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. 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,.
Amplifiers and filters
This occurs when the output voltage has dropped to 1/√2 or 0.707 of the maximum output voltage (exact: ) and the power has dropped by half (1/2 or 0.5) (exact: ). 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.