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FEEDBACK AMPLIFIERS Block diagram, Loop gain, Gain with feedback, Effects of negative feedback – Sensitivity and desensitivity of gain, Cut-off frequencies, distortion, noise, input impedance and output impedance with feedback, Four types of negative feedback connections – voltage series feedback, voltage shunt feedback, current series feedback and current shunt feedback, Method of identifying feedback topology and feedback factor, Nyquist criterion for stability of feedback amplifiers. OSCILLATORS Classification, Barkhausen Criterion - Mechanism for start of oscillation and stabilization of amplitude, General form of an Oscillator, Analysis of LC oscillators Hartley, Colpitts, Clapp, Franklin, Armstrong, Tuned collector oscillators, RC oscillators - phase shift – Wienbridge - Twin-T Oscillators, Frequency range of RC and LC Oscillators, Quartz Crystal Construction, Electrical equivalent circuit of Crystal, Miller and Pierce Crystal oscillators, frequency stability of oscillators TUNED AMPLIFIERS Coil losses, unloaded and loaded Q of tank circuits, small signal tuned amplifiers Analysis of capacitor coupled single tuned amplifier – double tuned amplifier - effect of cascading single tuned and double tuned amplifiers on bandwidth – Stagger tuned amplifiers – large signal tuned amplifiers – Class C tuned amplifier – Efficiency and applications of Class C tuned amplifier - Stability of tuned amplifiers – Neutralization - Hazeltine neutralization method. WAVE SHAPING AND MULTIVIBRATOR CIRCUITS RC & RL Integrator and Differentiator circuits – Storage, Delay and Calculation of Transistor Switching Times – Speed-up Capaitor - Diode clippers, Diode comparator Clampers. Collector coupled and Emitter coupled Astable multivibrator - Monostable multivibrator - Bistable multivibrators - Triggering methods for Bistable multivibrators - Schmitt trigger circuit. BLOCKING OSCILLATORS AND TIMEBASE GENERATORS UJT sawtooth waveform generator, Pulse transformers – equivalent circuit – response - applications, Blocking Oscillator – Free running blocking oscillator - Astable Blocking Oscillators with base timing – Push-pull Astable blocking oscillator with emitter timing, Frequency control using core saturation, Triggered blocking oscillator – Monostable blocking oscillator with base timing – Monostable blocking oscillator with emitter timing, Time base circuits - Voltage-Time base circuit, Current-Time base circuit - Linearization through adjustment of driving waveform. REFERENCE Text Books
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1.Sedra / Smith, Micro Electronic Circuits Oxford University Press, 2004. 2.S. Salivahanan, N. Suresh Kumar and A. Vallavaraj, Electronic Devices and Circuits, 2nd Edition, TMH, 2007. Reference Books 1.Millman J. and Taub H., Pulse Digital and Switching Waveforms, TMH, 2000. 2.Schilling and Belove, Electronic Circuits, 3rd Edition, TMH, 2002. 3.Robert L. Boylestad and Louis Nasheresky, Electronic Devices and Circuit Theory, 9th Edition, Pearson Education / PHI, 2002. 4.David A. Bell, Solid State Pulse Circuits, Prentice Hall of India, 1992. 5.Millman and Halkias. C., Integrated Electronics, TMH, 1991.
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Block diagram
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UJT A Unijunction transistor is a three terminal semiconductor switching device.this device has a unique characteristics that when it is triggered , the emitter current increases regeneratively until is limited by emitter power supply the unijunction transistor can be employed in a variety of applications switching pulse generator saw tooth generator etc.
