# Linear Integrated Circuit Questions and Answers – Closed-Loop Frequency Response and Circuit Stability

This set of Linear Integrated Circuit Multiple Choice Questions & Answers (MCQs) focuses on “Closed-Loop Frequency Response and Circuit Stability”.

1. Open loop configuration is not preferred in op-amps because
a) First break frequency is too large
b) First break frequency is very small
c) Second break frequency is too large
d) All of the mentioned

Explanation: The bandwidth of the op-amp is simply the first break frequency. As the value of bandwidth is very small, the open loop configuration is of little use or not preferred in op-amp.

2. Calculate the value of open loop frequency response curve at any point beyond break frequency in 741C op-amp?
a) 1000000Hz
b) 1000Hz
c) 10000000Hz
d) 100000Hz

Explanation: For a 741C op-amp the gain=200000 and the break frequency = 5Hz. Therefore, the unity gain bandwidth or the product of the coordinates (gain and frequency) of any point beyond the break frequency = 200000×5Hz =1MHz =1000000Hz.

3. Frequency response of µA709C for various closed loop gain is shown. Choose the curve that has wider bandwidth and high gain?
a) Curve 4
b) Curve 3
c) Curve 2
d) Curve 1

Explanation: To get a high gain and relatively wider bandwidth, the compensating components for curve 1 should be used. Curve 1 has the gain equal to 60dB and bandwidth nearly equivalent to 10MHz.

4. When does a system said to be stable?
a) Output reaches a minimum value at finite time
b) Output reaches a maximum value at any time
c) Output reaches a fixed value at finite time
d) Output reaches a fixed value at any time

Explanation: A circuit or a group of circuits connected together as a system is said to be stable, if its output reaches a fixed value in a finite time.

5. Why unstable systems are considered to be impractical?
a) None of the mentioned
b) Output decreases with time
c) Output reaches fixed value
d) Output increases with time

Explanation: A circuit is said to be unstable if its output increases with time instead of achieving fixed value. In fact the output keeps on increasing until the system breaks down. Therefore, unstable systems are impractical and need to be made stable.
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6. How is the criterion for the system determined?
a) Graphical method
b) Theoretical method
c) Analytical method
d) All of the mentioned

Explanation: The criterion for stability of system is tested practically using one of the above mentioned methods.

7. A standard block diagram of closed loop system composes of
a) Two blocks
b) Single block
c) Three blocks
d) None of the mentioned

Explanation: In closed loop system (control system) the standard form of representing a system is composed of two blocks.
(i) Forward block and
(ii) Feedback block.

8. “Transfer function” in control system refers to
a) Feedback block
b) Content of each block
c) Forward block
d) Input and output blocks

Explanation: The transfer function in control system refers the content of each block and it depends on the complexity of a system.

9. Which method is considered to be a graphical method in testing the system?
a) Bode plot
b) Routh-Hurwitz criteria
c) Circuit testing
d) None of the mentioned

Explanation: A graphical method used in determining the stability / testing the system is the bode plots. It is composed of magnitude versus frequency and phase angle versus frequency plots.

10. Measure taken to increase the bandwidth of an op-amp.
a) Increase the frequency for the configuration
b) Reduce the gain of the configuration
c) Closed loop configuration is used
d) Open loop configuration is used

Explanation: Closed loop configuration is preferred to increase the bandwidth of an op-amp.

11. Name the block connected in the feedback path
a) Feedback block
b) Forward block
c) Output block
d) Summation of feedback, forward and output block

Explanation: The block in the feedback path is referred to as the feedback blocks, which connects the block between the output signal and the feedback signal.

12. When the magnitude of (AoL )×( ß ) = 0dB, state the condition at which system become stable?
a) Phase angle is > -180o
b) Phase angle is < -180o
c) Phase angle is > +180o
d) Phase angle is < +180o

Explanation: Closed loop voltage gain AF = AoL/ (1+AoL×ß)
At lower frequency the contribution of AoL is zero. So, AoL×ß >0 and obviously AF < AoL and system is stable.

13. Mention the phase condition that leads to sustained oscillation
a) ∠-Aß =0
b) All of the mentioned
c) ∠-Aß = multiple of 2π
d) ∠Aß = odd multiple of π

Explanation: The circuit leads to oscillation if the characteristic equation (1+AoL×ß )=0
=> Loop gain, AoL×ß =1
Since AoL*ß is a complex quantity , the magnitude is |AoL×ß |=1 and the phase condition are ∠-Aß=0 (or multiple of 2π) or ∠Aß= π (or odd multiple of π).

14. Determine the state of the system a) Stable system
b) Unstable system
c) Casual system
d) Bounded system

Explanation: The given circuit is used in inverting mode and produce a phase shift of 180o. For two corner frequency, the maximum phase shift associated with gain AoL is -180o which makes total phase shift = 0 and for some value of ß, the magnitude of Aß becomes unity. In this case the amplifier begins to oscillate as both magnitude and phase conditions are satisfied. Thus, the circuit enter into verge of instability.

15. Find out the system stability when a system has three RC poles pairs
a) Attain stability at low frequency
b) Attain stability at high frequency
c) Attain instability at high frequency
d) Attain instability at low frequency

Explanation: For a system with three pole pairs, AoL have three corner frequencies and contribute to maximum of 270o phase shift, at this condition AoL×ß is negative and instability occurs at high frequencies.

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