Important Short Questions and Answers: Oscillators

OSCILLATORS

1. Mention two essential conditions for a circuit to maintain oscillations[

The conditions for oscillator to produce oscillation are given by Barkhausan criterion.  They are:

i)             The total phase shift produced by the circuit should be 360^o or 0^o

ii)          The Magnitude of loop gain must be greater than or equal to 1 (ie)|Aβ|≥1

2.           What is the major disadvantage of a Twin-T oscillator?

Twin –T oscillator is operated only at one frequency.

3. Differentiate oscillator from amplifier.

Oscillators

1. They are self-generating circuits. They generate waveforms like sine, square and triangular waveforms of their own. Without having input signal.

2. It have infinite gain

3. Oscillator uses positive feedback.

Amplifiers

1. They are not self-generating circuits. They need a signal at the input and they just increase the level of the input waveform.

2. It have finite gain

3. Amplifier uses negative feedback.

4. State Barkhausen criterion for sustained oscillation. What will happen to the oscillation if the magnitude of the loop gain is greater than unity?

The conditions for oscillator to produce oscillation are given by Barkhausan criterion.  They are :

i)             The total phase shift produced by the circuit should be 360^o or 0^o

ii)          The Magnitude of loop gain must be greater than or equal to 1 (ie)|Aβ|≥1

In practice loop gain is kept slightily greater than unity to ensure that oscillator work even if there is a slight change in the circuit parameters.

5.           Why an LC tank circuit does not produce sustained oscillations. How can this can be overcome?

We know that the inductor coil has some resistance and dielectric material of the capacitor has some leakage.so small part of the originally imparted energy is used to overcome these losses. As a result, the amplitude of oscillating current goes on decreasing and becomes zero when all energy is consumed as losses. So a LC tank circuit does not produce sustained oscillations.

To maintain sustained oscillations, energy must be supplied to the circuit at the same rate at which it is dissipated. In an oscillator, the function of transistor and power supply source is to feed energy to the circuit to overcome the losses at right time.

6. Draw the electrical equivalent circuit of crystal.  and mention its series and parallel resonance frequency.

The crystal actually behaves as aseries RLC circuit in parallel with C_M. . Because of presence of C_M , the crystal has two resonant frequencies.

One of these is the series resonant frequency f_s.In this case impedance is very low.

·              The other is parallel resonance frequency f_p. In this case impedance is very high.

7. What are the advantages and disadvantages of RC phase shift oscillators?

 Advantages:

i.             It is best suited for generating fixed frequency signals in the audio frequency range.

ii.           It requires no transformer or inductor, hence less bulky.

iii.        Simple Circuit.

*Pure sine wave output is possible.

Disadvantages:

i)             It requires a high β transistor to overcome losses in the network.

ii)          These oscillators are not suitable for high frequency operation.

iii)        Frequency of oscillation can not be changed easily. To change the frequency of oscillation, the three capacitor or resistors should be changed simultaneously. This is inconvenient.

8. What is the necessary condition for a Wien bridge oscillator circuit to have sustained oscillations?

Then for oscillations to occur in a Wien Bridge Oscillator circuit the following conditions must apply.

ü   With no input signal the Wien Bridge Oscillator produces output oscillations.

ü   The Wien Bridge Oscillator can produce a large range of frequencies.

ü   The Voltage gain of the amplifier must be at least 3.

ü   The network can be used with a Non-inverting amplifier.

ü   The input resistance of the amplifier must be high compared to R so that the RC network is not overloaded and alter the required conditions.

ü   The output resistance of the amplifier must be low so that the effect of external loading is minimised.

9.           Define piezoelectric effect.

The piezo electric Crystals exhibit a property that if a mechanical stress is applied across one face the electric potential is developed across opposite face and viceversa. This phenomenon is called piezo electric effect.

10. What is the principle behind operation of a crystal oscillator?

The principle behind the operation of crystal is piezoelectric effect. According to this effect, if a mechanical stress is applied across one face the electric potential is developed across opposite face and viceversa.

11. Draw an oscillator circuit with feedback network given below.

12. What are the advantages and disadvantages of wein bridge oscillators? Advantages:

1.           Provides a stable low distortion sinusoidal output over a wide range of frequency.

2.           The frequency range can be selected simply by using decade resistance boxes.

3.           The frequency of oscillation can be easily varied by varying capacitances C₁ and C₂ simultaneously. The overall gain is high because of two transistors.

Disadvantages :

1.           The circuit needs two transistors and a large number of other components.

2.           The maximum frequency output is limited because of amplitude and the phase-shift characteristics of amplifier.

13.      A weinbridge oscillator is used for operations at 9KHz. If the value of resistance R is 100KΩ, what is the value of C required?

14. A weinbridge oscillator is used for operations at 10KHz. If the value of resistance R is 100KΩ, what is the value of C required?

15.      A tuned collector oscillator in a radio receiver has a fixed inductance of 60µH and has to be tunable over the frequency band of 400KHz to 1200KHz. Find the range of variable capacitor to be used.

16.      Draw the feedback circuit of a colpitts oscillator. Obtain the value of the equivalent series capacitance required if it uses a L of 100mH and is to oscillate at 40KHz.

17.      In a Hartley oscillator if L₁=0.2mH,L₂=0.3mH and C=0.003µF. calculate the frequency of its oscillations.

18.      In an RC phase shift oscillator, if its frequency of oscillation is 955Hz and R₁=R₂=R₃=680KΩ.Find the value of capacitors.

19.      In an RC phase shift oscillator, if R₁=R₂=R₃=200KΩ and C₁=C₂=C₃=100pF. Find the frequency of the oscillator.

20.      A crystal has the following parameters L=0.5H, C=0.05pF and mounting capacitance is 2pF. Calculate its series and parallel resonating frequencies.

21.      Calculate the frequency of oscillation for the clap oscillator with C₁=0.1µF, C ₂=1µF, C₁=100pF and L=470µH.