The half-power point of an electronic amplifier stage is that frequency at which the output power has dropped to half of its mid-band value. That is a level of -3 dB. The half-power point is a commonly used specific definition of cutoff frequency, although not the only one.
Contents
This occurs when the output voltage has dropped by 1/√2 or 0.707 of the maximum output voltage (exact:
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.
3 dB
The half-power point is approximately 3 dB down, because
Using 3 dB rather than the correct value of 3.010... yields a power factor of
This is the same mathematical coincidence as
Antennas
The half-power point or 3 dB point of an antenna beam is the angle off boresight at which the antenna gain has fallen 3 dB below the peak.