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An experimental study of the small signal equivalent circuit of the lasing P/N junction itself is presented. The usual study examines the behavior of the packaged device through measurement of the dependence on dc current and on frequency of the modulus of the transfer function relating the stimulated power to the drive current. The present work features the study of the P/N junction itself through a new technique, the direct measurement of the modulus, and the argument of the ratio of the modulation of the stimulated power to the modulation of the spontaneous emission. The frequency modulation of the optical power is also studied, and the results compared with deductions from the present study of the spontaneous emission and from a published study of the amplitude and frequency transfer functions of a similar laser. All the studies support the new finding that the modulations of the junction voltage and the power are in phase with each other for frequencies up to the gigahertz range. The experimental values of the circuit elements show that the damping of the response is due to dissipation in the resistance representing the change in voltage with stimulated power. Comparison of the values of the circuit elements to the solutions of the multimode rate equations gives insight about the utility and limitations of existing formalism, in particular about the extent to which the formalism yields a single set of circuit elements which describes the transient response of all observables and about whether the observed responses are better modelled as those of a classical noise amplifier or a free running oscillator.