Electronics Devices & Circuits

Sinusoidal Oscillator Viva Questions

Sinusoidal Oscillator Viva Questions

Sinusoidal Oscillator Viva Questions, Viva Questions on Sinusoidal Oscillator, Short Questions on Sinusoidal Oscillator, Electronic Devices & Circuits Viva Questions, Engineering Viva Questions

Short Questions with Answers

Q.1. What is the Barkhausen criterion?

Ans. Barkhausen Criterion states that the frequency of a sinusoidal oscillator is determined by the condition that the loop-gain phase shift is zero.

Hence A\beta =1

It implies that

  • \left |A\beta \right | =1 i.e., magnitude of feedback factor = unity.
  • \phi =0 or multiple of 2π

Another interpretation of Barkhausen Criterion is

A_{fb}=\frac{A}{1+A\beta }=\frac{A}{1-1}=\infty

Since gain is infinity we will get fainter output with no input.

Q.2. What is a crystal oscillator?

Ans. In crystal oscillators, the usual electrical resonant circuit is replaced by a mechanically vibrating crystal. As a matter of fact, the crystal (usual quartz) has a high degree of stability in holding constant at whatever frequency the crystal is originally cut to operate. Therefore, the crystal oscillators are, used whenever great stability is required. For example, in communication transmitters, and receivers, digital clocks, etc.

Q.3. Write the expressions for two resonant frequencies of a crystal oscillator.

Ans. Two resonant frequencies are given by the expressions

  • series resonant frequency, f_{s}=\frac{1}{2\Pi \sqrt{LC}}
  • Parallel resonant frequency, f_{p}=\frac{1}{2\Pi}\sqrt{\frac{1+C/C_{M}}{LC}}

Q.4. What are the drawbacks of crystal oscillators?

Ans. Following are the drawbacks of crystal oscillators:

  • The crystal oscillators have a very limited tuning range (or not at all). They are used for frequencies exceeding 100 kHz.
  • The crystal oscillators are fragile and, therefore, can only be used in low power circuits.

Q.5. Write the advantages of a Wien bridge oscillator.

Ans. Advantages:

  • It provides a stable low distortion sinusoidal output over a wide range of frequency.
  • The frequency range can be selected simply by using decade resistance boxes.
  • The frequency of oscillation can be easily varied by varying capacitances C1 and C2 simultaneously.
  • The overall gain is high because of two transistors.

Q.6. what is an oscillator?

Ans. An oscillator may be defined as a circuit that generates an ac output signal of very high frequency without requiring any externally applied input signal or it may be defined as an electronic source of alternating current (or voltage) having sinusoidal or non-sinusoidal (square, sawtooth, or pulse) waveshape. It can also be defined as a circuit that converts d.c. energy into very high-frequency a.c. energy.

Q.7. What is the initial condition for oscillation to start?

Ans. The initial condition for oscillation to start is loop gain βA ≥ 1.

Q.8. What are the Barkhausen conditions of oscillations?

Ans. Barkhausen conditions for sustained oscillations are: (i) the loop gain of the circuit must be ≥ 1 and (ii) the phase shift around the circuit must be zero.

Q.9. Can every electronic oscillator be considered to be an amplifier with an infinite gain? If yes, wherefrom starting signal voltage come from?

Ans. Yes, starting signal voltage comes from more noise inherent in the transistor or variation in voltage of dc power supply.

Q.10. Why is L-C resonant circuit impractical at audio frequencies?

Ans. L-C resonant circuits are impractical at audio frequencies because components required in their construction for low-frequency operation are too bulky and heavy and expensive.

Q.11. How does the Clapp oscillator circuit differ from Colpitt’s oscillator circuit in construction?

Ans. Clapp oscillator circuit is a refinement of Colpitt’s oscillator circuit. The only difference is that the single inductor found in the tank circuit of Colpitt’s oscillator is replaced by a series L-C combination (inductor L in series with capacitor C).

Q 12. Why Clapp oscillator is preferred over Colpitt’s oscillator?

Ans. In a Colpitt’s oscillator, the resonant frequency is affected by the transistor and stray capacitances but in a Clapp oscillator, the transistor and stray capacitances have no effect on the oscillation frequency. Thus in the Clapp oscillator, the effect transistor parameters on the operation of the circuit are eliminated and the frequency stability is improved. That is why the Clapp oscillator is preferred over Colpitt’s oscillator.

Q 13. How does the Hartley oscillator differ from Colpitt’s oscillator in construction?

Ans. Hartley oscillator circuit is similar to Colpitt’s oscillator except that the phase-shift network consists of two inductors and one capacitor instead of two capacitors and one inductor.

Q.14. Why is a crystal oscillator used in communication transmitters and receivers?

Ans. Crystal oscillators are used in communication transmitters and receivers because of their greater frequency stability.

Q.15. Why do we use two transistors in a Wien bridge oscillator?

Ans. Wien bridge oscillator employs two transistors so as to obtain 360° or 0° phase shift between output and input voltages.

Q.16. Sometimes a lamp is used in one of the resistance arms of a Wien bridge oscillator. Why?

Ans. A lamp is used in one of the resistance arms of a Wien bridge oscillator so as to provide negative feedback, in proportion to the output current, in the circuit and thus ensure constant output over a range of frequencies.

Q.17. Why beat frequency oscillator (BFO) has become obsolete now?

Ans. The beat frequency oscillator has become obsolete now because its circuit is quite complicated.

Q.18. How does a signal generator differ from an ordinary oscillator?

Ans. The signal generator, like an ordinary oscillator, is a source of sinusoidal signals but it is also capable of modulating its sinusoidal output signal with other signals.

Q.19. Why the use of a regulated power supply is essential for signal generators?

Ans. The frequency of output voltage from a signal generator changes with the change in supply voltage so the use of a regulated power supply is essential.

Q.20. Why are buffer amplifiers used in signal generators?

Ans. Buffer amplifiers are used in the signal generators for isolating the oscillator circuit from the output circuit so that any change in the circuit connected to the output does not affect the frequency and amplitude of the Oscillator output.

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