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The behavior of a general free-running multivibrator circuit is investigated. The circuit contains two three-terminal active devices drawing control current and subject to saturation, and various passive circuit elements, some of which are parasitic. The operation of the multivibrator is described by a system of nonlinear equations which is a higher dimensional generalization of the van der Pol relaxation oscillator equations. The methods of singular perturbation theory are applied to show under what circumstances the multivibrator will, and under what circumstances it will not, oscillate. The period and waveform of the oscillations are also obtained.