BLOCKING OSCILLATORS AND TIMEBASE GENERATORS UNIT-V

                                                                         UNIT-V
                             BLOCKING OSCILLATORS AND TIMEBASE GENERATORS
PART –A
1. Define rise time of a pulse.                     (MAY 2004)
    The rise time is an important parameter related to this part of the response. It is defined by the time  required by the pulse to rise from 10% of its amplitude to 90% of its amplitude.

2. Define sweep speed error or slope error.       (JUN 2006)
    It is the ratio of difference in slope at beginning and end of sweep to the initial value of slope.
3. Define displacement error.           (JUN 2006&MAY 2011)
    It is defined as the maximum difference between the actual sweep voltage and linear sweep which passes  through the beginning and end points of the actual sweep.

4. What is UJT relaxation oscillator?               (MAY 2010)
        A unijunction transistor in conjunction with a capacitor and a charging resistor to construct an oscillator with an approximate ramp type output is known as UJT relaxation oscillator.

5. What is blocking oscillator? Why it is so called? (MAY 2008 , MAY 2010, NOV 2011)
    A tuned collector oscillator circuit which uses a regenerative (positive) feedback via a pulse transformer,producing a single pulse or pulse train is called a blocking oscillator. The oscillator cuts itself OFF or  BLOCKS after one or more cycles. Hence the name Blocking oscillator.

6. The transistor current sweep circuit will not provide precisely linear sweep. Why?       (DEC 2006)
    1. Lag of impulse term in current source 2.The inductance of iron core varies with current.  3. The driver   transistor which provides the inductor current does not operate with sufficient linearity.

7. What is a pulse transformer?           (MAY 2010)
       A pulse transformer is basically a transformer, which couples a source of pulses of electrical energy to the load keeping the shape and other properties of pulse unchanged. The voltage level of the pulse can be raised or
lowered by designing the proper turns ratio for the pulse transformer.

8. State the applications of pulse transformer.       (MAY 2010, MAY 2012)
    Pulse transformer can be used to 1.change the amplitude and impedance level of a pulse  2.invert the polarity of a pulse  3.produce a pulse
in a circuit having negligible dc resistance  4.effect dc isolation between source and load  5.couple between stages of pulse amplifiers  6.differentiate a pulse  7. acts as a coupling element in certain pulse generating circuit.

9. What are the applications of blocking oscillator?(MAY 2008 & MAY 2011)
1. The blocking oscillator can be used as a frequency divider or conter.
2. The blocking oscillator as a low impedance switch can be used to discharge a capacitor quickly.
3. The output of the blocking oscillator can be used as a gating waveform with a very small mark-space ratio.
 4.Blocking oscillator is capable of generating a pulse of large peak power. The average power is small since the duty cycle is low. 
5. The astable circuit is used as a master oscillator to supply triggers for synchronizing a system of pulse type waveforms-square waves,sweep voltages etc., 
6. The monostable circuit is used to obtain abrupt pulses from a slowly varying
    input triggering voltage. 
7. Using a tertiary winding, output pulses of either polarity may be obtained
    depending upon which end of the winding is grounded. Also, the output  winding may be isolated from ground  where required.

10. List various sweep circuits. Or What are the various methods of generating a time base waveform?                           (MAY 2008)
    1.Exponential charging circuit  2.Constsnt-current charging circuit  3.Miller circuit  4.Phantastron     circuit  5.Bootstrap circuit  6.Inductor circuit  7. Compensating networks.

11. What are current time base generator?        (DEC 2005)
         Current time base generator are the circuits which produce a current waveform which increases linearly with time. They are used in magnetic deflection system as television displays on large screen tubes and radars.

12. What are the drawbacks of time base generator? How can it be reduced?                            (DEC 2008)
        The main drawback of time base generator is the high percentage slope speed error. It can be reduced by  improving the linearity of the sweep by increasing the supply voltage or increasing the time constant of the      circuit.

13. Define Restoration time or Flyback time.     (JUN 2009, MAY 2012)
        The time required for the return for the sweep voltage to the initial value is called the restoration time, the return time or fly back time
.
14. Define sweep time.      (DEC 2008, JUN 2009, MAY 2012)
    The period during which voltage increases linearly is called sweep time.

15. How can the linearization of trapezoidal voltage can be obtained? Or How is linearization achieved in  current time base circuit?     (JUN 2010)
         It can be obtained by using three different circuit such as bootstrapping, using an op-amp or miller integrator and by changing a capacitor from a common current source such as common base transistor.

16. What is a sweep generator?                       (MAY 2010)
    A sweep generator is a circuit that produces a sweep waveform. Sweep waveforms are those which have at least one portion in it with respect to time. There are different types of sweep generators like Miller time base  sweep generator,Bootstrap sweep generator,etc.,

17. What do you mean by free running blocking oscillator?Why they are called so?                             (JUN 2010)
    Astable blocking oscillator is called as free running blocking oscillator.It produces train of pulses when triggered. The pulse width and the duty cycle of the blocking oscillator output can be controlled as per the       requirement. There are two types of astable blocking oscillators available:1.Diode controlled astable blocking oscillator  2.RC controlled astable blocking oscillator.   

18.Define transmission error.                  (MAY 2004)
    When a ramp voltage is transmitted through a high-pass RC network, its output falls away from the input.The transmission error is defined as the difference between the input and output divided by the input.

19. How will you form an astable blocking oscillator from a monostable blocking oscillator?                      (DEC 2005)
           A diode network can be replaced by RC network to obtain RC controlled astable blocking oscillator. Such RC network can be added either in the emitter circuit of a monostable blocking oscillator or in the base circuit       of a monostable blocking oscillator.

20. What do you mean by linear time base generator?    (JUN 2007)
    A linear time base generator is one that provides an output waveform, a portion of which exhibits a linear variation of voltage or current with time.

21. Compare diode controlled and RC controlled blocking oscillator.                                    (MAY 2008)
    Diode controlled blocking oscillator    RC controlled blocking oscillator
1.Low duty cycle operation is possible.    1. Low duty cycle operation is impossible
2.Time period and frequency of oscillations  can be easily varied by varying R1 or C1    2. Time period and frequency can not be easily varied. To change time period it is necessary to change the entire diode network to alter Vγ value.
3.The oscillations just before the pulse are missing and there is much less possibility of such oscillations.    3.There is possibility of oscillations just before the pulse for low duty cycle operation
4.The design equations are simpler.    4. The equations obtained are complicated and some are transcendental which require graphical solution.
5.The circuit is easy to design.    5. The circuit is difficult to design due to complexity of the equations.
6.The timing values are temperature dependent as diode network is temperature sensitive.    6. The time values does not depend on diode which is temperature sensitive. Hence the timing values are stable and temperature independent provided VBB >>Vγ of transistor and all the elements like R1,C1 etc., are temperature independent.
7. The overshoots after the pulse are absent and hence the waveforms are better and smooth.    7. In low duty cycle operation, voltage across capacitor discharges slowly. Due to this pulse at the collector has distorted waveform.

22. Give the uses of time base generator.          (MAY 2011)
         It is used in deflection plates of CRO to sweep the electron beam from left to right. It is also used in TV.

23. Give the expression for displacement error.
             (vs – vs’)max          Ts
ed =                                       =          
                         Vs                8RC   
24. Define sweep speed.
    The sweep speed is defined as the rate of change of sweep voltage with respect to time.

25. Give the expression for sweep speed error.
        Difference in slope at beginning and end of sweep                Ts
es =                                                                                                              =         
            initial value of slope                                 RC   

26. Give the expression for transmission error.
               Vs’ – Vs                Ts
et =                                        =          
                            Vs                2RC   
27. Give the expression for sweep speed for exponential charging.
    dvs         t
                    =           x  e-t/RC     
             dt         RC

28. Give the expression for sweep speed for constant current charging.
    dvs          I
                    =        
             dt           C

29. Give the expression for relation between transmission error, sweep speed error and displacement error.
                1               1
ed =            es =         et
                         8               4
30. What is the purpose of compensating network?
    A compensating circuit is added to improve the linearity of the Bootstrap and the Miller time base generators.

31. How can you suppress the transformer oscillation?
     In order to suppress the transformer oscillation without loading the blocking oscillator during the pulse interval a damping resistor R in series with a diode may be shunted across the transformer. The diode is  introduced with such a polarity that it does not conduct during the pulse interval, but does conduct during the overshoot.

32. What is the principle used in bootstrap circuit?
         Bootstrap generator provides a very linear output ramp. The amplitude of the ramp can approach the level of the supply voltage. The output ramp amplitude can be made adjustable over a fixed time period by       making resistor adjustable.

