Important Two Marks Questions with Answers on Feedback Amplifiers and Oscillators

TWO MARK QUESTION AND ANSWERS

1. Differentiate voltage and current feedback in amplifiers.

In voltage feedback, feedback voltage V_f is proportional to output voltage V_o and in current feedback, feedback voltage V_f proportional to the output current I_o.

2. Define positive and negative feedback?

Positive Feedback: If the feedback voltage (or current) is so applied as to increase the input voltage (i.e., it is in phase with it), then it is called positive feedback.

Negative Feedback: If the feedback voltage (or current) is so applied as to reduce the input voltage (i.e., it is 180° out of phase with it), then it is called negative feedback.

3. What are the advantages of negative feedback?

(or) Why do you prefer negative feedback in amplifier?

Advantages of Negative Feedback

1. Negative feedback stabilizes the gain of the amplifier.

2. There is a significant increase in the bandwidth of the amplifier.

3. Distortions in the amplifier output are reduced.

4. Input resistance increases for certain feedback configurations.

5. Output resistance decreases for certain feedback configurations.

6. Operating point is stabilized.

4. List out four different ways of connecting the feedback amplifier.

Different ways of connecting the feedback amplifier are

1. Voltage series feedback

2. Voltage shunt feedback

3. Current series feedback

4. Current shunt feedback

5. State the conditions in (1+A) which a feedback amplifier must satisfy inorder to be stable.

The two important and necessary conditions are

1. The feedback must be positive

2. Feedback factor must be unity ie A = 1.

6. State the three fundamental assumptions which are made in order that the expression  be satisfied exactly.

1. The input signal should be transmitted to the output through the internal amplifier A and not through the feedback network. Thus if A is set to zero by reducing hfe (or) gm of the transistor to zero, the output must drop to Zero.

2. The feedback signal travels from the output to the input through the ẞ network and not through the amplifier.

3. The reverse transmision factor β of the feedback network is independent of the load and the source resistance RL and RS.

7. What is the effect of complex loop gain and on input resistance in series voltage feedback?

In series voltage amplifier the complex loop gain decreases and the input resistance increases.

8. Mention the equation relating the gains and feedback factor in a single loop feedback amplifier.

9. Define desensitivity.

The reciprocal of sensitivity is called de-sensitivity D.

D = 1 + Aβ

Where sensitivity is defined as the fractional change in amplification with feedback divided by the fractional change without feedback and is equal to 1/1+Aβ

10. What is the effect of voltage shunt feedback on input resistance and output resistance?

In voltage shunt feedback amplifier both the input and output resistance decrease.

11. What are the ideal characteristics of voltage amplifier?

R₁ = ∞, R_o = 0 and V_o = A_V V

12. Define trans resistance amplifier.

It is an amplifier, driven by a source represented by its Norton's equivalent ie. the current source is connected with the source resistance R_S in shunt. The amplifier output circuit has been represented by its Thevenin's equivalent. This amplifier behaves as a transresistance amplifier if R_i << R_S and R_o << R_i.

13. What are the characteristics of transconductance amplifier?

R_i = R_i >> R_S, R_o >> R_i and its ideal characteristics are R_i = ∞, R_o = ∞ and I_o = G_m V_S.

14. If two stages of a multistage amplifier have gains of 20dB and 10dB respectively. What is the total voltage gain?

Total voltage gain = 20 + 10 = 30 dB

15. Define sampling and mixing.

Sampling is the process of taking a part of output voltage or current. The process of adding or subtracting the this sampled value to the input of the amplifier is called mixing.

16. What is Barkhausen Criterion?

For getting sustained oscillations we should satisfy the following two conditions.

(i) The loop gain should be unity

(ii) The loop phase shift should be 0° or 360°. ie., β_V A_V = 1. This criterion is known as Barkausen's criterion for oscillation.

