# Half-power point

(Redirected from 3dB-point)

The half-power point is the frequency at which the output power has dropped to half of its peak value; that is, at a level of -3 dB.[1] The half-power point is a commonly used definition for the cutoff frequency and can be used in a variety of contexts, including the characterization of electronic amplifiers , optical filters,[2].

## Amplifiers and filters

This occurs when the output voltage has dropped to 1/√2 or 0.707 of the maximum output voltage (exact: ${\displaystyle 20\log _{10}\left({\tfrac {1}{\sqrt {2}}}\right)\approx -3.0103\,\mathrm {dB} }$) and the power has dropped by half (1/2 or 0.5) (exact: ${\displaystyle 10\log _{10}\left({\tfrac {1}{2}}\right)\approx -3.0103\,\mathrm {dB} }$). A bandpass amplifier will have two half-power points, whilst a low pass amplifier will have only one. A high pass amplifier stage will have only the lower half-power point.

The bandwidth of an amplifier is usually defined as the difference between the lower and upper half-power points. This is therefore also known as the 3 dB bandwidth.

## Antennas

The half-power point of an antenna beam is the angle off boresight at which the antenna gain has fallen 3 dB below the peak. The angle between the 3 dB points is known as the beamwidth.[3]