33. What are the methods used to improve the linearity of the sweep circuits?
        a)By generating an impulse     b)By improving the linearity of current divider for yoke.c)By linearization of trapezoidal voltage.

34. Write the equation used to determine sweep frequency of a UJT relaxation oscillator. Calculate the frequency with R = 100 KΏ, C = 0.4 µF and intrinsic stand-off ratio 0.57.                        (DEC 2007)
                                             1
Sweep frequency =                                                               =   29.6
                                          2.3RClog10(1/1- η)

35. Draw the equivalent circuit of pulse transformer.   (NOV 2011)

36. Which are the two important elements of a blocking oscillator?
    1. Active element like transistor   2. A Pulse transformer

37. What is the function of pulse transformer in blocking oscillator?
    A pulse transformer is used to couple output of the transistor back to the input. The nature of such feedback through pulse transformer is controlled by relative winding polarities of a pulse transformer.


38. What is leading edge response?
    At start there is an overshoot and then the pulse settles down.The response till it settles down after theovershoot is called leading edge response.

39. What is trailing edge response?
    The response generally extends below the zero amplitude after the end of pulse width is called back swing. The portion of response from back swing till it settles down is trailing edge response.

40. What is flat top response?
    The portion of the response between the trailing edge and the leading edge is called flat top response.

41. What is droop?
    The displacement of the pulse amplitude during its flat response is called droop or a tilt.

42. What is the disadvantage of base timing monostable blocking oscillator?
    In monostable blocking oscillator, the pulse width is linear function of hFE which is temperature dependent. Also its value changes from transistor to transistor and hence pulse width gets affected due to  transistor replacement. This is the biggest disadvantage of the circuit and hence base timing circuit is not used if stable pulse width is required.

43. Which are the two types of astable blocking oscillator?
1.Diode controlled blocking oscillator 2. RC controlled blocking oscillator

44. What is mark space ratio?
    The ratio of time for which Q is ON to time for which Q is OFF is called mark space ratio. If this is unity, then the output is almost symmetrical square wave.

45. Define Duty cycle.
   The duty cycle is defined as the ratio of the time tp to the time period T. Mathematically it is given by  D = tp / T.

46. What do you mean by voltage time base generators?
    Circuits used to generate a linear variation of voltage with time are called voltage time base generators.

47. What are the applications of the voltage time base generators?
    1. In cathode ray oscilloscope 2.In radar   3.In television indicators 4.In precise time measurements and 5.In time modulation

48. List important sweep parameters.
    Important sweep parameters are: sweep speed error, displacement error and transmission error.
49. Name the different errors in generation of sweep waveforms.
    Different errors in generation of sweep waveforms are: sweep speed error, displacement error and  transmission error.

50. Give the expression for sweep voltage for exponential sweep circuit.
    Vs = V (1-e-t/RC)



PART – B
1. Draw the circuit diagram of a monostable transistor blocking oscillator with emitter timing. Explain its operation with the equivalent circuit during the pulse formation.                    (NOV 2011)
2. Explain the operation of a free running (astable) blocking oscillator.  (MAY 2005)
3. With necessary waveforms and circuit diagram explain the method of generating a linear sweep voltage using  Bootstrap circuit.(DEC 2008, DEC 2009, JUNE 2009)
4. Define slope, displacement and transmission errors with reference to a sweep signal.                                           (MAY 2005)
5. Define the errors that are used to measure the deviation from linearity of a sweep signal.                                     (MAY 2005)
6. With a neat circuit diagram and relevant waveforms, explain the performance of a triggered transistor blocking  oscillator.         (MAY 2008)
7. With suitable circuit diagrams, explain the following 1.UJT sawtooth generator 2.Miller sawtooth generator.   (MAY 2008, MAY 2010, MAY 2012)
8. A uni-junction transistor with η = .62(intrinsic stand-off ratio) is used in a relaxation oscillator circuit with R = 5 KΏ and C = 0.05 µF. 1.Determine the period and frequency of oscillation 2.Determine the new value of R, which must be changed in order to obtain a frequency of oscillation of 50Hz.  3. If C is increased by a factor of 10, how the value of R changes,if the frequency is to be 50Hz.                             (DEC 2006)
9. With neat circuit diagram, explain monostable blocking oscillators with base and emitter timing. Draw  necessary waveforms.(DEC 2009, MAY 2010, 2011)
10. Describe the working of UJT relaxation oscillator.(DEC 2009, JUN 2010)
11. Draw the circuit diagram and explain the operation of a Push-pull Astable blocking oscillator with emitter timing.               (JUN 2010)
12. Explain with circuit diagram and waveforms the voltage time base generator.
13. Draw the circuit of a simple current time base generator. Explain the working of the circuit with associated  waveforms. (DEC-2007 & NOV 2010)
14. For a diode controlled astable transistor blocking oscillator circuit with the following parameters L =5.2mH,C=90pF,Vcc=10V,R=500Ώ,Vr=6Vn=1 and VBB=0.5V. Calculate 1. The period and duty cycle 2.Peak voltages and currents  3. Current in magnetizing inductance at the end of one cycle. Neglect saturation  junction voltages.                (Nov./Dec-2006)
15. Explain the method of triggering the blocking oscillator using transistor.                                       (JUN 2006)
16. Draw the circuit of an astable blocking oscillator with base timing. Sketch the wave forms of collector voltage and magnetizing current. Explain the operation of the oscillator covering one full cycle, along with necessary      equations. Sketch the equivalent circuits (1) when there is magnetizing current and (2) when the magnetizing current is zero. Also, mention the advantages and disadvantages of this oscillator.                        (MAY 2012)
17. For a certain UJT sweep circuit, the resistance is 10 kΩ while the capacitance is 0.1 µF. The valley potential is  1.5V when VBB = 20 V. Assuming diode cutin voltage of 0.7 V and intrinsic stand off ratio as 0.6, calculate  the frequency of oscillations.                   (MAY 2012)
18. Explain bootstrap saw-tooth generator with circuit diagram and draw the output signal.          (NOV 2011)

WAVE SHAPING AND MULTIVIBRATOR CIRCUITS UNIT-IV

                                                                   UNIT-IV
                                     WAVE SHAPING AND MULTIVIBRATOR CIRCUITS

PART – A
1. What is a Clamper? What are its uses?   (NOV2011, MAY 2012)
    A clamper is a circuit which shifts (clamps) a signal to a different dc level, ie., it introduces a dc level to  an ac signal. The clamper is also known as dc restorer. It is used in television receivers to restore the dc  reference signal to the video signal.

2. State the basic action of monostable multivibrator.      (APR-2003)
       Monostable multivibrator is also known as one-shot multivibrator or univibrator. It has one stable state and one quasi-stable state. When an external trigger pulse is applied to the circuit, the circuit goes into the quasi-stable state from its normal stable state. After some time interval, the circuit automatically returns to its  stable state.The circuit does not require any external pulse to change from quasi-stable state to stable state. The time interval for which the circuit remains in the quasi-stable state is determined by the circuit components and can be designed as per the requirement.

3. What is Astable multivibrator?              (NOV 2010)
    An astable or free-running multivibrator generates square wave without any external triggering pulse. It has no stable states, ie., it has two quasi-stable states. It switches back and forth from one state to the other,      remaining in each state for a time depending upon the discharging of a capacitive circuit.

4. Where are bistable multivibrator applied? or State the applications of bistable multivibrator ?           (JUN 2007 & MAY 2011)
     a)Memory element in shift register, counters and so on.             b)To generate square wave of symmetrical shape by sending regular triggering pulse to the input.
        c)It is used as a frequency divider.

5. What are the applications of Monostable multivibrator?  (DEC-2007)
1.    It is used to function as an adjustable pulse width generator.
2.    It is used to generate uniform width pulses from a variable width input pulse train.

6. What is meant by unsymmetrical triggering?      (NOV-2005)   
This method uses two triggering inputs. The triggering signal from the first input is applied to set the  circuit in one particular state. The triggering signal from the second input is applied to reset the circuit to     the opposite state. This process is called set-reset triggering. The unsymmetrical triggering is used in     register and coding circuits.

7. What is meant by symmetrical triggering?
In symmetrical triggering method, each successive triggering signal changes the state of the flip-flop. It is     used in binary counting circuits.

8. What is the frequency of oscillation of astable multivibrator?  (NOV-2005)   
Time period=T=1.4RC , Frequency of oscillation=1/T=1/1.4RC

9. What are the advantages of dc-flip-flop?            (OCT-2004)
Extreme simplicity, Low power dissipation, Transistor with low breakdown voltages are used, one supply voltage of low value

10. What are applications of Schmitt trigger?(DEC 2007 & NOV 2010, MAY 2012)
    a) It is used for wave shaping circuits and as a voltage comparator
    b) It is used for the generation of rectangular waveform with sharp edges from a sine wave or any other waveform  
    c) It can be used as a voltage comparator.
    d) Triggering can be made more sensitive to small noise fluctuations present in the input signal.





