Linear Integrated Circuit Multiple Choice Questions on “Basic Principles of Sine Wave Oscillator – 2”.

1. What will be the phase shift of feedback circuit in RC phase shift oscillator?

A. 360^{o} phase shift

B. 180^{o} phase shift

C. 90^{o} phase shift

D. 60^{o} phase shift

Answer: B

Clarification: The RC feedback network provide 180^{o} phase shift and amplifier used in RC phase shift oscillator provide 180^{o} phase shift (op-amp is used in the inverting mode) to obtain a total phase shift of 360^{o}.

2. How many RC stages are used in the RC phase shift oscillator?

A. Six

B. Two

C. Four

D. Three

Answer: D

Clarification: The RC stage forms the feedback network of oscillator. It consist of three identical RC stages, each of 60^{o} phase shift so as to provide a total phase shift of 180^{o}.

3. Calculate the frequency of oscillation for RC phase shift oscillator having the value of R and C as 35Ω and 3.7µF respectively.

A. 1230 Hz

B. 204 Hz

C. 502Hz

D. 673 Hz

Answer: C

Clarification: The frequency of oscillation of RC phase shift oscillator is,

f_{o}=1/(2πRC√6) = 1/(2×3.14×√6×3.7µF×35Ω)

=> f_{o}= 1/ 1.9921×10^{-3} = 502Hz.

4. What must be done to ensure that oscillation will not die out in RC phase shift oscillator?

A. Gain of amplifier is kept greater than 29

B. Gain of amplifier is kept greater than 1

C. Gain of amplifier is kept less than 29

D. Gain of amplifier is kept less than 1

Answer: A

Clarification: For a sustained oscillation in RC phase shift oscillator the gain of the inverting op-amp should be at least 29. Therefore, gain is kept greater than 29 to ensure the variation in circuit parameter will not make |Aß|<1, otherwise oscillation will die out.

5. Calculate the feedback voltage from phase shit network.

A. V_{f} = ( V_{o}R^{3}S^{3}C^{3}) / (1+ 6SRC+5S^{2}C^{2}R^{2}+S^{3}R^{3}C^{3})

B. V_{f} = ( V_{o}R^{3}S^{3}C^{3}) / (1+ 5SRC+6S^{2}C^{2}R^{2}+S^{3}R^{3}C^{3})

C. V_{f} = ( V_{o}R^{3}S^{3}C^{3}) / (1+ 5SRC+5S^{2}C^{2}R^{2}+S^{3}R^{3}C^{3})

D. V_{f} = ( V_{o}R^{2}S^{3}C^{3}) / (1+ 6SRC+5S^{2}C^{2}R^{2}+S^{3}R^{3}C^{3})

Answer: B

Clarification: Applying KVL equation to the circuit, we get

=> I_{1}(R+(1/SC.)-I_{2}=V_{o} -> Equ1

=> -I_{1}R+ I_{2}(2R+(1/SC.)- I_{3}R=0 -> Equ2

=> 0- I_{2}R+ I_{3}(2R+(1/SC.=6 -> Equ3

WKT, V_{f} = I_{3}× 2R,

Solving Equ 1, 2, and 3 for I_{3}

=> I_{3}= ( V_{o}R^{2}S^{3}C^{3})/ (1+ 5SRC+6S^{2}C^{2}R^{2}+S^{3}R^{3}C^{3})

=> V_{f} = I_{3}× 2R

=( V_{o}R^{3}S^{3}C^{3}) / (1+ 5SRC+6S^{2}C^{2}R^{2}+S^{3}R^{3}C^{3}).

6. Which type of op-amp is avoided for high frequencies?

A. LM318

B. Op-amp 741

C. LF 351

D. None of the mentioned

Answer: B

Clarification: Op-amp741 is generally used for low frequencies < 1 kHz.

7. Find out the constant values of α and ß in phase shift oscillator.

A. α = √6, ß = -1/29

B. α = 6, ß = -1/29

C. α = √6, ß = 1/29

D. α = 6, ß = -1/29

Answer: A

Clarification: From phase shift network, we obtain

ß= 1/(1-5 α^{2})+j α(6- α^{2}) -> Equ1

For Aß=1, ß should be real and the imaginary terms must be zero

α(6- α^{2}) =0

=> α=√6

Now substituting α^{2}=6 in Equ 1, we get

ß=-1/29 (Negative sign indicates that the feedback network produces a phase shift of 180^{o}).

8. A phase shift oscillator is designed to oscillate at 155Hz. Determine the value of R_{f}. (Take C=0.30µF)

A. 399Ω

B. 3.98MΩ

C. 13.9kΩ

D. 403kΩ

Answer: D

Clarification: R = 1/(2πC√6×f_{o})

=> R= 1/7.153×10^{-4}= 1398=13.9kΩ.

9. The value of feedback resistor in phase shift oscillator is 180kΩ. Find its input resistance?

A. 52kΩ

B. 151kΩ

C. 209kΩ

D. 6.2kΩ

Answer: C

Clarification: To obtain sustained oscillation in phase shift oscillator.

=> |A|=29 or |R_{f} / R_{1}|=29

=> R_{1}|= R_{f} / 29 = 180kΩ/29= 6.21kΩ,

10. Determine the frequency of oscillation (f_{o}) in phase shift oscillator?

A. f_{o} = √6/ωRC

B. f_{o} = 0.56/ωRC

C. f_{o} = 0.065/ωRC

D. f_{o} = 6/ωRC

Answer: C

Clarification: The frequency of oscillation of phase shift oscillator is given as

f_{o} = 1/(2π×RC×√6) = 1/15.38×RC

=> f_{o} = 0.065/RC.

11. The condition for zero phase shift in wein bridge oscillator is achieved by

A. Connecting feedback to non-inverting input terminal of op-amp

B. Balancing the bridge

C. Applying parallel combination of RC to the feedback network

D. All of the mentioned

Answer: B

Clarification: In wein bridge oscillator, the feedback signal in the circuit is connected to the non-inverting input of op-amp. So, feedback network does not provide any feedback and the condition of zero phase shift around the circuit is achieved by balancing the bridge.