Construction
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It consists of an N type silicon bar with an electrical connection on each end the leads to these connection are called base leads. Base 1 B1 Base 2 B2 the bar between the two bases nearer to B2 than B1. A pn junction is formed between a p type emitter and Bar.the lead to the junction is called emitter lead E. Operation
The device has normally B2 positive w.r.t B1 If voltage VBB is applied between B2 and B1 with emitter open. Voltage gradient is established along the n type bar since emitter is located nearer to B2 more than half of VBB appears between the emitter and B1. the voltage V1 between emitter and B1 establishes a reverse bias on the pn junction and the emitter current is cut off. A small leakage current flows from B2 to emitter due to minority carriers If a positive voltage is applied at the emitter the pn junction will remain reverse biased so long as the input voltage is less than V1 if the input voltage to the emitter exceeds V1 the pn junction becomes forward biased. under these conditions holes are injected from the p type material into the n type bar these holes are repelled by positive B2 terminal and they are attracted towards B1 terminal of the bar. This accumulation of holes in the emitter to B1 region results in the degrees of resistance in this section of the bar the internal voltage drop from emitter to b1 is decreased hence emitter current Ie increases as more holes are injected a condition of saturation will eventually be reached at this point a emitter current limited by emitter power supply only the devices is in on state. If a negative pulse is applied to the emitter, the pn junction is reverse biased and the emitter current is cut off. The device is said to be off state. Characteristics of UJT
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The curve between Emitter voltage Ve and emitter current Ie of a UJT at a given voltage Vbb between the bases this is known as emitter characteristic of UJT Initially in the cut off region as Ve increases from zero ,slight leakage current flows from terminal B2 to the emitter the current is due to the minority carriers in the reverse biased diode . Above a certain value of Ve forward Ie begins to flow , increasing until the peak voltage Vp and current Ip are reached at point P. After the peak point P an attempt to increase Ve is followed by a sudden increases in emitter current Ie with decrease in Ve is a negative resistance portion of the curve The negative portion of the curve lasts until the valley point V is reached with valley point voltage Vv.and valley point current Iv after the valley point the device is driven to saturation the difference Vp-Vv is a measure of a switching efficiency of UJT fall of Vbb decreases Advantages of UJT It is a Low cost device It has excellent characteristics It is a low-power absorbing device under normal operating conditions
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TRANSFORMER EQUIVALENT CIRCUIT: The influences of a transformer’s parameters can best be understood by considering the equivalent circuit in below. This circuit shows a typical output pulse waveform. Assuming that this output pulse is the result of injecting an ideal rectangular input pulse, one can see that a number of parameters are distorted. Overshoot, droop, back swing, rise time, etc. appear as unwanted signal distortion on the output pulse. Assuming the pulse transformer is properly matched and the source is delivering an ideal rectangular pulse, the transformer should have low values of leakage inductance and distributed capacitance while having a high open circuit inductance. This will limit the deterioration of the pulse shape. Also, the fact that the source will never produce an ideal rectangular pulse adds to the problems of distortion.
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Einstein College of Engineering ECE TWO MARK QUESTIONS EC-41-ELECTRONIC CIRCUITS II
1. Define feedback? A portion of the output signal is taken from the output of the amplifier and is combined with the normal input signal. This is known as feedback.
2. Define positive feedback? If the feedback signal is in phase with input signal, then the net effect of the feedback will increase the input signal given to the amplifier. This type of feedback is said to be positive or regenerative feedback.
3. Define negative feedback? If the feedback signal is out of phase with the input signal then the input voltage applied to the basic amplifier is decreased and correspondingly the output is decreased. This type of feedback is known as negative or degenerative feedback.
4. Define sensitivity? Sensitivity is defined as the ratio of percentage change in voltage gain with feedback to the percentage change in voltage gain without feedback. dAf/A=(dA/A)(1/1+βA)
5. What are the types of feedback? i. Voltage-series feedback ii. Voltage-shunt feedback iii. Current-series feedback iv. Current-shunt feedback
6. What are the basic amplifiers? The basic amplifiers are
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Voltage amplifier Current amplifier Transconductance amplifier Transresistance amplifier.
7. What are the components of feedback amplifier? The components are sampling network, Feedback network, and mixer network.
8. What are two types of sampling? Voltage sampling or node sampling Current sampling or loop sampling 9. State the two types of mixing? Series mixing Shunt mixing
10. What is transfer gain? It is the ratio of the output signal to the input signal. It is denoted by A. A=Xo/Xi
11. List out the characteristics of feedback amplifier? Desensitivity Nonlinear distortion Noise distortion Frequency distortion
12. What is the effect of input resistance due to series mixing? The input resistance increases due to series mixing irrespective of the type of sampling. The feedback signal opposes the source signal and the input current decreases and due to this input resistance increases.