11. Define Schmitt trigger.                (JUN-2006)
    Schmitt trigger is a wave shaping circuit, used for generation of a square from a sine wave input. It is a bistable circuit in which two transistor switches are connected regeneratively.

12. Compare the transistors used in multivibrator circuits with those that are used in conventional amplifiers?            (DEC 2004)
    In the multivibrators, the transition of transistor states from ON to OFF and OFF to ON is important. To avoid the distortion, this transistion must be as fast as possible.Hence the transistors used in multivibrators have small switching times ie.,small turn on and turn off times as compared to those used in conventional circuits. Such transistors are called switching transistors.

13. State the roe of commutating capacitors in Bistable multivibrator.                                                           (JUN-2009)
      In bistable multivibrator,trigger is required to change the state. The time required to change the state is called the transistion time.The speed-up capacitors are used to reduce the transition time without affecting the stable states.These capacitots allow fast rise and fall times and avoids the distortion in the output.

14. State the expression for gate width of monostable multivibrator.                                                                            (JUN 2009)
            T = τ ln(2)
      The time constant τ for the charging path of capacitor C is RC,  So,T=0.69RC.Thus the gate width is independent of transistor characteristics, supply voltage and resistance value.

15. Define quasi stable state.                 (MAY 2010)
    The stable state of monostable multivibrator in which circuit remains for some time and returns to stable state,is called Quasi stable state.

16. Differentiate clippers from clampers.            (MAY2010)
CLIPPERS    CLAMPERS
The circuit with which the waveform is shaped by removing (or clipping) a portion of the input signal without distorting the remaining part of the alternating waveform is called a clipper    A clamper is a circuit which shifts (clamps) a signal to a different dc level, ie., it introduces a dc level to an ac signal.




17. Draw the circuit of an integrator using Rand C as circuit elements and find its output voltage V0, if Vi  is the input voltage.      (MAY 2011)                            

                             

V0=1 /RC ∫Vi dt
      
 V0=Vi [1-e-t/RC]

18. Why is monostable multivibrator is called delay circuit? (MAY 2011)
    The time between the trasistion from quasi –stable state to stable state can be pre determined and hence I can be used to introduce time delays with the help of fast transition. Due to this application, it  is called as delay circuit.
   
19. What is self biased multivibrator?              (MAY2011)
      If the multivibrator automatically makes the successive transitions from one quasi – stable state to other without any external triggering pulse., then it is known as self-biased multivibrator. Ex:  Astable Multivibrator.

20. What is free running multivibrator             (NOV 2010)
    An astable or free-running multivibrator generates square wave without any external triggering pulse. It has no stable states, ie., it has two quasi-stable states. It switches back and forth from one state to the other,       remaining in each state for a time depending upon the discharging of a capacitive circuit.

21. What is linear wave shaping?
    The process by which the shape of a nonsinusoidal signal is changed by passing the signal through the network consisting of linear elements is called linear wave shaping.

22. Mention the various nonsinusoidal waveforms.
    1.Step      2.Pulse      3.Square       4.Ramp

23. What is high pass RC circuit?(or)What is a differentiator? Explain its feature and use.
    A circuit which consists of a series capacitor and a shunt resistor is called a differentiating or high pass RC circuit. At very high frequencies, the capacitor acts as a short circuit and all the higher frequency components appear at the output with less attenuation than the lower frequency components. Hence this circuit is called high- pass filter. Due to this feature, it is used as coupling circuit to provide dc isolation between input and output.   
24. What is low pass RC circuit? (or)What is an integrator? Explain its feature and use.
    A circuit which consists of a series resistor and a shunt capacitor is called an integrator or low pass RC circuit. At very high frequencies, the capacitor acts as a virtual short circuit and the output falls to zero. Hence     this circuit is called low - pass filter. Due to this feature, it is used when it is required to extend the range of  operation of some electronic circuit to a higher frequency.

25. What is tilt or sag?
    The decay in the amplitude of the output voltage waveform when the input level is maintained constant is  called tilt or sag or decay.

26. Define Duty cycle.
    It is the ratio of the ON period (TON) to the total period (T = TON + TOFF).Therefore duty cycle = TON / T.

27. Define Rise time.
    It is the time required by the output response to rise from 10% to 90% of its maximum value.

28. Define Delay time.
    In a transistor switching circuit, the collector current(output response) does not immediately respond to the input pulse. There is a delay and the time that elapses during this delay together with time needed for the  current to rise to 10% of its maximum(saturation) value ie., IC(Sat)= VCC/RC   is called the delay time.

29. Define Fall time.
It is the time required by the output response to fall from 90% to 10% of its maximum value.

30. Define storage time.
    When the input signal returns back to its initial state at t = T, the collector current gain fails to respond  immediately. The interval which elapses between the transition of the input voltage waveform and the time when collector current (ic) dropped to 90% of its maximum value is called the storage time.   



31. Define speedup capacitor.
         Speedup capacitors or commutating, transpose capacitors are small capacitors introduced in shunt with the coupling resistors of the binary of a switching circuit in order to reduce the transition time. Since these      capacitors assist the binary in making abrupt transition between starter, they are known as speedup capacitors.

32. What is clipper circuit?
    The circuit with which the waveform is shaped by removing (or clipping) a portion of the input signal without distorting the remaining part of the alternating waveform is called a clipper.Clippers are also called as      limiters, amplitude selectors or slicers.

33. What are the types of clippers?
1.Positive Clipper  2.Negative Clipper  3.Biased Clipper  4.Combination Clipper.

34. What are the uses of clippers?
Clippers are used in radars, digital computers, radio and television receivers.

35. What are the types of Clampers?
    1. Positive clamper        2. Negative clamper

36. What is a diode comparator?
A nonlinear circuit which is used to compare an input signal Vi(t) with a reference voltage VR. The   comparator output is independent of the signal until it attains the reference level. When the signal and reference level become equal, there will be a sharp pulse at the comparator output.

37. Define Multivibrator.
        A multivibrator is a two stage switching circuits in which the output of the first stage is fed to the input of the second stage and vice-versa. The outputs of two stages are complementary.

38. What are the various types of multivibrator?
    There are three types of multivibrator. 1. Bistable multivibrator 2.Monostable multivibrator     3.Astable multivibrator.

39. What is bistable  multivibrator?
    A bistable multivibrator has two stable states. The multivibrator can exist indefinitely in either of the two stable states. It requires an external trigger pulse to change from one stable state to another.The circuit remains in one stable state unless an external trigger pulse is applied.The bistable multivibrator is also referred as flip-  flop, Eccles-Jordon circuit,trigger circuit or binary.

40. What are the applications of Astable multivibrator?
1.    Used as square wave generator, voltage to frequency converter and in pulse synchronization, as clock for binary logic signals, and so on.
2.    Since it produces square waves, it is a source of production of harmonic frequencies of higher order.
3.    Used in the construction of digital voltmeters and SMPS.
4.    Can be operated as an oscillator over a wide range of audio and radio frequencies.

41. What is meant by triggering of multivibrator?
    To provide proper pulse input to achieve a transistion from one state to the other state of the multivibrator,is called the triggering of multivibrators.

42. Define UTP and LTP.
       UTP is a point at which the transistor enters into conduction state i.e. from OFF to ON state. LTP is a  point at which the transistor enters from ON to OFF state.

43. What is hysteresis voltage and hysteresis in Schmitt trigger?
     The difference between UTP and LTP is known as hysteresis voltage(VH). VH is also known as Dead zone of the Schmitt Trigger. The lagging of the lower threshold voltage from the upper threshold voltage is       known as hysteresis.

44. What is the difference between an oscillator and multivibrator?
         Multivibrator operates in non linear region of transfer characteristics and oscillator operates in linear or  active region of its transfer characteristics.

45. Give the expansions of UTL, UTP, LTL, LTP.
       UTL  - upper trigger level        UTP - upper trigger point
       LTL - lower trigger level        LTP - Lower trigger point

46. What are the other names of monostable multivibrator?
          It is commonly known as oneshot,single cycle, a single step circuit or univibrator.
.
47. What is a differentiator? Explain its feature and use.
    A circuit which consists of a series capacitor and a shunt resistor is called a differentiating or high pass RC circuit. At very high frequencies, the capacitor acts as a short circuit and all the higher frequency components appear at the output with less attenuation than the lower frequency components. Hence this circuit  is called high- pass filter. Due to this feature, it is used as coupling circuit to provide dc isolation between input and output.   
48. What is bistable  multivibrator?
    A bistable multivibrator has two stable states. The multivibrator can exist indefinitely in either of the two stable states. It requires an external trigger pulse to change from one stable state to another.The circuit remains in one stable state unless an external trigger pulse is applied.The bistable multivibrator is also referred as flip-  flop, Eccles-Jordon circuit,trigger circuit or binary.

