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.