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Rif>Ri Where Rif = input resistance with feedback Ri = input resistance without feedback
13. What is the effect of input resistance due to shunt mixing? The input resistance decreases due to shunt mixing irrespective of the type of sampling. The feedback signal opposes the source signal and the input current (Ii) decreases as a consequence ViIi reduces leading toa reduction in input resistance. Rif
14. What happens to output resistance due to current sampling? The output resistance increases due to current sampling. Rof>Ro Rof = input resistance with feedback Ro= input resistance without feedback
15. What happens to output resistance due to voltage sampling? The output resistance decreases due to current sampling. Rof
16. What happens to output resistance due to current sampling? The output resistance increases due to current sampling. Rof>Ro Rof = input resistance with feedback Ro= input resistance without feedback
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17. Write the expression for input and output resistance of voltage series feedback amplifier. Input resistance with feedback, Rif = Ri(1+βA) Output resistance with feedback, Rof = Ro(1+βA)
18. Give the properties of negative feedback. i. Negative feedback reduces the gain ii. Distortion is very much reduced
19. Give the effect of negative feedback on amplifier characteristics. Types of feedback Characteristics
Current-series
Voltage-series
Voltage-shunt
Current-shunt
Voltage gain
Decreases
Decreases
Decreases
Decreases
Bandwidth
Increases
Increases
Increases
Increases
Input resistance Increases
Increases
Decreases
Decreases
Output resistance
Decreases
Decreases
Increases
Increases
20. What is Oscillator circuit? A circuit with an active device is used to produce an alternating current is called an oscillator circuit.
21. What is type of feedback used in oscillators? Positive feedback
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22. Differentiate Oscillators and Feedback Amplifiers.
Parameters
Feedback Amplifiers
Oscillators
Input signal
Needed
Not needed
Type of feedback
negative
Positive
Gain
Reduces
Increases leading to oscillations
Example
Voltage series feedback amplifier
Hartley oscillator
23. What are the classifications of Oscillators? *Based on wave generated: i. Sinusoidal Oscillator, ii. Non-sinusoidal Oscillator or Relaxation Oscillator Ex: Square wave, Triangular wave, Rectangular wave etc. *According to principle involved: i. Negative resistance Oscillator, ii. Feedback Oscillator. *According to frequency generated: i. Audio frequency oscillator 20 Hz – 20 kHz ii. Radio frequency Oscillator 30 kHz – 30 MHz iii. Ultrahigh frequency Oscillator 30 MHz – 3 GHz iv. Microwave Oscillator 3 GHz – above.
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24. On the basis of circuit components classify oscillators. (i) RC Oscillators (ii) LC Oscillators
25. Define Barhausen Criterion. The product βAv is greater than one this is called Barhausen criterion. Avf =Av/1- βAv
1- βAv <0 βAv >1 this is the condition for feedback Oscillator. An Oscillator which follows Barkhausen criterion is called the Feedback Oscilltor.
26. What are the types of feedback oscillators? * RC-Phase shift Oscillator, * LC-Oscillators i. Tuned collector Oscillator ii. Tuned emitter Oscillator iii. Tuned collector base Oscillator iv. Hartley Oscillator v. Colpits Oscillator vi. Clap Oscillator
27. What are the conditions for oscillation? The total phase shift of an oscillator should be 360o. For feedback oscillator it should satisfies Barhausen criterion.
28. Define Piezoelectric effect. When applying mechanical energy to some type of crystals called piezoelectric crystals the mechanical energy is converted into electrical energy is called piezoelectric effect.
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29. Draw the equivalent circuit of crystal oscillator.
30. What are the two types of crystal Oscillator? Pierce crystal oscillator Miller crystal oscillator
31. What is Miller crystal oscillator? Explain its operation. It is nothing but a Hartley oscillator its feedback Network is replaced by a crystal. Crystal normally generate higher frequency reactance due to the miller capacitance are in effect between the transistor terminal.
32. What is Pierce crystal oscillator? It is nothing but a Colpitts oscillator its feedback Network is replaced by a crystal.
33. State the frequency for RC phase shift oscillator. The frequency of oscillation of RC-phase shift oscillator is F=1/2∏RC√ (4k+6) Where k=2.639.