49. What is an integrator? Explain its feature and use.
    A circuit which consists of a series resistor and a shunt capacitor is called an integrator or low pass RC circuit. At very high frequencies, the capacitor acts as a virtual short circuit and the output falls to zero. Hence 
    this circuit is called low - pass filter. Due to this feature, it is used when it is required to extend the range of  operation of some electronic circuit to a higher frequency.

50. Define UTP .
       UTP is a point at which the transistor enters into conduction state i.e. from OFF to ON state.


PART – B
1. How high pass RC circuit be used as a differentiator?       (MAY 2006)
2. How low pass RC circuit be used as a integrator?       (DEC 2004)
3. Define and illustrate the following nonsinusoidal waveforms:1.Step 2.Pulse  3.Square  4.Ramp
4 .Derive and draw the response of high pass and low pass RC circuit to following waveforms:
     1. Step 2.Pulse  3.Square  4.Ramp                 (JUN 2006, MAY 2012)
5. With circuit diagram and waveforms explain the operation of RC circuits as Integrators and Differentiators for  square wave input.
6. Explain how RC circuits are used as integrators and differentiators. Sketch the output for a square wave input.
7. Draw the circuit diagram of an emitter-coupled monostable multivibrator and explain its operation with relevant  waveforms.         (JUN 2006)
8. Explain the unsymmetrical triggering and symmetrical triggering of bistable multivibrator.         (DEC 2007)
9. Draw the circuit diagram of an emitter coupled astable multivibrator.           (DEC-2009, NOV 2011)
10. Design a monostable multivibrator for the following specifications: VCC = 10 V,VBB = 6 V, IC(on) = 1 mA, duration of output pulse = 14 seconds,hFEmin = 100, ICBO = 0,VBE(off) = -0.5V. 
11. Design a Schmitt trigger circuit for VCC = 10 V, UTP = 5V, LTP = 3V. Assume hFEmin = 100 and IC(on) =1 mA. 
12. Design a collector coupled astable multivibrator for the following specifications: Output voltage 10V peak,IC(on) =1 mA, hFEmin = 100, ICBO = 0, output to be a positive pulse, the duration of which is 20µsec,the time      between pulses to be 10 µsec.                    (MAY 2004)
13. With circuit diagram and waveforms explain the operation of a Schmitt trigger using two transistors for a sinusoidal input.(MAY 2010, 2011 & NOV 2010, MAY 2012)
14. With circuit diagram and necessary waveforms, explain the operation of a saturated collector coupled astable  multivibrator.  (DEC 2008)
15. With circuit diagram and necessary waveforms, explain the operation of a saturated collector coupled monostable multivibrator.   (DEC 2008)
16. Explain the effect of emitter coupling in monostable multivibrator.(JUN 2006)
17. Explain the ON and OFF state of the transistors in a collector coupled symmetrical triggered Bistable multivibrator.        (JUN 2006)
18. Explain the effect of speed up capacitor in bistable multivibrator.(JUN 2006)
19. Determine the value of capacitors to be used in an astable multivibrator to provide a train of pulse 2µs wide at a repetition rate  of 100KHz if R1=R2=20KΏ.                             (JUN 2009, MAY 2012)
20. Explain circuits used to trigger a transistor monostable multivibrator circuit                                    (DEC 2008)
21. Describe the parallel biased clipper operation with necessary diagrams.                                   (MAY 2010)
22. Design and draw a saturated collector coupled monostable multivibrator for the following specifications: Vcc=10V, Vbb=5V, pulse duration=12ms, Ic=2mA and hfe=100.                                  (MAY 2011)
23. A 10 Hz symmetrical square wave, with a peak to peak amplitude of 2 V, is impressed upon a high pass RC circuit whose 3-dB frequency is 5 Hz. Calculate and sketch the output waveform. What is the peak to peak output amplitude?                                           (MAY 2012)
24. Explain schmitt circuit as (i) Amplitude comparator (2) Squaring Circuit (3) Flip-flop circuit.          (NOV 2011)   

TUNED AMPLIFIER UNIT-III

                                                                       UNIT – III
                                                               TUNED AMPLIFIER

PART –A
1. What is tuned amplifier?         (MAY 2011)
An amplifier with tuned circuit load is known as tuned amplifier. They use a circuit that will amplify the frequency band required and reject all other unwanted signals.

2. What are the applications of tuned amplifiers?   (MAY 2008)
The application of tuned amplifier is to obtain a desired frequency and to reject all frequency in radio and TV broadcasting and wireless communication system.

3. What happens to the circuit above and below resonance?
    Above resonance the circuit acts as capacitive and below resonance the circuit acts as inductive.

4. Write down the expression for the bandwidth of a tuned circuit in terms of quality factor and resonant  frequency.    (MAY 2010)
 Resonance frequency fr= 1 / (2π (LC)   &  Bandwidth = fr / QL

5. A series resonant circuit has the following constants: L=220µH, C=300pF, R=20Ω. Calculate resonant  frequency.    (MAY 2010)
    Resonance frequency fr= 1 / (2π (LC),   fr= 1 / (2π X  (220 X 10-6 X         300 X 10-12) = 619.5 KHz

6. A tuned circuit has resonant frequency of 1600 kHz and bandwidth of 10kHz. What is the value of its Q-factor?
Q= Fr  / BW = 1600 X 103 / 10 X 103 = 160

7. Determine the bandwidth of a 3 stage cascaded single tuned amplifier if the resonant frequency is 455 KHz and the loaded Q of each stage is 10.                      (JUN 2009, NOV 2010, MAY 2011)
    fo = 455 X 103, Qi =10.      
    BWn= BW1 (21/n - 1), BW1 = 455 X 103 / 10 = 45.5 KHz,
    Therefore for 3 stage BW3 = BW1 (21/3 - 1) = 45.5 X 103  X  (21/3 - 1)              = 23.2 KHz.

8. State the reason for instability in tuned amplifier.        (MAY 2010)
    If the reactance of the collector to base capacitance (Cbc) at RF is low enough it provides the feedback  path from collector to base. With this circuit condition, if some feedback signal manages to reach the input from output in a positive manner. This is the reason for instability of the tuned amplifier.

9. Define Q-factor?                     (MAY 2010)
     It is defined as the ratio of maximum energy stored per cycle to that of maximum energy dissipated per cycle.

10. What is the need for neutralization in tuned amplifiers?(DEC2008 & NOV 2010, NOV2011)
    To prevent oscillations in tuned RF amplifiers it is necessary to reduce the stage gain to a level that  ensured stability. So, to achieve this stability neutralization is needed.

11. What are the advantages of tuned amplifiers?      (MAY 2012)
    (i)They amplify defined frequencies. (ii)Signal to noise ratio at output is good. (iii)They are suited for radio transmitters and receivers

12. What are the disadvantages of tuned amplifiers?         (MAY 2012)
    (i) The circuit is bulky and costly (ii) The design is complex. (iii) They are not suited to amplify audio frequencies.

13. What is a stagger tuned amplifier?          (NOV 2010)
    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.

14. Mention the application of Class C tuned amplifier?    (MAY 2008)
    Class C tuned amplifier can be used as a mixer or frequency converter.

15. What is neutralization?      (MAY 2008 & MAY 2011,NOV 2011)
       It is a phenomenon by which a signal can be transmitted from the input to the output alone and not vice- versa.

16. Briefly explain narrow band neutralization?        (JUN 2009)
    A process of cancelling the instability effect due to the collector to base capacitance of the transistor in tuned circuits by introducing a signal which cancels the signal coupled through the collector to base capacitance is called narrow band neutralization.

17. What is meant by stagger tuned amplifier?      (NOV 2010)
    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.


18. What is meant by Q-factor?             (MAY 2010)
     It is defined as the ratio of maximum energy stored per cycle to that of maximum energy dissipated per cycle
    Q = 2π(Maximum energy stored per cycle / Energy dissipated per cycle )

19. Define unloaded Q?
        It is the ratio of stored energy to the dissipated energy in a reactor or resonator.

20. What is loaded Q?
    The loaded Q or QL, of a resonator is determined by how tightly the resonator is coupled to its  terminations.

21. What is double tuned amplifier?
    Double tuned amplifier uses two inductively coupled tuned circuit per stage,   both tuned circuits being tuned to the same frequency.

22.What are the method to reduce instability?
 a)Neutralization      b) Unilateralization      c) Mismatch technique.

