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Considerable difficulty is encountered in directly measuring the parameters of a transistor at very high frequencies. An approximate high-frequency equivalent circuit for a transistor is presented, and several techniques for measuring the alpha-cutoff frequency are discussed. An indirect technique is presented involving the measurement of the maximum frequency at which the transistor can oscillate, and it is shown that this leads to a simple and accurate determination of the alpha-cutoff frequency. This maximum frequency of oscillation can also be used to predict the approximate gain obtainable from a transistor at high frequencies, and the efficiency which may be realized from the transistor used as a high frequency oscillator.