Sinusoidal Oscillators Interview Questions and Answers:

1. What is an oscillator?

Ans. Oscillator may be defined as a circuit which 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 nonsinusoidal (square, sawtooth or pulse) waveshape. It can also be defined as a circuit that converts dc energy into very high frequency ac energy.

2. What is Barkhausen criterion for the feedback oscillator.

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.

3. What do you mean by Barkhausen criteria?

Ans. The introduction of positive feedback in an amplifier causes an increase in voltage gain. If the loop gain Aβ ≥ 1, the Barkhausen criterion for oscillation is satisfied, and an output remains even without the input.

For linear oscillation. in which the output is sinusoidal with little distortion, the loop gain. Aβ, should be only slightly greater than unity.

The other condition that is required to be fulfilled for sustained oscillations is that the phase shift around the circuit must be zero.

4. Explain why positive feedback and not negative feedback is necessary to produce oscillations.

Ans. With negative feedback, feedback factor Aβ becomes less than unity, AβV_in less than V_in and therefore output signal will die out while with positive feedback Aβ exceeds unity, AβV_in exceeds V_in and oscillations are produced. That is why positive feedback is necessary to produce oscillations.

5. Every electronic oscillator can be considered to be an amplifier with infinite gain. Explain where you get the starting signal voltage from.

Ans. Due to the thermal energy, electrons are at random vibrations. These vibrations give rise to small potentials. All oscillators use positive feedback. The small voltage due to movement of electrons due to thermal energy is repeatedly amplified in the oscillator circuit and potential buildup occurs. This results in oscillations. To make these oscillations, dc biasing voltage is essential.

6. Explain how amplitude and frequency stability are improved in an oscillator.

Ans. Amplitude of oscillations build up infinitely if the loop gain is little above unity. So amplitude should be controlled, maintaining the oscillation. This is accomplished by using a parallel combination of condenser and resistance or a condenser with a resistance connected across the input.

The stability of frequency can be improved by

• enclosing the circuit in a constant temperature chamber.
• maintaining constant supply voltage V_CC by using zener diodes or IC voltage regulators.
• reducing problem of inter-element capacitances by connecting swamping capacitor across offending inter-element capacitor.
• by isolating the oscillator circuit from the source of mechanical vibrations. It will avoid the instability of frequency of oscillation due to mechanical vibration.

7. Why are L-C resonant circuits 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, heavy and expensive.

8. How does Clapp oscillator circuit differ from Colpitt’s oscillator circuit in construction?

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

9. Why Clapp oscillator is preferred over the 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 Clapp oscillator the effect of transistor parameters on the operation of the circuit is eliminated and the frequency stability is improved. That is why, Clapp oscillator is preferred over Colpitt’s oscillator.

10. How do’s Hartley oscillator differ from Colpitt’s oscillator in construction?

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

11. Why is the Armstrong oscillator less common than the Colpitts, Clapp and Hartley oscillators ?

Ans. Armstrong oscillators use transformers which occupy large space and are bulky and costly. Hence Armstrong oscillators are less common than other oscillators (Colpitts, Clapp or Hartley oscillators).

12. Why is crystal oscillator used in radio transmitter?

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

13. Normally, crystal controlled oscillators are not available beyond 10 MHz, why?

Ans. When alternating voltage is applied to a crystal, mechanical vibrations are set up. These vibrations having a natural resonant frequency dependent on the crystal. The crystal frequency depends upon the temperature and therefore, it is restricted due to temperature. As infinite Q is unattainable and also Q depends on frequency f, so frequency is controlled. That is why crystal controlled oscillators are not available beyond 10 MHz.

14. Why do you use three R-C sections in R-C phase shift oscillator?

Ans. In a phase shift oscillator, if the loading of the phase-shift network on the amplifier can be assumed to be negligible, a phase shift of 180° between the amplified output voltage v_out and the input voltage v_in is produced by the amplifier. The three section R-C phase-shift network is provided to produce an additional phase shift, which is a function of frequency and equals 180° at some frequency of operation. Thus, at the frequency of operation, the total shill from the gate around the circuit and back to the gate will be exactly zero.

15. Why 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.

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 a negative feedback, in proportion to the output current, in the circuit and thus ensure constant output over a range of frequencies.

17. Why is -ve feedback provided in Wien-bridge oscillator?

Ans. Negative feedback is provided in Wien-bridge oscillator to stabilize the amplitude of the output.