23. What are the different types of neutralization methods?
    a)Hazeltine method    b)Rice method  c) cross method  d) coil method

24. What is meant by parasitic oscillation?
    If an amplifier tends to sustain oscillation at a frequency other than the driving frequency, it is called as parasitic oscillation. It occurs due to positive feedback produced by inter electrode capacitance, load inductance, stray capacitance.

25. Why is GBW product constant?
    GBW is defined as the product of the mid band gain, which is a constant and the bandwidth which is also a constant. The product of two constant should also be a constant.

26. What are the different methods of coupling the load to a tuned amplifier?
    a)capacitive coupling             b)Inductive coupling.

27. What are the types of small signal tuned amplifier?
    a)single tuned
    b)Double tuned
    c)Stagger tune


28. What is a single tuned amplifier?
    An amplifier circuit that uses a single parallel tuned circuit as a load is called single tuned amplifier.

29. What are advantages of double tuned over single tuned?
       (i) Possess flatter response having steeper sides. (ii) Provides larger 3 db bandwidth (iii) Provides large gain-bandwidth product.

30. What are advantages of stagger tuned amplifier?
    The advantage of stagger tuned amplifier is to have better flat, wideband characteristics.

31. Define Gain bandwidth product of a tuned amplifier?
    The gain bandwidth product of a tuned amplifier is a figure of merit defined in terms of midband gain and upper 3-db frequency as GBW=ﺍaimﺍ=gm/2Πc

32. How will you classify Tuned amplifier?
        a)Small signal tuned amplifier     b)Large signal tuned amplifier

33. What are the factors that govern the selectivity of a single tuned amplifier?
        The factors that govern the selectivity of a tuned amplifier are a)Quality factor(Q)       b)Bandwidth(BW)

34. What is meant by impedance matching?
         It is a technique by which matching the output impedance of the amplifier to that of load or the impedance of the next stage is carried out. It is achieved using a transformer and it results in maximum gain.

35. Why is impedance matching needed for the tuned amplifier?
         In order to transfer maximum power from one stage to the other or to the load which ensured the maximum gain impedance matching is needed for the  tuned amplifier.

36. What is the effect of Q on stability?
         Higher the value of Q, it provides better selectivity, but smaller bandwidth and large gain. Hence it provides less stability.

37. If the quality factor of a resonant circuit is doubled, What is the effect of bandwidth?
    Quality factor is inversely proportional to the bandwidth. So when the quality factor of a resonant circuit is doubled ,the B.W is halved.


38. 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.

39. What is unloaded Q?
    It is the ratio of stored energy to the dissipated energy in a reactor or resonator.

40. What are the applications of mixer circuits?
    Used in radio receivers. Used to translate signal frequency to some lower frequency

41. What is up converter?
    When the mixer circuit is used to translate signal to high frequency, then it is called up converter.

42.What are the different coil losses?
Hysteresis loss
Copper loss
Eddy current loss.

43.What are the advantages of double tuned over single tuned?
1. Possess flatter response having steeper sides
2. Provides larger 3 db bandwidth
3. Provides large gain-bandwidth product.

44.Why quality factor is kept as high as possible in tuned circuit?
    When quality factor is high inductor losses are less. Another important point is that when Q is high, Bandwidth is low and we get better selectivity. Hence Q is kept as high as possible in tuned circuit.

45. What is narrow band neutralization?
A process of cancelling the instability effect due to the collector to base capacitance of the transistor in tuned circuits by introducing a signal which cancels the signal coupled through the collector to base capacitance is called narrow band neutralization.

46.A tuned amplifier is designed to receive AM broadcast of speech signal at 650KHZ. What is needed Q for amplifier.
Solution:
             BW = 2fm = 2*3 = 6 KHZ
               Q  = fr / BW   =  650 / 6 =108.33 kHZ.



47. What is the effect of cascading single tuned amplifiers on bandwidth?
Bandwidth reduces due to cascading single tuned amplifiers.

48. Write short notes on parasitic oscillation?
    If an amplifier tends to sustain oscillation at a frequency other than the driving frequency, it is called as parasitic oscillation. It occurs due to positive feedback produced by inter electrode capacitance, load inductance, stray capacitance.

49. What do you meant by tuned amplifiers?
The amplifiers which amplifies only selected range of frequencies with the help of tuned circuits are called tuned amplifiers.

                                                                         PART – B
1.     Draw the circuit diagram and equivalent circuit of a capacitor-coupled single tuned amplifier and explain the frequency response derive the expression for its gain and cutoff frequency. (MAY 2009, NOV 2010, MAY 2012)
2.     Derive the equation for the gain bandwidth product of a single tuned amplifier circuit.             (MAY 2010)
3.     A tank circuit has a capacitor of 100pF and an inductor of 150µH.The series resistance is 15Ω. Find the impedance and bandwidth of a resonant circuit.                                      (MAY 2010)
4.     With a circuit, explain the narrow band neutralization technique.   (MAY 2010)
5.     With circuit diagram and frequency response characteristics compare double tuned and stagger tuned amplifiers.       (JUNE 2009)
7.     Draw the circuit of class C tuned amplifier and explain its operations with relevant waveforms. Discuss also its  frequency response.   (MAY 2008, May2011, MAY 2012)
8.     Discuss the Hazeltine method of neutralization with circuit.  (MAY 2008, MAY 2011, NOV 2011, MAY 2012)
9.     What is the effect of cascading single tuned amplifier on bandwidth? Derive the expression for it? 
10.     Draw and explain the circuit of synchronous tuned amplifier with the help of frequency response?
11.     Draw and explain the double tuned amplifier in detail.       
12.     Explain the stabilization technique used in tuned amplifier?     
13.     If class c tuned amplifier has RL=6k and required tank circuit Q=80.Calculate the values of L & C of the tank circuit. Assume Vcc=20V ,Resonant frequency=5MHZ and worst case power.
14.     A single tuned amplifier is designed with following values.fr=500KHZ,BW=25KHZ.The total output    
    resistance Rt=18K and C=35Pf.Find the L&C of the tuned circuit.
15.     A single tuned RF amplifier has tank circuit of 47pF capacitance & a series combination of 1 µH  inductance and 22ohm resistance. The values of R0=48K and C0=10pf.If Ri of the next stage is 18K. Find all relevant parameters.
16.     The BW of a single tuned amplifier is 25KHZ.Calculate the BW if such three stages are cascaded. Also calculate the BW for 4 stages.
17.     (a) Briefly explain the principle of Stagger tuning.      (MAY 2011)
           (b) Discuss the effect of bandwidth on cascading single tuned amplifiers.
18.     A single tuned transistor amplifier is used to amplify modulated RF carrier of 600 kHz and bandwidth of  15kHz. The citcuit has total output resistance Rt = 20kΩ and output capacitance C0=50 pF. Calculate the values of inductance and capacitance of tuned circuit.                  (MAY 2012)
19.     Obtain the bandwidth of a n stage cascaded identical single tuned amplifiers in terms of the bandwidth of a   single- stage tuned amplifier.
20. Explain the effect of changing Q of a coil used in tank circuit on its bandwidth.              (NOV 2011)

OSCILLATORS UNIT-II

                                                     UNIT II OSCILLATORS
                                                                PART - A
1. Mention the essential conditions satisfied by an oscillator circuit?     OR (MAY 2011, 2010, 2012)
    State the Barkhausen criterion.   (JUN 2009 & NOV 2010, 2011)
           The essential conditions for maintaining oscillations are:
    1. A  = 1 ,i.e the magnitude loop gain must be unity.
    2. The total phase shift around the closed loop is zero or 360 degrees.
              These conditions are commonly known as Barkhausen criterion.

2. How are oscillators classified based on the frequency?   (MAY 2010)
    (i) Audio Frequency Oscillator          (ii) Radio frequency oscillator.

3. In a Hartley oscillator, if L1=0.2 mH, L2=0.3 mH and c=0.003 µF. Calculate the frequency of its oscillations.                          (MAY 2012)
F r = 1 / ( 2π (LC) = 1 / ( 2π ((0.2+0.3) X10-3 X(0.003X10-6)) =129 kHz.

4. Give expression for fs and fp of crystal oscillator.       (MAY 2008)
      fs = 1 / ( 2π (LC) )   and   fP = 1 / ( 2π (LCeq) )  Where, Ceq =  CMC / CM+C.

5. Define piezo-electric effect.        (MAY 2010 & MAY 2011)
          Piezo-electric effect means the crystal reacts to any mechanical stress by producing electric charge. In converse process, it produces mechanical stress with respect to an electric field input.

