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A method for estimating the S-parameters of active circuits using hand analysis is introduced. This method involves the determination of S-parameters from the poles of voltage-gain transfer function. It is found that the information on the frequency responses of input/output return loss, input/output impedance, and reverse isolation is all hidden in the poles or equivalently in the denominator of the voltage-gain transfer function of a circuit system. The method has been applied to three commonly used RF circuit configurations and one fabricated CMOS wide-band amplifier to illustrate the usefulness of the proposed theory.