Design Project: Simple Component Curve-Tracer Circuit

Question 1

Question 1

What do each of the oscilloscope axes represent, in real-life terms (variables and units) when used in conjunction with this curve tracer circuit?

Answer

Horizontal axis: voltage applied across terminals of device under test (1 volt displayed = 1 volt).

Vertical axis: current through device under test (1 volt displayed = 1 mA)

Question 2

Question 2

What mode does the oscilloscope have to be set in, in order to use with this curve tracer circuit?

Answer

The oscilloscope needs to be set in the “X-Y” mode.

Question 3

Question 3

If quantitative accuracy is desired from this curve tracing circuit, the shunt resistor should be precisely 1 kΩ in size. Explain how a precise shunt resistance may be built without the use of precision components (i.e. a special 1 kΩ precision resistor).

Answer

Hint: there’s a saying in the electronics world that goes something like this: “Don’t make it precise, make it adjustable!”

Question 4

Question 4

When testing this circuit for the first time, the student connects a 1 kΩ resistor to the two test leads and gets this result from the oscilloscope display:

The student was actually expecting something that looked like this:

Upon inspection, nothing appears to be wrong with the wiring of the curve tracer circuit. Explain what the problem is so the student is able to achieve the expected results.

Answer

One of the oscilloscope’s two input channels must be inverted for the curve tracer to work as expected. Most oscilloscopes come equipped with an “invert” control on the second input channel that is used for this purpose.

Follow-up question: is the curve tracer circuit configured for an AC test or a DC test, based on the appearance of the oscilloscope trace?

Question 5

Question 5

Explain why the wave-shape of the excitation voltage is irrelevant to the operation of this curve tracing circuit. The transformer happens to output the same sine wave shape exhibited by the AC line power, but the circuit would work equally wall with a triangle wave, sawtooth wave, or badly distorted sine wave.

Answer

All that is needed is some sort of wave-shape that sweeps the oscilloscope beam to and fro. Since the relationship between vertical displacement and horizontal displacement on the oscilloscope’s trace is purely a function of the test component’s characteristic curve, we really don’t care what form of AC voltage excites it.

Design Project: Simple Component Curve-Tracer Circuit

Discrete Semiconductor Devices and Circuits

Worksheet Topics

Design Project: Simple Component Curve-Tracer Circuit

Discrete Semiconductor Devices and Circuits

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