Two useful definitions for RF amplifiers are available gain and maximum available gain. Available gain is measured with conjugate match at both input and output ports. Then Available gain = Available power at the output port/Available power from the source. This is the maximum gain obtainable from the amplifier. The maximum available gain ( MAG) is often used as a transistor or FET figure of merit. It is defined as the theoretical power gain of the device with its reverse transfer admittance set to zero. The source and load admittances are conjugately matched. The MAG = Absolute value(yf)**2/4gigo where gi and go are the real parts of the input and output admittances. yf is the forward transfer admittance.