Interview Questions on RC Oscillator

Jan 11 • Resources • 4679 Views • 4 Comments on Interview Questions on RC Oscillator

Q1. What is a phase shift oscillator?
Ans. Phase shift oscillator is a sine wave generator that uses a resistive-capacitive(RC) network as its frequency determining device. It consists of an inverting amplifier element such as a transistor or op amp with its output fed back to its input through a phase shift network consisting of resistors and capacitors.The feedback network ‘shifts’ the phase of the amplifier output by 180 degrees at the oscillation frequency to give positive feedback.

Q2. What are the applications of RC phase shift oscillator?
Ans. RC phase shift oscillators are mostly used at audio frequencies. Many electronic organs makes use of this oscillator such as electronic musical instruments like pianos.

These are also used in other equipment that emits beeps. Example, many GPS unit beeps when it performs an action.
This is also used in voice synthesis.

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Q3. Why an R.C. phase shift oscillator not used in high frequency?
Ans. As the frequency goes up, the value of R and C goes down. As the value of R and C go down, the sensitivity the frequency to changes in R and C; to temperature coefficient, to tolerance, and to parasitics; goes up. At a certain point it is next to impossible to construct an RC oscillator because we cannot stablize the component’s values to keep a constant frequency.

Q4. What is the frequency of RC phase shift oscillator?
Ans. To find the resonant frequency (fr) of an RC phase shift oscillator, use the following formula:
fr=1/2πRC√2n
The calculation of resonant frequency of RC phase shift oscillator is complex. The calculations are greatly simplified by setting all the resistors (except the negative feedback resistor) and all the capacitors to the same values. In the diagram, if R1 = R2 = R3 = R, and C1 = C2 = C3 = C, then:
f(oscillation)=1/2πRC√6

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Q5. What phase shift does RC phase shift oscillator requires?
Ans. To obtain the regenerative feedback in the phase-shift oscillator, we need a phase shift of 180 degrees between the output and the input signal. An RC network consisting of three RC sections provides the proper feedback and phase inversion to provide this regenerative feedback. Each section shifts the feedback signal 60 degrees in phase.

Q6. How is phase angle determined in RC phase shift oscillator?
Ans. Since the impedance of an RC network is capacitive, the current flowing through it leads the applied voltage by a specific phase angle. The phase angle is determined by the amount of resistance and capacitance of the RC section. If the capacitance is a fixed value, a change in the resistance value will change the phase angle. If the resistance could be changed to zero, we could get a maximum phase angle of 90 degrees.

Q7. Is it possible to get a maximum phase angle of 90 degrees in RC phase shift oscillator?
Ans. If the resistance could be changed to zero, we could get a maximum phase angle of 90 degrees. But since a voltage cannot be developed across zero resistance, a 90-degree phase shift is not possible. With a small value of resistance, however, the phase angle or phase shift is less than 90 degrees.

Q8. Give a brief description of RC phase shift oscillator design?
Ans. In the phase-shift oscillator, therefore, at least three RC sections are needed to give the required 180-degree phase shift for regenerative feedback. The values of resistance and capacitance are generally chosen so that each section provides about a 60-degree phase shift. Resistors RB, RF, and RC provide base and collector bias. Capacitor C E bypasses ac variations around the emitter resistor R E. Capacitors C1, C2, and C3 and resistors R1, R2, and R B form the feedback and phase-shifting network. Resistor R2 is variable for fine tuning to compensate for any small changes in value of the other components of the phase-shifting network. When power is applied to the circuit, oscillations are started by any random noise (random electrical variations generated internally in electronic components). A change in the flow of base current results in an amplified change in collector current which is phase-shifted the 180 degrees. When the signal is returned to the base, it has been shifted 180 degrees by the action of the RC network, making the circuit regenerative.

Q9. What is oscillation criterion?
Ans. Oscillation criterion is used in most of the oscillators. And in general terms it is written as
R(feedback)=29.R

Q10. Why there is only 60 degree phase shift in RC phase shift oscillator?
Ans. In order for an oscillator to be stable, its total phase shift must be 180 degrees. The most common design of an RC phase shift oscillator consists of three identical RC stages in series, which means that each stage contributes 60 degrees of phase shift.

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