The available power gain, , is defined as the ratio of the power available from the
amplifier to the power available from the source , i.e., the load is
conjugately matched to the output port of the amplifier, ,

For a given set of S-parameter, depends only on . Hence is very useful for investigating the effect of the source on the amplifier power gain. Note that we can find using the equation of by replacing with and with .

In general, and . Hence is the smallest in value. If both the input and output ports are conjugately matched then , where is the maximum power gain.