What is P3dB?

The 3 dB Compression Point or P3dB is the power level at which the gain of an amplifier decreases by 3 dB from its ideal linear gain. This is similar to P1dB, where the output power level deviates from the ideal power level by 1 dB.

Let's assume an amplifier has a gain of 10 dB. This means, an input of 10 dBm will result in an output of 20 dBm (10 dBm + 10 dBm Gain). And an input of 20 dBm will result in an output of 30 dBm. However, every amplifier saturates at a particular power level and no longer behaves linearly. So the point at which the output power deviates from its linear value by 3 dB is known as the 3 dB Compression point. In the above example, lets assume this happens at 30 dBm, so an input power of 30 dBm would result in an output power of 37 dBm (Instead of 40 dBm), this power level is known as P3dB.

Once an amplifier reaches its P1dB it goes into compression and becomes a non-linear device, producing distortion, harmonics and intermodulation products. Amplifiers should always be operated below the compression point.