Small Signal Gain vs Large Signal Gain in an Amplifier

If we draw the Input power vs Output power for an amplifier on a graph, we can see that for a specific frequency range the output power of the amplifier is proportional to the input power (initially) and we get a straight line (Linear Relationship) i.e. the amplifier provides a constant gain, which is independent of power level. The gain in this linear region is called small signal gain.

But as the input power level increases, the amplifier approaches saturation i.e., the output power of the amplifier no longer increases by the specified gain value and starts to saturate. The gain starts dropping and drops till the amplifier reaches saturation i.e. as input power increase beyond this point, output power remains constant. The amplifier no longer operates linearly, and the gain of the amplifier in this this non-linear region is called large signal gain.

Many power amplifier datasheets specify both these values. So based on the power level at which you plan to use the amplifier, you can decide which type of gain would be relevant. For example, if you plan to use the device at a lower power which falls in the linear region, then the gain you will get is the small signal gain, however if you plan to use the device at a higher power level around the compression point (P1dB) then the large signal gain would be relevant.