# Analog Circuits Questions and Answers – General Frequency Consideration

This set of Analog Circuits Multiple Choice Questions & Answers (MCQs) focuses on “General Frequency Consideration”.

1. The frequency f1 and f2 from the below picture are respectively called ___________

a) lower cut-off frequency and upper cut-off frequency
b) upper cut-off frequency and lower cut-off frequency
c) left frequency, right frequency
d) there is no specific name

Explanation: The frequencies are called lower cut-off frequency and upper cut-off frequency. At these frequencies power of the signal becomes half of its original value, 3dB less than the maximum and they are also called 3dB cut-off frequencies.

2. Bandwidth of amplifier is __________
a) Difference between upper cut-off frequency and lower cut-off frequency
b) Sum of upper cut-off frequency and lower cut-off frequency
c) Average of upper cut-off frequency and lower cut-off frequency
d) Independent to cut off frequency

Explanation: 3dB bandwidth of an amplifier is the difference between upper cut-off frequency and lower cut-off frequency. The unity gain bandwidth is the difference between frequencies where gain is 1.

3. At 3dB cut-off frequency the voltage gain will be __________
a) 100% of maximum gain
b) 70.7% of maximum gain
c) 80.7% of maximum gain
d) 47.5% of maximum gain

Explanation: 3dB cut-off frequency is the frequency at which the power becomes half of its maximum value. That is the voltage gain becomes 0.707 times maximum voltage gain.

4. At 3dB cut-off frequencies power will be __________
a) Half of maximum value
b) Quarter of maximum value
c) 70.7% of maximum value
d) Same as maximum value

Explanation: 3dB cut-off frequency is the frequency at which the power becomes half of its maximum value. That is the voltage gain becomes 0.707 times maximum voltage gain. The importance of 3dB points is that for a human ear, it will not notice the fall in power of the signal up to 50% of maximum power.

5. A voltage amplifier has a voltage gain of 100. What will be gain at 3dB cut-off frequencies?
a) 70.7
b) 80.7
c) 45.7
d) 50

Explanation: 3dB cut-off frequency is the frequency at which the power becomes half of its maximum value. That is the voltage gain becomes 0.707 times maximum voltage gain. Therefore if voltage gain is 100 then gain at 3dB frequencies will be
100 x 0.707 = 70.7.

6. What is the roll-off rate of single order filter?
b) 5dB/octave
d) 10dB/octave

Explanation: At lower and higher frequencies the gain is decreasing. This fall or decrease in gain is known as roll-off rate. It is commonly specified in dB/octave or dB/decade. For a single order filter, the roll-off rate is 6dB/octave or 20dB/decade.

7. -6dB is equivalent to __________ power gain.
a) 0.5
b) 0.25
c) 0.75
d) 0.8

Explanation: The relation between dB and power gain is
dB = 10 log(power gain)
That is power gain for -6dB is
10(-6 /10) = 0.251.

8. Voltage gain of 1,00,000 is equivalent to __________
a) 10dB
b) 1000dB
c) 100dB
d) 50dB

Explanation: dB = 20 log(voltage gain)
Therefor 100000 voltage gain is equivalent to
20 log(1,00,000)dB = 100 dB.

9. If the output power from an audio amplifier is measured at 100W when the signal frequency is 1kHz, and 1W when the signal frequency is 10kHz. Calculate the dB change in power.
a) -10dB
b) -20dB
c) -30dB
d) 15dB

Explanation: The initial power gain in dB = 10 log (output power)
= 10 log(100) = 20dB
The final power gain in dB = 10 log(output power)
= 10 log(1) = 0 dB
So change in power = final power – initial power
= 0-20 = -20dB.

10. If an electronic system produces a 48mV output voltage when a 12mV signal is applied, calculate the decibel value of the systems output voltage gain.
a) 12dB
b) 6dB
c) 20dB
d) 4dB

Explanation: Gain of the system is = output voltage/input voltage
= 48/12 = 4
Gain in dB = 20 log(voltage gain)
= 20 log(4) = 12.04 dB.

Sanfoundry Global Education & Learning Series – Analog Circuits.

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