6.  State the Barkhausen criterion.   (JUN 2009 & NOV 2010, 2011)
           The essential conditions for maintaining oscillations are:
    1. A  = 1 ,i.e the magnitude loop gain must be unity.
    2. The total phase shift around the closed loop is zero or 360 degrees.
        These conditions are commonly known as Barkhausen criterion.
7. What are the advantages and disadvantages of RC phase shift oscillator?                            (MAY 2008).

    Advantages:  (i) Simple circuit (ii) Produce output over audio frequency range (iii) It is fixed frequency  oscillations (iii) Produces sinusoidal output waveform.

    Disadvantages: (i) Change in value is not permissible  (ii) Poor frequency stability .

8. What is the difference between the oscillator and amplifier? (DEC 2010)            
                                    
AMPLIFIER    OSCILLATOR
1.An amplifier strengthens the input signal without 
   any change in its waveform and frequency.    1.An y circuit which is used to generate a  
   periodic voltage without an ac input signal.
2.It needs an external signal either to start or   
   maintain the process of energy conversion.    2. It does not need any external signal either to 
   start or maintain the process of energy
   conversion.
3. Energy conversion is controlled by input signal.    3. Energy conversion is controlled by oscillator
    circuit itself.

09. Give the equivalent circuit of quartz crystal and mention its series and parallel resonant frequency.                      (MAY 2011)



10. Make a table of comparison of RC phase shift oscillator and wein-bridge oscillator bringing out the similarities and differences.                                                         (MAY 2012)
S.No.    RC Phase Shift Oscillator    Wien Bridge Oscillator
1.    It is used for low frequency range    It is used for low frequency range
2.    The feedback network consists of three RC sections    The feedback network consists of Lead –Lag Network.
3.    It introduces 180 ̊ phase shift    It does not introduce any phase shift
4.    Op-amp is used in inverting mode    Op-amp is used in non-inverting mode
5.    The amplifier gain condition is ,|A| ≥ 29    The amplifier gain condition is ,|A| ≥ 3
6.    The frequency of oscillation is F r = 1 / 2RC 6         The frequency of oscillation is F=1/2RC

11. Draw the electrical equivalent circuit crystal and mention the significance of each component.        (DEC 2009)
    Resistance R is used to represent internal frictional losses during vibrations, Inductance L is used to   indicate the inertia and Capacitor C is used to represent the stiffness.

12. Why LC tank circuit does not produce sustained oscillations. How can they overcome?(DEC 08)   
    Every time when energy is transferred from C to L and L to C, the losses occur due to which amplitude of oscillating current keeps on decreasing everytime when energy takes place. Hence actually exponentially  decaying oscillations will be produced. This is known as damped oscillations. The care of proper polarity is  taken by feedback network. Thus LC tank circuit along with transistor amplifier can be used to obtain  oscillators called LC oscillators. Due to supply of energy, the oscillations get maintained

13. In a wein bridge oscillator if the value of R is 100KΩ, and the frequency of oscillation is 10KHz find the   value of capacitor C.                                           (DEC 2006, NOV 2011)
    f r = 1 / 2πRC, Therfore, C= 1/ 2πR f r , C= 1/ (2π X 100 X 103 X 10 X 103) = 159 pF

14. What is the condition for sustained oscillation in Hartley oscillator?                                                                                    (AP-96)
           The condition for sustained oscillation is  h fe    [L1 + M] / [L2 + M].Here M is the coefficient of mutual inductance

15. Find the frequency of oscillations of a wien bridge oscillator with R1=R2=220KΩ,c1=c2=250pF.                        (MAY 2010)
     For Wien bridge oscillator frequency of oscillation is given by f r = 1 / 2πRC ; only if R1 =  R2  = R  and C1 =  C2 = C. f r = 1 / 2πRC, f r = 1 / (2π X 220 X 103 X 250 X 10-12 ) = 2.89 KHz.

16. Give expression for the frequency of oscillation of wien-bridge oscillator.                                      (JUN 2009)   
    For Wien bridge oscillator frequency of oscillation is given by f r = 1 / 2πRC ; only if R1 =  R2  = R  and  C1 =  C2 = C.


17. What is the negative resistance oscillator?         (JUN 2009)
     An oscillator is which a parallel-tuned resonant circuit is connected to a transistor. So that the combination acts as the negative resistance needed for continuous oscillation.

18.How does crystal oscillator maintain frequency stability?(DEC 2007 & JUN 2009)
    A crystal can by shown to be high-Q resonant circuit. Due to the high Q of the crystal, the crystal  oscillator is very stable or accurately fixed in value.

19. Give the block diagram of an oscillator.




20. Write short notes on Phase shift oscillator.
    In phase shift oscillator, the required phase shift of 180 degree in the feedback loop from output to input  is obtained by using R and C components instead of tank circuit. The phase shift of RC section is  = tan –1(1/  CR)

21. Write the expression for the frequency of oscillation in a phase shift oscillator.
            In a phase shift oscillator the frequency of oscillation is given by,
            F r = 1 / 2RC 6     at frequency  = 1 / 29.


22. Write notes on Wien bridge oscillator.
          A  Wien bridge oscillator circuit consists of a two stage RC coupled amplifier , which provides a phase shift  of 360 degree or 0 degree. A balanced bridge used as the feedback network consists of a lead lag network and  a voltage divider.

23. How will you determine the frequency of oscillation of Hartley oscillator?
The frequency of oscillation of Hartley oscillator can be determined by using the equation.
                            F r = 1 / ( 2π (LC) )  where  L = L1 +L2 +2M.
 Where M is the coefficient of mutual inductance between coils L1 & L2.

24. How is frequency of oscillation expressed for colpitts oscillator?
            For a colpitts oscillator the frequency of oscillation is expressed as,   
                F r = 1 / (2π (LC)) where C = C1C2  / (C1 + C2 ).

.25. Write notes on crystal oscillator.
    Crystal oscillator is basically a colpitts oscillator in which the inductor is replaced by the crystal. Generally a piezo-electric crystal,usually quartz is used as a resonant circuit replacing an LC circuit.

26. Name the piezo-electric materials used for crystal oscillator.
            The piezo-electric materials used for crystal oscillators are quartz, tourmaline and   Rochelle salt, which exhibits a property called piezo-electric effect.

27. What are the factors that affect the frequency of any oscillator?
           The factors that affect the frequency of oscillator are
      1.Change in temperature
      2.Variation in power supply
      3.Unstable transistor parameters
      4.Variation in biasing and loading condition.

28. What is the purpose for providing both positive and negative feedback in wein bridge oscillator?
    The positive feedback ensures sustained oscillations, a condition necessary in all oscillations. However   the purpose of providing negative feedback in this of of oscillator  is to ensure constant output. Any increase  or decrease in the oscillator ouput is taken care of by the negative feedback.

29. Why is quartz crystal commonly used in crystal oscillators?
    Quartz crystal  is commonly used in crystal oscillators because it is inexpensive and  readily available in  nature.

30. What is the main advantage of wein bridge oscillators over RC phase shift oscillators?
    Wein bridge oscillators are more amenable to variable frequency operation as it easier to vary two      capacitors simultaneously than varying three capacitors.

31. Why are LC oscillators preferred over RC oscillators at high frequencies range in MHZ?
    Firstly, because L and C have reasonable values at higher frequencies and secondly because of     comparatively much better frequency stability at these frequencies, LC oscillators preferred over RC  oscillators.

32. 18. What is the condition for sustained oscillation in colpitts oscillator?
           The condition for sustained oscillation in colpitts oscillator is given by h fe = C2 / C 1.

33. How does an oscillator differ from an amplifier as far as circuit configuration is concerned?
     An oscillator is an amplifier with a positive feedback in which the amount of output to be feedback is a function of the amplifier gain.

34. What is relaxation oscillator?
    An oscillator whose fundamental frequency of the output is determined by the time of charging or Discharging the capacitor or coil through a resistor producing forms that may be rectangular or saw- tooth.

35. What is frequency stability? 
    The ability of an oscillator to maintain the desired frequency, usually expressed as percent deviations from the assigned frequency value.

36. What is Hartley oscillator?
      An oscillator in which the parallel tuned tank circuit is connected between base and collector, the tank coil has an intermediate tap at emitter potential. So the base emitter portion of the coil provides at the necessary feedback voltage.

37. How is it that R-C phase shift, Wein Bridge, Hartley, Colpitts and other oscillators of their categories  produce sinusoidal output?
      All these oscillators have a frequency selective network in the feedback path. These feedback networks satisfy Barkhausen criterion as regards loop phase-shift only at a single frequency which implies that the output will be sinusoidal.


38. 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.

39.What happens to the circuit above and below resonance?
    Above resonance the circuit acts as capacitive and below resonance the circuit acts as inductive.

