Significance of 3dB Frequency

• The 3dB frequency is the frequency at which the power is reduced by one half.
• Another way of saying it is the frequency at which the voltage is reduced to 70.7% of its full
value.
• An audio amplifier will have a lower and an upper 3dB frequency, so will a bandpass filter.
• A high‐pass filter will have only one 3dB frequency, that of the lowest frequency for which it is
much use; and conversely for a low‐pass filter.
• A power ratio of 2 is equal to 3 dB. ( take the log of 2 check above equation) Conversely, a
power ratio of 1/2 is equal to ‐3 dB. (NOTE: The ‐3 dB point is often referred to as a "half‐power
point," as when describing, say, a frequency response curve)
• A voltage or current ratio of 2 is equal to 6 dB. Conversely, a voltage or current ratio of 1/2 is
equal to ‐6 dB. Since power is E^2/ R.
• The 3dB point, or 3dB frequency, is the point at which the signal has been attenuated by 3dB (in
a bandpass filter). This is generally considered the point for determining the filter's bandwidth.
The bandwidth is defined as the difference between the upper and lower 3dB points.
• For example, if you have a filter which has a lower 3dB point of 720 MHz, and an upper 3dB
point of 730 MHz, you have a bandwidth of 10 MHz.