Automatic Plotting Device for the Second Derivative of Langmuir Probe Curves

Abstract

An electronic device for automatically plotting the second derivative of probe curves is described in detail. A small sinusoidal signal is superimposed on the dc probe voltage which is swept manually. The second harmonic in the probe current is filtered, amplified by a low noise cascode amplifier, rectified by a phase-true rectifier, and plotted by an X-Y recorder. The components, the critical points in their wiring, and the performance of the system are described. A quantitative comparison is made with related devices. The system described here is somewhat simpler than the competing ones and has the advantage of being applicable to the first and the third derivatives as well.