40. A wein bridge oscillator is used for operations at 9KHZ. If the value of the resisteance R is 100 K OHM, What is the value of C required?

Solution:
          f = 1/ 2πRC
         9 * 103 = 1/ (2π *100* 103 C)
         C = 0.245nF

41. What is meant by tickler oscillator?
    The small secondary winding is called tickler coil and the oscillators is also called tickler oscillator.

42. What are the two types of RC oscillators?
       The two types of RC oscillator are
      1. RC Phase shift oscillator and              2.Wien bridge oscillator.

43. 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

44. Disadvantage of wien bridge oscillators.
•    It cannot be used to generate high frequencies.
•    The circuit needs two transistors and a large number of components.
The maximum frequencies is limited due to the amplitude and the phase shift charecteristics of the amplifiers.

45. Define oscillator.
    Any circuit which is used to generate a.c voltage without a.c input signal is called an oscillator.  An amplifier with positive feedback has infinite gain and gives an a.c output without a.c input signal.

46. Give the classification of oscillators on the basis of waveform generated.
    According to the waveforms generated the oscillators are classified into:
     1. Sinusoidal oscillator   2.Relaxation oscillator.

47. How will you classify oscillators with respect to the fundamental mechanism  involved?
       With respect to the fundamental mechanism involved oscillators are classified as:
      1. Negative resistance oscillators       2.Feedback oscillators.

48. What are the types of sinusoidal oscillator with respect to the circuit used?
         With respect to the circuit used, sinusoidal oscillators may be classified as   1. LC tuned oscillator and        2.RC phase shift oscillator.


49. Advantage of wien bridge oscillators.
•    High gain and stability.
•    Low distortion sinusoidal output
•    Circuit is easy to design.
•    Constant output.

50. Draw the electrical equivalent circuit crystal and mention the significance of each component.    
Resistance R is used to represent internal frictional losses during vibrations, Inductance L is used to   indicate the inertia and Capacitor C is used to represent the stiffness.




                                                                  PART – B
1. Explain Barkhausen criterion to be satisfied for sustained oscillations.                               (DEC 2007)   
2. Explain RC Phase shift oscillator with neat circuit diagram. Derive its frequency of oscillation.                                        (NOV 2010, MAY 2012)
3. Design an RC phase shift oscillator to generate 5 KHz sine wave with 20V peak to peak amplitude. Draw the designed circuit. Assume hfe=150.(DEC 2009 & MAY 2011)
4. Derive the magnitude condition and frequency of oscillation of a wein bridge oscillator.  (MAY 2010)
5. Describe the general analysis of feedback network of LC oscillator.       (MAY 2006)
6. Explain the working of colpitt’s oscillator and derive the frequency of oscillation.         (MAY 2010 ,2011& NOV 2010,2011)
7. In colpitts oscillator C1=0.001µF, C2=0.01 µF and L1=10 µH. Find the frequency of oscillation, feedback   factor and voltage gain. (DEC 2009, NOV 2011)
8. Explain about Hartley oscillator and derive its frequency of oscillation.   
                                                                                               (DEC 2006)
9. Write in detailed note on negative resistance Oscillator using tunnel diode.                  (MAY 2003)10. (a)What is piezoelectric effect? Draw and explain a.c equivalent of a crystal.
       (b) Describe the crystal oscillator with neat diagram.       (MAY 2007)
11. With neat diagram explain the operation of Pierce crystal Oscillator.(DEC 2008)
12. Explain the working of Miller type crystal Oscillator. Give two applications.      (MAY 2008)
13. With the circuit diagram derive an expression for frequency of oscillations of Clapp oscillator. Explain how   Barkhausen criterion is satisfied.                                        (DEC 2009)
14. With neat circuit diagrams explain the working principle of the following oscillators. State also the expression   for the frequency in each case.
       (1) Tuned collector oscillator  (2) Franklin oscillator     (3) Armstrong oscillator     (MAY 2012)
15. Derive the transfer function of a phase lead – lag network and hence obtain the frequency of oscillation of a wein bridge oscillator.   (NOV 2011)

FEEDBACK AMPLIFIERS UNIT-I

UNIT – I

FEEDBACK AMPLIFIERS

PART - A

1. Classify negative feedback amplifiers?        (MAY 2010 & NOV 2010)

            (i) Voltage series feedback (ii) Voltage shunt feedback (iii) Current series feedback (iv) Current shunt  feedback.

2. What are merits of negative feedback amplifiers?(MAY2010&JUN 2009)

            The performance measures of negative feedback amplifiers are as follows(i) Improved stability   ii) Reduction in gain    iii) Reduction in distortion and noise iv) Increase the input Impedance and bandwidth  v) Decrease the output impedance

3. Define Feedback factor?                 (MAY 2010, MAY 2012)

            The ratio of feedback voltage to output voltage is known as feedback factor or feedback ratio.

4. Discuss the effect of feedback on noise.            (MAY 2011)

            Effect of feedback on noise can be explained with the following consideration:Let A₁ and A₂ be two amplifier feedback circuit .N be the noise introduced after stage1.The output voltage can be expressed as,

 V₀  =                                           A₁A₂V_s          =          NA₂

                                             1 + A₁A₂2                 1 + A₁ A₂2

Thus the overall noise of two stage amplifier is reduced by the factor 1 + A₁A₂b.

5. “The gain bandwidth product of an amplifier is not altered, when negative feedback is introduced”-Justify the statement.    (NOV 2004)

            When negative feedback is employed in an amplifier, its bandwidth increases by the same factor (1 + Ab) by which its gain reduces .Hence the product of gain and bandwidth called gain bandwidth product  remains the same.

6. Draw the block diagram of voltage feedback amplifier and give its input and output resistance.                         (MAY 2005)

 

7. Define loop gain. (NOV 2007)

            The product of the feedback factor b and the gain of the amplifier multiplied by –1 (for negative feedback) is called loop gain or return ratio or the loop transmission feedback factor.

8.  What is loop gain? (May 11)

            A path of a signal from input terminals through basic amplifiers, through the feedback network and back to the input terminals from a loop. The gain of this loop is the product –Aβ. This gain is known as loop gain.

9.  What is return ratio? (May 11)

            A path of a signal from input terminals through basic amplifiers, through the feedback network and back to the input terminals from a loop. The gain of this loop is the product –Aβ. This gain is known as loop gain or return ratio.

10. Compare the input and output resistance of the voltage and current shunt feedback amplifiers.                       (NOV 2008, NOV 2011)

	Input Resistance	Output Resistance
Voltage Shunt	Rif = Ri / (1+Aβ)	R0f = R0 / (1+Aβ)
Current Shunt	Rif = Ri / (1+Aβ)	R0f = R0 .(1+Aβ)

11. If an amplifier has the gain of 400 and feedback ratio of 0.1, find the gain with negative feedback.                (NOV 2008 , 2010, 2011)

            Negative feedback gain Af = A / (1+Aβ), Given  A=400, β=0.      

Therefore, gain = 400 / (1+(400 X 0.1)) = 9.7

12. State Nyquist stability criterion.                                   (MAY 2010)

            The amplifier is unstable if this curve encloses the point (-1+j0), and the amplifier is stable if the curve does not enclose this point.

13. Define sensitivity in feedback amplifiers .                    (MAY 2011)

            The fractional change in the amplification with feedback divided by the fractional change without feedback is called the sensitivity.

 

15. Define desensitivity.   (MAY 2004 & NOV 2010, NOV 2011, MAY 2012)

            The fractional change in the amplification with feedback divided by the fractional change without feedback is called the sensitivity. The reciprocal of the sensitivity is called the desensitivity.  D= 1+Aβ.

16.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.

17. Give the effect of negative feedback on amplifier characteristics.

Characteristics	Type of feedback
	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	Increases	Decreases	Decreases	Increases

18. Give an example for voltage-series feedback.

        The Common collector or Emitter follower amplifier is an example for voltage series feedback.

 

19. 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.

20. 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.

21.Write the expression for input  resistance of voltage series feedback amplifier.

                               Input resistance with feedback, Rif  = Ri 

 22. Write the expression for output  resistance of voltage series feedback amplifier.

               Output resistance with feedback, Rof = Ro

 23. What type of feedback used oscillators?

                                    Positive feedback.

24. What is the purpose of mixer network in feedback amplifiers?

            The mixer network is used to combine feedback signal and input signal at input of an amplifier.

25. Define feedback factor or feedback ratio.

            The ratio of feedback voltage to output voltage is known as feedback factor.

26. What is nyquist diagram?

            The plot which shows the relationship between gain and phase shift as a function of frequency is called nyquist diagram.

27. State the Nyquist criterion for stability of feedback amplifiers.

            * The amplifiers is unstable if the curve enclose the point -1+j0. The system is called unstable system.

