Oscillators Interview Question Answer

Important Questions of Oscillator Circuits

It includes meaning of tank circuit, feedback circuit, oscillator frequency, damped oscillation, undamped oscillation, Barkhausen criterion, classifications of oscillators, Hartley and colpitt oscillators, principle of rc phase shift oscillator, advantages of RC phase shift oscillator, Piezo electric crystal, frequency of the crystal and advantages and disadvantages of crystal oscillators.

Describe the function of oscillator or tank circuit?    

Oscillator

• The function of the oscillator circuit is to generate sinusoidal or non – sinusoidal oscillation of desired frequency.

Is the oscillator an energy converter circuit? True or false?       

• Yes, the oscillator is an energy converter circuit. 
• It receives DC energy at the input and converts it to AC energy of desired frequency. 
• The output frequency depends upon the constant of the oscillator circuit.

Which type of feedback circuit is used in the oscillator?    

• Positive feedback

Describe the effect of the value of inductance and capacitance on the oscillator frequency.         

Effect of inductance and capacitance

• The frequency of oscillation of the tank circuit is inversely proportional to the inductance and capacitance. 
• Lower value of inductance and capacitance, higher the oscillation frequency.
• The period of oscillation will be large if the higher value of capacitance is used. 
• The time requires for the complete one cycle of oscillation depends upon value of inductance. 
• Higher value of inductance means more time requires for completing one cycle.

Explain the following terms:  Damped oscillation and Un - damped oscillation           

Damped oscillation

• It is an electrical oscillation whose magnitude goes on decreasing with time is called as damped oscillation.

Un-damped oscillations

• It is an electrical oscillation whose magnitude remains constant with respect to time is called as undamped oscillation.

Why the amplitude of damped oscillations decreases with time?

• The amplitude of damped oscillations decreases with respected to time because some energy is lost and energy losses take place during each oscillation. 

How the damped oscillations are converted into undamped oscillations?   

• The oscillator circuit produces damped oscillation. 
• If some energy is supplied at correct time intervals to meet the energy losses by means of external electronics circuit, the damped oscillations convert into undamped oscillations.

What is Barkhausen criterion?

Barkhausen Criterion

The following relation is called as Barkhausen criterion:

         m_vA_v = 1

Where m_v = Feedback fraction ( 0 <  m_v  < 1 )

            A_v = Voltage gain of the amplifier without feedback

The above relation must be satisfied in order to produce continuously Un - damped oscillations.

Describe the significance of the Barkhausen Criterion.      

Significance of Barkhausen Criterion

The voltage gain of the positive feedback amplifier is infinite if the product of    

            m_v A_v = 1

                A_vf  =  1 / ( 1 –  m_vA_v )

• However, it is not possible to achieve infinite voltage gain. 
• It means that definite value of output voltage is achieved even if input signal is zero. 
• Therefore, once the oscillator circuit gets input it will generate oscillations at the output even if the input reduced to zero.

What are the essential conditions to produce oscillations?

The following are the essential conditions for producing oscillations

• Tank circuit
• Amplifier circuit and
• Feedback circuit

Describe the name of different types of oscillators.    

Classification of oscillators

• Colpitt oscillator
• Hartley oscillator
• Wien bridge oscillator
• Phase shift oscillator
• Crystal oscillator

Describe the disadvantages of Hartley and Colpitt oscillators.   

Disadvantages of Hartley and Colpitt oscillators

• The Hartley and Colpitt oscillators cannot be used for very low frequency because they become bulky and expensive due higher rating of inductance and capacitance. 
• They suffer from distorted waveforms and frequency in stability.

Describe the working Principle of RC phase shift oscillator.      

Principle of RC Phase shift oscillator

• The phase shift 180⁰ in the RC phase shift oscillator is produced by series combination of the RC circuit and other phase shift of 180⁰ is produced by the transistor circuit. (The RC phase shift oscillator consists of series combination of R and C, instead of conventional inductance and capacitance.  
• The value of R is varied such that voltage across R is lead by C by angle 60. There are three section of RC series circuit produces total phase difference of 180 degree.)

Describe the advantages of RC phase shift oscillator.

Advantages of RC phase shift oscillators

• It produces good stability.
• It can be used for very low frequency.
• Inductor or capacitor is not required therefore it is less bulky than Hartley and Colpitt oscillator

State the applications of Piezo electric crystal.  

Applications of Piezo electric crystal

• The Piezo electric crystal is useful where frequency is nearly constant with low tolerance.  
• The broadcasting stations have frequencies which are very close to each other. 
• The frequency of the crystal oscillator remains constant with respect to temperature changes. 
• Therefore, the Piezo electric crystal is more reliable than RC / LC oscillators.

In which type of oscillator both positive and negative feedback circuits are used? 

Wien bridge oscillator

Describe the disadvantages of Wien bridge oscillator.        

Disadvantages of Wien bridge oscillator

• It is not used for high frequency.
• It requires large number of components as well as two transistors.

What are the limitations of the LC and RC oscillators?     

Limitations of LC and RC oscillators

• The frequency dependent parameter L and C changes with variation of frequency and temperature. 
• The operating frequency changes if any component of the feedback network is changed.

Which materials have property of Piezo electric crystal?  

• The Quartz, Rochelle salt and tourmaline have property of Piezo electric crystal.

Describe the Principle of operation of Piezo electric crystal.       

Principle of Piezo - electric Crystal

• When alternating voltage is applied to Piezo electric crystal it will vibrate at the frequency of the applied voltage and vice versa (It means that when crystal is compressed or placed under mechanical strain, it produce alternating voltage.)

Which parameters greatly affect the frequency of the crystal?  

• The frequency of the crystal inversely proportional to thickness of the crystal. 
• Lesser the thickness of the crystal, greater the frequency and vice versa.

Describe the advantages and disadvantages of crystal oscillators.

Advantages

• High frequency stability
• High quality factor as compared to LC tank circuit

Disadvantages

• Only used for low power applications
• The frequency of oscillation cannot be changed 

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