Design Project: Simple Component Curve-Tracer Circuit

The resistor’s specified value of 1 kΩ should make perfect sense when students see the voltage:current ratio for the vertical axis!

It is important for students to realize that the oscilloscope will not produce the desired display if configured for the normal “time-domain” display. For the circuit to be able to work, the curve tracer must provide the horizontal sweep, hence the dual inputs to the oscilloscope.

Though it may have been a while since you last discussed how to achieve precise (adjustable!) values of resistance from non-precision resistors, students should at least remember the general concept if not the exact implementation.

This circuit provides an excellent opportunity for students to discuss and review the common grounds of oscilloscope inputs, and why one of the oscilloscope’s inputs must be wired “backward” in order to yield the expected trace from lower-left to upper-right.

Dedicated curve tracer circuits typically use a sawtooth waveform (much like the waveform used to sweep an oscilloscope beam horizontally in the normal “time-domain” mode), so that the brightness of the trace is relatively even throughout. This is why a square-wave excitation would not work well: most of the trace would be extremely dim and hard to see, with the endpoints of the curve being the only bright spots!sawtotj