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)
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