17. Classify the different sinusoidal oscillators.

i. Hartley oscillator

ii. Colpitts oscillator

iii. RC phase shift oscillator

iv. Wien bridge oscillator

v. Clapp oscillator

vi. Crystal oscillator.

18. What are the factors needed to choose type of oscillators?

The factors needed to choose type of oscillators are

i. The nature of generated wave form.

ii. The frequency of generated signals

iii. The type of associated circuit of components.

iv. The fundamental mechanism involved.

19. What is resonant circuit oscillator?

The oscillator using resonant LC tank circuits are more often used for sources of radio frequency energy are called as resonant circuit oscillator.

20. What is the difference between amplifier and oscilator?

In an amplifier circuit, the frequency waveform and magnitude of ac power generated is controlled by ac signal voltage applied at the input of the amplifier in the other hand in an oscillator the frequency, waveform and magnitude of ac power generated is controlled by the circuit itself i.e. no external controlling voltage is required. Thus an oscillator may be considered as an amplifier which provides its own input signal.

21. Why RC phase shift oscillators are needed?

For the generation of low frequency signlas the LC circuits become impracticable and the RC phase shift oscillators are more suitable with the advantage of IC technology RC network is the only feasible solution. It is very difficult to make an inductance that too of very high value in an IC. Therefore RC oscillators are increasingly popular.

22. Which oscillators use both positive and negative feedback? Why?

Wien bridge oscillator uses both positive and negative feedback. The positive feedback ensures sustained oscillation, the negative feedback ensures constant output ie., any increase or decrease in the oscillator output is taken care of by this negative feedback.

23. What are the advantages of crystal oscillator?

i. Simple circuit since no tuned circuit is needed other than the crystal itself hotel is used.

ii. Different frequencies of oscillations can be obtained by simply replacing one crystal another. Hence it makes it easy for a radio transistor to work at different frequencies.

iii. Since the frequency of oscillation is set by the crystal, changes in the supply VS and transistor parameters does not affect the frequency of oscillation.

24. Name two low frequency oscillator.

i. RC phase shift oscillator

ii. Wien bridge oscillator

25. Name two high frequency oscillators.

i. Hartley Oscillator

ii. Colpitt's Oscillator

26. Why crystal oscillators are superior than other oscillators?

Crystal oscilators are superior than other oscillators because of their great mechanical strength, simplicity of manufacture and it obeys the Piezoelectric effect accurately.

27. What is meant by un-sustained oscillation? When it will occur?

The electrical oscillations whose amplitude goes on decreasing with time are called un-sustained oscillation. The electrical system in which these oscillations are generated has losses and some energy is lost during each oscillations. Further, no means are provided to compensate for the losses and consequently the amplitude of the generator wave decrease gradually.

28. Write the expresion for frequency of oscillatioon in RC-phase shift oscillator.

 

REVIEW QUESTIONS

1. Explain the concept of negative feedback in amplifier. Derive the expression for voltage gain, input impedance and output impedance. (AUC DEC'11)

2. With a neat diagram explain the construction of BJT RC phase shift oscillator.

(AUC DEC'11)

3. Explain the construction and working of Hartley oscillator. (AUC DEC'11)

4. Draw the block diagram of a voltage series feedback amplifier and derive the equation for input impedance, output impedance and the voltage gain. (AUC MAY'09)

5. Calculate the voltage gain, input and output resistances of a voltage series feedback amplifier having A_V = 300, R_i = 1.5 k, R_o = 50 k and β = 1/15. (AUC MAY'09)

6. Differentiate oscillator with amplifier. (AUC MAY'09)

7. Draw the circuit of a Hartley oscillator and derive the condition for the frequency of oscillation. (AUC MAY'09)

8. Draw and explain the operation of a Hartley oscillator. (AUC NOV'10)

9. (i) Discuss the various topologies of feedback amplifier. (AUC APR'09)

(ii) Discuss the operation of a Colpitts oscillator in detail. (AUC NOV'10)

10. Explain the effects of negative feedback in amplifiers. (AUC APR'09).