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The stability criteria of two-ports are briefly reviewed, and the activity of two-ports is then discussed at some length. It is found that the real power available from an absolutely stable two-port for a fixed input excitation and passive load cannot exceed a maximum value. This maximum value is only obtained when the two-port is unilateral and has its parameters on the stability boundary. A "figure of merit" for active two-ports can now be defined in terms of "maximum absolutely stable power gain."