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34. What is a tuned amplifier? The amplifier with a circuit that is capable of amplifying a signal over a narrow band of frequencies are called tuned amplifiers.
35. What is the expression for resonant frequency? fr=1/2∏√LC
36. What happens to the circuit above and below resonance? Above resonance the circuit acts as capacitive and below resonance the circuit acts as inductive.
37. What are the different coil losses? Hysteresis loss Copper loss Eddy current loss
38. What is Q factor? It is the ratio of reactance to resistance.
39. What is dissipation factor? It is referred as the total loss within a component i.e1/Q
40. What is the classification of tuned amplifiers? Single tuned Double tuned Stagger tuned
41. What is a single tuned amplifier?
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An amplifier circuit that uses a single parallel tuned circuit as a load is called single tuned amplifier.
42. What are the advantages of tuned amplifiers? They amplify defined frequencies. Signal to noise ratio at output is good They are suited for radio transmitters and receivers
43. What are the disadvantages of tuned amplifiers? The circuit is bulky and costly The design is complex. They are not suited to amplify audio frequencies.
44. What is neutralization? The effect of collector to base capacitance of the transistor is neutralized by introducing a signal that cancels the signal coupled through collector base capacitance. This process is called neutralization.
45. What is a stagger tuned amplifier? It is a circuit in which two single tuned cascaded amplifiers having certain bandwidth are taken and their resonant frequencies are adjusted that they are separated by an amount equal to the bandwidth of each stage. Since resonant frequencies are displaced it is called stagger tuned amplifier.
46. What are the advantages of stagger tuned amplifier? The advantage of stagger tuned amplifier is to have better flat, wideband characteristics.
47 What are the different types of neutralization? 1. Hazeltine neutralization 2. Rice neutralization
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3. Neutrodyne neutralization.
48. What is rice neutralization? It uses center tapped coil in the base circuit. The signal voltages at the end of tuned base coil are equal and out of phase.
49. What is unloaded Q? It is the ratio of stored energy to the dissipated energy in a reactor or resonator.
50. What are the applications of mixer circuits? Used in radio receivers. Used to translate signal frequency to some lower frequency
51. What is up converter? When the mixer circuit is used to translate signal to high frequency, then it is called up converter.
52. What is a Multivibrator? The electronic circuits which are used to generate nonsinusoidal waveforms are called Multivibrators.
53. Name the types of Multivibrators? Bistable Multivibrator, Monostable Multivibrator,Astable Multivibrator
54. How many stable states do bistable Multivibrator have? Two stable states.
55. When will the circuit change from stable state in bistable Multivibrator ?
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When an external trigger pulse is applied, the circuit changes from one stable state to another.
56. What are the different names of bistable Multivibrator? Eccles Jordan circuit, trigger circuit, scale-of-2 toggle circuit, flip-flop and binary.
57. What are the applications of bistable Multivibrator? It is used in the performance of many digital operations such as counting and storing of the Binary information. It also finds applications in the generation and processing of pulse – type waveforms.
58. What are the other names of monostable Multivibrator? One-shot, Single-shot, a single-cycle, a single swing, a single step Multivibrator, Univibrator.
59. Why is monostable Multivibrator called gatting circuit? The circuit is used to generate the rectangular waveform and hence can be used to gate other Circuits hence called gating circuit.
60. Why is monostable Multivibrator called delay circuit? The time between the transition from quasi-stable state to stable state can be predetermined and hence it can be used to introduce time delays with the help of fast transition. Due to this application is Called delay circuit.
61. What is the main characteristics of Astable Multivibrator The Astable Multivibrator automatically makes the successive transitions from one quasi- stable State to other without any external triggering pulse.
62. What is the other name of Astable Multivibrator- why is it called so? As it does not require any external pulse for transition, it is called free running
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Multivibrator.
63. What are the two types of transister bistable Multivibrator? i. Fixed bias transistor circuit ii. Self bias transistor circuit.
64. Why does one of the transistor start conducting ahead of other? The characteristic of both the transistors are never identical hence after giving supply one of the Transistors start conducting ahead of the other.
