Half power point
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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 level.
This occurs when the output voltage has dropped by 3dB and the power has dropped by half. A bandpass amplifier will have 2 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 BW 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 BW.
[edit] Antennas
The half power point or 3dB point of an antenna beam is the angle off boresight at which the antenna gain has fallen 3dB below the peak.
