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Online MCQ Assignment
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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

Answer: a [Reason:] The frequencies are called lower cut-off frequency and upper cut-off frequency because at these frequencies power of the signal becomes half of its original value. So 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) Iindependent to cut off frequency

Answer: a [Reason:] 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 were 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

Answer: b [Reason:] 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

Answer: a [Reason:] 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

Answer: a [Reason:] 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

Answer: a [Reason:] 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

Answer: b [Reason:] 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

Answer: c [Reason:] 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

Answer: b [Reason:] 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) 6dB
b) 10dB
c) 20dB
d) 4dB