65. What are the two stable states of bistable Multivibrator? i. Q1 OFF (cut off) and Q2 ON (Saturation) ii. Q2 OFF (Cut off) and Q1 On (Saturation)
66. What finally decides the shape of the waveform for bistable multivibrator? The spacing of the triggering pulses
67. How are the values R1, R2 and VBB chosen in bistable Multivibrator? It is chosen in such a way that in one state the base current is large enough to drive the transistor into saturation while in other state the emitter junctions is well below off.
68. What is the self biased Multivibrator? The need for the negative power supply in fixed bias bistable Multivibrator can be eliminated by rising a common emitter resistance RE. The resistance previous the necessary bias to keep one transistor or and the other OFF in the stable state such type of biasing is called self biasing and the circuit is called self biased bistable Multivibrator.
69. What are the other names of speed up capacitors. i. Commutating Capacitors
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ii. Transpose capacitors
70. Define transition time? It is defined as the time interval during which conduction transfers from one transistor to other.
71. What is the value of commutating capacitor. It lies in the range of tens to some hundreds of Pico farads.
72. Define resolving time. The smallest allowable interval between triggers is called resolving time.
73. Give the expression of fmax with respect to resolving time Fmax = 1/resolving time.
74. Define gate width The pulse width is the time for which the circuit remains in the quasi stable state. It is also called gate width.
75. What are the advantages of monostable Multivibrator. - used to introduce time delays as gate width is adjustable - used to produce rectangular waveform and hence can be used as gating circuit.
76. What are the applications of astable Multivibtrator. - used as a clock for binary login signals - used as a square wave generator, voltage to frequency converter.
77. What is a complementary Multivibrator
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It is turning half the circuit upside down. So one transistor is n-p-n while the other is p-n-p. The circuit is called complementary Multivibrator circuit.
78. What is UTP of the Schmitt trigger When Vi reaches to VBE1 +VE the Q1 gets driven to active region. This input voltage level is called upper threshold point.
79. What is the other name for UTP It is also called input turn on threshold level.
80. What is LTP Schmitt trigger. The level of Vi at which Q1 becomes OFF and Q2 on is called lower threshold point.
81. Define transfer Characteristics The graph of output voltage against input voltage is called transfer characteristics of Schmitt trigger.
82. What is the important application of Schmitt trigger? - It is used as an amplitude comparator - It is used as a squaring circuit.
83. Define Blocking Oscillator? A special type of wave generator which is used to produce a single narrow pulse or train of pulses. 84. What are the two important elements of Blocking Oscillator? Transistor and pulse transformer
85. What are the applications of blocking Oscillator?
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It is used in frequency dividers, counter circuits and for switching the other circuits.
86. Give the expression for co-efficient of coupling
K=M/LpLs
M-> Mututal Inductance Lp -> Primary Inductance Ls -> Secondary Inductance
87. Give the formula for transformation ratio n= Ns/Np = transformation ratio Ns= Secondary Turns; Np= Primary turns
88. Define rise time It is defined by the time required by the pulse to rise from 10% of its amplitude to 90% of its amplitude.
89. Define overshoot. It is the amount by which the output exceeds its amplitude during first attempt. 90. Define flat top response. The position of the response between the trailing edge and the leading edge.
91. Define droop or a tilt The displacement of the pulse amplitude during its flat response is called droop or a tilt.
92. What are the applications of pulse transformer.
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i. to invert the polarity of the pulse ii. to differentiate pulse
93. When do the core saturates? When L->o as B-> Bm, the core saturates
94.What is the other name of astable Blocking Oscillator Free running blocking Oscillator
95. What are the two types of astable Blocking Oscillator? 1. Diode controlled Astable Blocking Oscillator. 2. Re controlled Astable Blocking Oscillator.
96. Define Sweeptime in sawtooth generator The period during which voltage increases linearly is called sweep time.
97.What is the other name of sawtooth generator? Ramp generator
98. Define Displacement error in the sawtooth generator? It is defined as the maximum differenece between the actual sweep voltage and linear sweep which passes through the beginning and end points of the actual sweep.
