull class B amplifier is 78.5%. (16) 45

46 Draw the Push Pull class B amplifier Explain the operation of it Derive the Created max. efficiency by of Push PDFTiger Pull class B amplifier is 78.5% 15. a. Derive the Ripple factor for following filters: L, C, LC, CLC Draw the half wave rectifier cicuit with L,C,LC,CLC filter Derive the ripple factor expression 15.b. Discuss about electronically regulated DC power Supply. Draw the Regulated Power Supply Explain the operation B.E./B.Tech.D EGREE EXAMINATION, APRILA4AY Third Semester Electronics Unregistered and Communication Version Engineering EC ELECTRONIC CIRCUITS - I Time : Three hours Maximum : 100 marks Questions and Answer PART A - (10 x2 = 20 marks) 1. Define quiescent point. When we bias a transistor we establish Collector current (Ic)and Collector to Emitter voltage (Vce) conditions for the transistor. These conditions are known as operating condition or dc operating point or quiescent point. 2. What is bias compensation using thermistor? It refers to the use of temperature-sensitive devices such as thermisotrs, etc., which provide compensating Created voltages by and PDFTiger currents to maintain the operating point stable. 3. Define the four h parameters. h11 = hi = input resistance 46

47 h12 = hr = reverse transfer voltage ratio h21 = hf = forward transfer current ratio h22 = ho = Output conductance 4. What does bootstrapping mean? The name arises from the fact that, if one end of the resistor R 3 changes in voltage, the other end of R 3 moves through the same potential difference; it is as if R 3 is pulling itself up by its bootstraps. 5. What is cross over distortion? How it can be eliminated? For making transistor ON, it is necessary that V BE voltage must exceed 0.7V. Due to this, in class B amplifier while crossing over from one half cycle to other, as long as input is below 0.7V, none of the transistor is ON and output is Zero. Due to this, there is distortion in the output, which is called the cross over distortion. To over come Created this distortion, by a small PDFTiger forward bias is kept applied to the transistors so that when input is zero, this additional forward bias can make the transistor ON immediately elimination cross over distortion. 6. Draw the power flow diagram of the transistor.(out of portion) 7. Draw the high frequency equivalent of CE amplifier. 8. Draw the frequency response of video amplifier..(out of portion) 9. Define TUF. 47

48 The factor which indicates how much is the utilization of the transformer in the circuit. dc power delivered to the load TUF = ac power rating of the transformer 10. Define voltage regulation. It is defined as variation of dc output voltage with change in dc load current %regulation = ((VnL - VL)/VL) * 100 % Part B (5x16=80marks) (i)with the help of a neat diagram explain the voltage divider biasing method for JFET. (8) Draw voltage divider bias of FET write about voltage divider bias of FET derive ID and Vds (ii)define stability factor.derive and explain the condition to avoid thermal runaway.( 8 ) Ans: Define stability factor Define thermal runway Write about how thermal runway occuring Compensation techniques either using diode, thermistor to avoid thermal runway Or 48

49 (b) (i) For the voltage divider bias circuit in figure calculate the base voltage (Vb), base current (Ib), emitter voltage (Ve), emitter current (Ie), collector voltage (Vc) and collector to emitter voltage (Vce). The current gain can vary from 30 to 300. Calculate the worst case base current. ( 10) Ans: Draw voltage divider bias circuit Write the Thevenin equivalent network for the network to the left of the base terminal to be found. To find Rth: Fig 1.12 From the above circuit, Rth = R1 \\R2 = R1 R2 / (R1 + R2) To find Eth Fig 1.13 From the above circuit, Eth = VR2 = R2VCC / (R1 + R2) 49

50 In the above network, applying KVL ( Eth VBE) = IB [ Rth +( β+ 1) RE ] IB = ( Eth VBE) / [ Rth +(β + 1) RE ] Analysis of Output loop KVL to the output loop: VCC = ICRC + VCE + IERE IE~ IC = β*i B VE = IERE Thus, VCE = VCC IC (RC + RE) The above procedure will be used to calculate the IE, IC, IB, VE and VCE for various values of β = 100, 30, 300 Finally check for which β value IC and VCE varying too much from Ic and VCE at β=100. For that value, find Ib this will be the worst case IB. (ii) Write short notes on temperature compensation methods. (6) Write why we need tempersature compensation. Write different methods of tempersature compensation 12. (a) (i) Draw the circuit diagram for a differential amplifier using BJTs. Describe common mode and differential modes of working. (8) Draw the Differential amplifier diagram Explain about Common mode of operation 50