Piecewise-linear modeling of I-V characteristics with SPICE

This paper presents a technique for piecewise-linear modeling of arbitrary nonlinear I-V characteristics with SPICE. In particular, I-V characteristics (including those exhibiting negative resistance) that lend themselves to piecewise-linear approximation are easily modeled using six elemental building blocks; three for voltage-controlled I-V characteristics and three for current-controlled I-V characteristics. The elemental building blocks are implemented with resistor, diode, independent voltage source, and independent current source SPICE primitives. Two of the elemental building blocks use the ability of SPICE to accept negative values for the resistance and diode saturation current parameters. The technique is applied to model a unijunction transistor and a tunnel diode. Two negative resistance oscillator examples which use these models are included; a current-controlled negative resistance sinusoidal oscillator (unijunction transistor) and a voltage-controlled negative resistance relaxation oscillator (tunnel diode). These examples have been used to teach the fundamentals of negative resistance oscillators and nonlinear effects to sophomores and juniors