Measurement System for Derivative Studies

A system for measuring I-V, dI/dV, and d2I/dV2 characteristics of two-terminal devices is described. An operational amplifier integrator is used to produce a sweep voltage. For derivative measurements the dc bias voltage is modulated by a constant ac voltage signal. Current is detected by a near-zero input impedance circuit with adjustable gain. A simple circuit utilizing an IGFET at zero drain voltage as a voltage controlled resistor compensates for harmonic content in the oscillator signal. It also enhances the detectability of weak harmonic signals in the presence of a strong fundamental signal by rejecting more than 90% of the strong signal. The system has been used to measure devices with impedances from a few ohms to over 105 Ω.