Analog Circuits Questions and Answers – Frequency Filters – 2

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This set of Analog Circuits Question Paper focuses on “Frequency Filters – 2”.

1. Find which one is a sallen-key topology.
a) analog-circuits-question-papers-q1a
b) analog-circuits-question-papers-q1b
c) analog-circuits-question-papers-q1c
d) analog-circuits-question-papers-q1d
View Answer

Answer: a
Explanation: Sallen-key topology is an electronic filter topology used to implement second-order active filters. It requires a single op-amp for gain control and 4 passive RC components to accomplish the tuning. These are also called positive feedback circuits since the output flows back into non-inverting terminal too.
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2. A first-order Butterworth low pass filter is an interconnection of ____________ and ___________
a) Single low pass RC circuit, Voltage follower
b) Low pass RC circuit, Band-pass RC circuit
c) Low pass RC circuit, LC feedback
d) Single low pass RC circuit, Power amplifier
View Answer

Answer: a
Explanation: A first-order Butterworth LPF is an interconnection of a single low pass RC circuit and a voltage follower.
analog-circuits-question-papers-q2

3. Considering a second-order Butterworth LPF using an op-amp, where damping factor = 1.414 find the value of R3, given the following circuit. (R2=5kΩ)
analog-circuits-question-papers-q3
a) 6.71 kΩ
b) 4.22 kΩ
c) 2.93 kΩ
d) 5 kΩ
View Answer

Answer: c
Explanation: Maximum gain, A = 3 – α = 1.586
For the above circuit, the gain is 1 + R3/R2 = 1.586
R3/R2 = 0.586
R3 = 2.93 kΩ.

4. For the following circuit, R2=220kΩ, R1=10kΩ, find the maximum gain and the phase shift at the cutoff frequency.
analog-circuits-question-papers-q4
a) Maximum gain = 22, Phase shift = 45°
b) Maximum gain = 23, Phase shift = 135°
c) Maximum gain = 22, Phase shift = 225°
d) Maximum gain = 23, Phase shift = 45°
View Answer

Answer: d
Explanation: Above is a high pass filter, with maximum gain = 1+R2/R1 = 23
Phase shift = tan-1(1) = 45°.
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5. Find the cutoff frequency for the following circuit. (R1=R2=20kΩ)
analog-circuits-question-papers-q5
a) 25 kHz
b) 3987 rad/sec
c) 2500 Hz
d) 25000 rad/sec
View Answer

Answer: d
Explanation: Cutoff frequency ωC = 1/RC
ωC = 1000000/20×2 = 25000 rad/s.

6. Given a second-order Butterworth HPF, find the maximum gain magnitude.
a) 1.586 dB
b) 4 dB
c) 2.66 dB
d) 1 dB
View Answer

Answer: b
Explanation: For a second-order Butterworth HPF, the maximum gain AMAX = 3 – α.
α is the damping factor = 1.414
AMAX = 1.586
In dB, the gain is 20 log(1.586) = 4dB.

7. Consider the following circuits.
analog-circuits-question-papers-q7
analog-circuits-question-papers-q7a
If a band-pass filter is created by using the above two circuits in cascade, find the correct relation from the choices below.
a) R4C2 >> R1C1
b) R4C2 << R1C1
c) R6R5 = R2C3
d) R4C2 = R1C1
View Answer

Answer: a
Explanation: A band-pass filter can be created as an interconnection of an LPF and an HPF. For this to work, the LPF cut-off frequency should be much greater than the HPF cut-off frequency.
Thus, 1/2π R1C1 >> 1/2π R4C2
Thus R4C2 >> R1C1.
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8. Which of these is incorrect for a band-stop filter?
a) An adder is required when designing it using LPF and HPF
b) LPF and HPF are connected in parallel
c) The HPF cut-off frequency should be much higher than LPF cut-off frequency
d) The LPF and HPF are connected in series
View Answer

Answer: d
Explanation: When designing a band-stop filter, a HPF and LPF are connected in parallel, and their output goes into the input of an adder to get the desired the output. For correct output, the cut-off frequency of HPF should be much higher than that of the LPF.

9. Which of these is wrong for an all-pass filter?
a) It is used for phase equalization in a communication system
b) It is used in landline communication
c) Its phase shift is -2tan-1RC, between 0 to -180°
d) It can be made using a single op-amp
View Answer

Answer: c
Explanation: An all-pass filter passes all frequencies but provides a different phase shift to each frequency present in the circuit. It is used for phase equalization or delay equalization and is used in landline communication. Following is a circuit of an all-pass filter using a single op-amp.
analog-circuits-question-papers-q9
However, its phase shift is -2tan-1(ωRC)

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He is Linux Kernel Developer & SAN Architect and is passionate about competency developments in these areas. He lives in Bangalore and delivers focused training sessions to IT professionals in Linux Kernel, Linux Debugging, Linux Device Drivers, Linux Networking, Linux Storage, Advanced C Programming, SAN Storage Technologies, SCSI Internals & Storage Protocols such as iSCSI & Fiber Channel. Stay connected with him @ LinkedIn