99. What is constant current charging? A capacitor is charged with a constant current source.
100. What is the miller circuit Integrator is used to convert a step waveform into ramp waveform.
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Einstein College of Engineering ECE PART B
1. Explain the relevant information ,how the negative feedback improves stability reduce noise and increase input impedance? Draw the circuit diagram. Explain detail the following transfer gain. stability of gain. The transfer of gain of the amplifier is not constant as it is depends upon the factors such as operating point temperature, etc. This lack of stability can be reduced by introducing negative feedback. The signal feed back reduces the amount of the noise signal and non linear distortion. The factor (1+A) reduces both input noise and resulting non linear distortion for considerable improvement. Thus, noise and non linear distortion also reduced by same factor.
2. Explain voltage shunt feed back amplifiers? Draw the circuit diagram. Draw the equivalent circuit. Find the input and output impedance after feed back. 3. Explain current series feed back amplifiers? Draw the circuit diagram. Draw the equivalent circuit. Find the input and output impedance after feed back. 4. Explain the classification of amplifiers? Explain the following in detail.
Voltage amplifier. Current amplifier. Transconductance amplifier. Trans resistance amplifier.
5. Explain current shunt and voltage shunt feed back amplifiers? Draw the circuit diagram. Draw the equivalent circuit. Find the input and output impedance after feed back. 6. With simple diagrams explain the operation of negative resistance oscillator using tunnel diode? Draw the circuit diagram and graph. Draw the characteristics of tunnel diode. Get the expression for time period ‘t’.
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Draw the wave form for negative resistance oscillator. 7. Explain RC phase shift oscillator?. Draw the circuit diagram Draw the equivalent circuit. Derive the minimum value of hfe for oscillation. 8. Explain Clapp’s oscillator and derive the expression for frequency of oscillation. Also explain how frequency stability can be improved Clapp’s oscillator? Draw the circuit diagram Draw the equivalent circuit. Derive the frequency of oscillation. 9. Explain Hartley oscillator and derive the equation for oscillation? Draw the circuit diagram Draw the equivalent circuit. Derive the frequency of oscillation. 10. Explain pierce crystal oscillator and derive the equation for oscillation? Draw the circuit diagram Draw the equivalent circuit. Derive the frequency of oscillation. 11. Explain in detail about single tuned amplifier Draw the circuit diagram Draw the equivalent circuit. Derive the expression for band width 12. Explain in detail about stagger-tuned amplifier Draw the circuit diagram Draw the equivalent circuit. Derive the expression for band width 13. Compare single tuned and stagger tuned amplifier Compare the circuit diagram Compare the equivalent circuit. Compare the expression for band width 14. Explain the different types of neutralization?
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Explain Hazeltine neutralization Explain Rice neutralization. Explain Neutrodyne neutralization 15. Explain bistable Multivibrator and its types? General form of bistable Multivibrator circuit. fixed Bias transistor bistable Multivibrator circuit self Bias transistor biastable Multivibrator circuit Applications
16. Explain about speedup capacitors or commutating capacitors Practical self biased bistable Multivibrator Explanation about the circuit
17. Explain about Monostable Multivibrator Explanation about the circuit diagram Pulse width of collector coupled Monostable Multivibrator Wareforms Applications
18. Explain about collector coupled astable Multivibrator Explanation about the circuit diagram Waveforms Distration & its eliminator Applications
19. Explain emitter coupled astable Multivibrator Operation and Mathematical analysis Practical circuit
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Advantages and disadvantages of the Multivibrator
20. Write in detail about Schmitt Trigger circuit? Circuit diagram Operation of the circuit Schmitt trigger wareforms. Hysterisis Applications
21. Explain about pulse transformer? Ideal pulse transformer model Practical equivalent circuit Pulse response characteristics Applications of pulse transformer
22. Explain Monostable blocking oscillator using emitter timing? Circuit Diagram Mathematical analysis Expression for pulse width Triggering circuit for monostable blocking oscillator
23. Write about the core saturation method Circuit diagram Waveforms of ic and iB when core Saturates.
24. Write about astable blocking oscillator. Diode controlled astable blocking Oscillator
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RC controlled astable blocking Oscillator
25. Write about UJT sawtooth generator Operation Circuit diagram
26. What will happen when a step input voltage is applied to the high pass RC Circuit? Derivation The output Waveform
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