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The purpose of this paper is three-fold. First, the subject of maximum available power gain at high frequencies is discussed briefly. Also, maximum gain for a four-terminal network driven by a generator having a purely resistive internal impedance is calculated in terms of small-signal parameters of the network. Then a theoretical model of a junction transistor comprising the ideal one-dimensional model plus a base impedance, which may be complex and frequency-dependent as in the case of grown-junction transistors, is introduced for the network to obtain an expression for maximum available power gain in terms of fundamental device parameters. Experimental results, which are given for a number of grown-junction transistors, tend to confirm the theoretical expression. Finally, an idealized model of a grown-junction transistor is introduced, and theoretical power gain is calculated in terms of physical parameters. Such calculations show, for example, that 30 db of gain should be available at 5 mc and that such transistors should be capable of oscillating up to several hundred mc.