Half power point

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.

This occurs when the output voltage has dropped by 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.

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