Test: General Frequency Consideration - Electronics and Communication Engineering (ECE) MCQ

 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.

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.

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.

 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.

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.

 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.

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.

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

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.

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.

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.