Important Short Questions and Answers: Tuned Amplifiers

TUNED AMPLIFIERS

1.    What is tuned amplifier? What are the various types of tuned amplifiers?

A tuned amplifier amplifies a certain range of frequencies (narrow band of frequencies) in the radio frequency region and rejects all other frequencies.

Types:

The various types of tuned amplifiers are

i)             Single tuned amplifier

ii)          Double tuned amplifier

iii)        Stagger tuned amplifier & synchronously tuned amplifier.

2.    Define tuned amplifier.

A tuned amplifier is defined as an amplifier circuit which amplifies a certain range of frequencies (narrow band of frequencies) in the radio frequency region and reject all other frequencies.

3.    Why tuned amplifier cannot be used at low frequency?

For low frequencies the size L and C are large. So the circuit will be bulky and expensive, hence the tuned amplifiers cannot be used at low frequency.

4.    What is the other name for tuned amplifier?

Tuned amplifiers used for amplifying narrow band of frequencies hence it si also known as “narrow band amplifier” or “Band pass amplifier”.

5.    Mention The Two Applications of tuned amplifiers.

i)             They are used in IF amplifiers in Radio and TV receivers.

ii)          They are used in wireless communication systems.

6. State two advantages and two disadvantages of tuned amplifiers. Advantages:

i)             They amplify defined frequencies

ii)          Signal to noise ratio (SNR) at output is good.

iii)        They are suited for radio transmitters and receivers.

Disadvantages:

i)             They are not suitable to amplify audio frequencies.

ii)          Circuit is bulky and costly.

iii)        The design is complex.

7.           What is Single tuned and double tuned amplifier?

Single tuned amplifier:

A amplifier circuit that uses  a single parallel tuned circuit as a load is called single tuned amplifier.

Double tuned amplifier:

The amplifiers having two parallel resonant circuit in its load are called double tuned amplifiers.

8.           What are the advantages of double tuned amplifier over single tuned amplifier?

i)             Provides higher gain

ii)          Provides large 3dB bandwidth.

iii)        Possess flatter response having steeper sides.

9.    What are the different coil losses?

i)             Hysteresis loss

ii)          Copper loss

iii)        Eddy current loss

10. What are the differences between single tuned and synchronously tuned amplifiers?

11. What is Stagger tuned amplifier

If two or more tuned circuits which are cascaded are tuned to slightly different resonant frequencies, it is possible to obtain an increased bandwidth with a flat passbandwith steep sides. This technique is known as stagger tuning and the amplifier using this technique is called as stagger tuned amplifier.

12. What are the different types of neutralization?

i)             Hazeltine neutralization

ii)          Neutrodyne neutralization

iii)        Rice neutralization

13. Why neutralization required in tuned amplifiers? & Draw the circuit for Narrow Band neutralization.

In order to prevent oscillations in tuned RF amplifiers it was necessary to reduce the stage gain to a level that ensured circuit stability. This can be accomplished in several ways such as lowering the Q of the tuned circuits, stagger tuning ,loose coupling between the stages. Instead of losingthe circuit performance to achieve stability, a circuitin which the troublesome effect of the collector to base capacitance of the transistor was neutralised by introducing a signal which cancels the signal coupled through the collector to base capacitance.

14. Define loaded and unloaded Q.

 Unloaded Q:

It is defined as the ratio of stored energy to dissipated energy in a reactor or resonator. For an inductor or capacitor

Where  X= reactance: 

R_s= series resistance

Loaded Q:

The loaded Q or Q_L of a resonator is determined by how tightly the resonator is coupled to its terminations.

15. What is the effect of cascading n stages of identical single tuned amplifiers (synchronously tuned) on the overall 3db bandwidth?

The bandwidth of n stage cascaded single tuned amplifier is given as

From the above equation it is clear that the overall 3dB bandwidth reduces.

16. Mention the bandwidth of a double tuned amplifier

Q is the Quality factor of the coil alone

b- is a constant

17. Where is the Q-point placed in a class C type amplifier? What are its applications?

In a class C type amplifier the Q-point is placed below the X-axis.

Applications:

·              The Class C amplifiers are used to amplify the signals at radio frequencies.

·              They are also used in mixer circuits.

18. Brief the relation between bandwidth and Q-factor.

The quality factor determines the 3dB bandwidth for the resonant circuit. The 3dB bandwidth for resonant circuit is given by

BW=f_r/Q

Where f_r= centre frequency of a resonator

BW= f₂-f₁

19.      What is narrow band neutralization?

The 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.

20.      Mention two important features of stagger tuned amplifier.

i)             It has better flat, wide band characteristics.

ii)          Increased bandwidth

21. What is the need for neutralization circuits?

In tuned RF amplifiers, the inter-junction capacitance C_bc of the transistor becomes dominant (i.e) its reactance is low, it provides the feedback signal from collector to base. If some feedback signal manages to reach the input from output in a positive manner with proper phase shift, then amplifier keeps oscillating, thus stability of amplifier gets affected. Hence neutralization is employed.

22. Draw a class C tuned amplifier circuit and what is its efficiency.

23. Derive the resonance frequency for the tank circuit shown:

At resonance X_L=X_C

24.      A tuned circuit has resonant frequency of 1600 KHz and bandwidth of 10 KHz. What is the value of its Q-factor?

25.      A tuned amplifier has its maximum gain at a frequency of 2 MHz and has a bandwidth of 50 KHz.calculate the Q-factor.

26.      An inductor of 250µH has Q=300 at 1MHz. Determine R _s and R_p of the inductor.

27.      A parallel resonant circuit has an inductance of 150µH and a capacitance of 100pF. Find the resonant frequency.