What is P1dB?

When operating in the linear region, an amplifier ideally provides constant gain over a specific frequency range. For example, if an amplifier has a gain of 10 dB, a 1 dBm input will result in a 11 dBm output and a 10 dBm input will result in a 20 dBm output. In both cases the input is amplified by 10 dB. As the input power level increases, there comes a point where the output power of the amplifier no longer increases proportionally with gain. Instead, the gain begins to drop, and the amplifier enters compression.

The 1 dB compression point (P1dB) is the output power level at which the amplifier’s gain decreases by 1 dB from its small-signal (linear) gain. In other words, if the expected output (based on linear gain) is 20 dBm, but the actual measured output is 19 dBm, the amplifier has reached its 1 dB compression point. At this point, the amplifier is no longer behaving linearly and begins producing harmonic distortion and intermodulation products.

As the input power level continues to increases, the amplifier output power starts to saturate and the amplifier enters the compression region.

P1dB Illustration - Input power Vs Output power of an Amplifier

Amplifiers should typically be operated several dB below P1dB to ensure linear performance. Operating near or above P1dB results in distortion, degraded signal quality, and regulatory compliance issues.

P1dB is one of the most important specifications for RF and microwave amplifiers because it defines the usable linear output power range.