The popular emitter-coupled multivibrator and its MOS variant are analyzed using the classical discontinuity theory. Accurate formulas for predicting their relaxation oscillation are obtained solving the nonlinear circuit equation, which is derived finding the voltage-current characteristic of the nonlinearity connected to the timing capacitor. Then, using a small-signal equivalent circuit accounting for the intrinsic capacitances of the devices, we investigate the high-frequency behavior of these circuits finding the oscillation start-up condition. We show that they can exhibit a completely different behavior, similar to a series LC-tuned oscillator, without using an inductor. Numerical simulations are performed showing that the presented formulas are more accurate than the existing ones and that both behaviors are possible