            * The amplifiers is stable if the curve enclose the point -1+j0. The system is called stable system.

28. What is the effect of voltage series feedback on circuit impedance?

            The effect of voltage series feedback on circuit impedance increases the input impedance but decreases the output impedance.

29. What is the effect of  negative feedback on noise?

            The noise is reduced with the negative feedback.

30. What is the effect of  negative feedback on frequency distortion?

            The frequency distortion is reduced with  the negative feedback.

31. Which type of negative feedback always produces an increase in input resistance and decrease in output resistance?

            Negative voltage series feedback always produces an increase in input resistance but decrease in output resistance.

32. What are the types of distortion in an amplifiers?

* Frequency

* Non linear

33. Why is positive feedback rarely used in practice?

            Positive feedback is rarely used in practice because it has the disadvantage of increased distortion and instability.

34. Give two examples of Voltage series feedback amplifier.

            The examples of voltage series feedback amplifier are (i) emitter follower   (ii) source follower.

35. Distinguish between series and shunt feedback.

SERIES FEEDBACK	SHUNT FEEDBACK
i)The feedback signal is connected  in series with the input signal.	i) The feedback signal is connected in parallel with the input signal.
ii) It increases the input resistance.	ii) It decreases the input resistance.

36. Discuss the effect of feedback on output impedance.

            Any amplifier with low output impedance is capable of delivering power to the load without much loss and it can be easily achieved with the help of of feedback techniques. In this case, the output impedance of the feedback amplifier is given as

       Z_of  =                                                   Zo

                                                              1 + Ab

37. Explain how reduction in distortion is carried out in case of feedback amplifier.

            Let D^| be the distortion in amplifier without feedback and D be the distortion in amplifier with       feedback. After amplification, the distortion is in anti phase with the original distortion voltage D and      expressed as                

                                       D^| =        D

                                                 1 + Ab

38. Discuss the disadvantage of negative feedback.

            The main disadvantage of negative feedback is that it reduces the input to the amplifier which in turns reduces the output of the amplifier.

39. What are the two types of feedback?

             If the sampled output adds to the input of the amplifier, it is called positive or direct or regenerative feedback. A feedback is said to be negative or inverse or degenerative if the sampled output reduces the input to the amplifier.

40. Compare the amplifier gain with and without feedback.

            Gain of the amplifier without feedback is,A= X₀ / X_i        Where X₀ – Output voltage, X_i – Input Voltage Gain of the with feedback (negative) is given as,A_f  =  A /(1 + Ab) Where  A = amplifier gain without feedback, b= feedback gain, A_f  = gain with feedback.

41. Define sampling and mixing.

            Sampling is the process of taking a part of output voltage or current. The process of adding or subtracting

this sampled value to the input of the amplifier is called mixing.

42. What are the types of distortion that occur in amplifiers?

            Non-linear distortion, phase distortion, frequency distortion and noise distortion are the types of distortion had occur in amplifiers.

43. Give some applications of voltage feedback.

            (i) Public address system, (ii) transistor radio receivers etc..,

44. What type of feedback is employed in a common collector amplifier?

            Negative voltage series feedback is employed in a common collector amplifier.

45. What is the basic difference between feedback in biasing circuits and amplifier circuits?

            In biasing circuits, we employ dc negative feedback while in amplifiers, it is ac negative feedback.

46. Why negative feedback is employed in high gain amplifiers?

            The negative feedback reduces the gain of the amplifiers but improves the circuit performance in terms of stability distortion, etc. Hence it is employed in high gain amplifier.

47. What is phase margin?

            The phase margin evaluated at the frequency to where the magnitude of A(jw) b reaches unity. The phase margin is defined as the difference between the angle of A(jw) b at w and the angle-180°.

48. What is gain margin?

            The stability of a feedback loop can also be assessed by examining its bode plot and evaluating one of two related parameters. One parameter called the gain margin is defined as the difference between unity and magnitude of A(jw) b at w 180°.

49. What is feedback?

            When we sample a part of the output voltage or current of an amplifier by a suitable circuit and apply this to the input so as to modify the performance and operation of the circuit, this is called feedback.

50. What is a feedback amplifier?

            If an amplifier output voltage or current is sampled and this sampled output is applied to the input through a two port network, the network becomes a feedback  amplifier.

51. List the types of feedback.

            Broadly classifying, the feedback can be divided as positive or regenerative or direct and negative or  degenerative or inverse. Based on the type of sampling and mixing, it can be further classified into voltage  series or shunt and current series or shunt.

52. What is negative feedback?

            When the effect of the feedback signal on the action of the circuit is opposite to that produced by the input   signal, then the feedback is called negative, degenerative or inverse.

PART - B

1. (a) Draw the block diagram of current sampling - series mixing feedback configuration and derive an expression for transfer gain, input resistance and output resistance.            (MAY 2010)

    (b) How the negative feedback amplifier improves stability, reduces noise and increases the input impedance?

2. (a) Identify four possible topologies of a feedback network.

    (b) Identify the o/p signal & f/b signal for each topology.

    (c) Identify the transfer gain & feedback factor for each topology.             

                                                                                    (MAY 2010)

3. (a) Write notes on nyquist criterion.

    (b) Writs notes on a feedback amplifier, using a block diagram.                                                                                                  (MAY 2010)

4. With block diagram of current series feedback and derive the expression for R_if and R_of..                                                                                                           (DEC 2009)

5. Design a two stage voltage series feedback amplifier so as to reduce the gain to 75. Assume β=0.01, Vcc=20,h_fe1=h_fe2=100. Draw the designed circuit.                                                                                  (DEC 2009)

6. (a) Using block diagram, derive the closed loop forward transfer ratio of feedback system in terms of the open gain.                   (NOV 2011)

    (b) Discuss the effect of negative feedback on the frequency response of an amplifier.                                                  (JUN 2009,  MAY 2012)

 7. Draw the circuit of emitter follower. Identify the type of negative feedback. Calculate the gain, input and output resistance with and without feedback.                                                     (DEC 2008 & NOV 2010)

8. (a) Discuss the classification of feedback amplifiers with schematic(topology). How is impedance level modified in each type?                                                                                  (MAY 2008, MAY 2012)

    (b) Derive expression for Avf with positive feedback and state condition for stability in negative feedback  amplifiers.

9. Draw the circuit of a current series feedback amplifier and explain. Derive expressions for input and output  impedance. How does it improve the stability of the amplifier.                                                                        (DEC 2007)

10. Give the block diagram of feedback amplifier and discuss the effect of negative feedback with respect to closed loop gain, bandwidth and distortion.                                                                                        (DEC 2006)

11. (a) Describe briefly the general characteristics of negative feedback amplifier.

      (b) Draw the basic circuit of the voltage shunt feedback amplifier and describe the concepts involved in such an amplifier.                   (DEC 2005)

12. (a) Using a block diagram, derive the expression of closed loop forward transfer ratio with positive and negative feedbacks introduced in an amplifier.         (MAY 2005, NOV 2011, MAY 2012)

(b) List out the steps that are carried out in obtaining the complete analysis of a feedback amplifier.

13. Define and explain the following types of amplifier (i) voltage amplifier (ii) Current amplifier (iii) trans-conductance amplifier (iv) trans-resistance amplifier. Derive a general expression for gain with feedback. What is an ideal feedback amplifier?                                               (APR 1995)

14. Derive the expressions for A_vf, R _if,  R_of, R_o for voltage series feedback transistor amplifier?

15. Perform the general analysis of current shunt feedback amplifier.

16. Explain the general analysis of Voltage shunt feedback.

17. (a) Using an example, explain voltage series feedback and analyze the circuit to determine the gain,

input and output resistance.

      (b) Explain Nyquist criterion to analyze the stability of feedback amplifiers.     (MAY 2011)

18. The gain bandwidth product of an amplifier is not altered, when negative feedback is introduced. Justify the statement.                             (NOV 2011)

19. For, a feedback amplifier, derive the expressions for (1) the gain with feedback (2) Lower cut off  frequency (3) Upper cut off frequency. (MAY 2012)

20. If an amplifier has a bandwidth of 300 kHz and a voltage gain of 100, what will be the new bandwidth and gain if 10% negative feedback is introduced? What will be the gain – bandwidth product before and after feedback? What should be the amount of feedback if the bandwidth is to be limited to 800 kHz?

21. A current series feedback amplifier is shown in the figure below:

It has the following parameters: R1 =20K Ω, R2=20K Ω, h_ie = 2 K Ω, R_L=1K Ω, R_e=100 Ω, h_fe=80, h_re=h_oe=0 Calculate A_v, β, R_if  and A_{vf .}      (MAY 2012)