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A tunnel diode switching circuit is discussed which is useful as well as representative and yet is simple and amenable to detailed analysis. A stability diagram is given showing how the equilibrium states of the circuit change in character as the circuit element values and diode bias are varied. It is shown that when the circuit is arranged to be monostable but with three equilibrium states it exhibits a true threshold for triggering and is, therefore, an ideal pulse amplitude discriminator and pulse regenerator. The ultimate speed capability of this circuit is investigated. Current and voltage waveforms similar to those actually observed are obtained by numerical integration of the nonlinear circuit equations. The dependence of pulse height, width, and cycle time on inductance is shown, and a scaling diagram is given showing the inductance and resistance that should be used with any diode in order to have an optimum cycle time. Several available diodes with 10 to 20 mA peak current may be used with practical loads (about 10 ohms and 1 nH) at repetition periods of 0.3 to 0.5 ns, or pulse rates of 2 to 3 Gc/s. However, a cycle time of 60 ps or pulse rate greater than 10 Gc/s is predicted if a 2 ohm, 20 pH load could be combined with a 100 mA, 1.8 pF junction, values corresponding to the best available diodes. Since the latter values of resistance and inductance are awkwardly low, it is concluded that pulse rates of 2 to 3 Gc/s are practical, and that substantially higher rates are possible but require improved circuits or diodes.