Testing a device to determine if it is Functional
It can be useful not only to know whether a device is func-
tional, but if it has failed, whether it is a short circuit, open circuit, or some other state.
Testing a device to determine it meets its Specifications
One can quickly determine whether a device meets
certain performance requirements, such as current gain.
Matching Devices by Comparison
In certain specialized applications it is useful to be able to match device
characteristics. For example, one wishes to use a pair of matched JFETs for the input to a low-noise
differential amplifier. The two JFETs should be matched, but a matched pair are not available commercially.
For low volume production or a one-off scientific instrument, a group of single JFETs can be sorted and
matched according to their curve-tracer measurements, and then used in pairs.
Testing a Two-Terminal Circuit
Two terminal circuits, such as a constant current or constant voltage device,
can be constructed from component parts. The curve tracer is ideal for measuring the properties of these
circuits. As another example, a
negative resistance
device (Lambda Diode) can be synthesized using two
junction FETs. The curve tracer can be used to plot its VI characteristic.
Testing Unknown Device
If you obtained a large quantity of a particular transistor, it would be possible to de-
termine its principal characteristics, such as polarity (NPN or PNP) and current gain.
Why do we need a curve tracer to determine device characteristics? Isn’t the information
in the datasheet?
The data sheet for a semiconductor device will specify some or all of the maximum, minimum and typical values
for some parameter. For example, the forward voltage drop of a diode will be specified at some value of forward
current. The datasheet may also show a typical curve of forward voltage vs current. However, in the process of
electronic design and troubleshooting it’s often important to be able the
exact
behaviour of a given device.
For example, the forward voltage of a silicon diode is often quoted a 0.6 volts. A curve tracer shows that the
forward voltage of an MR851 diode, when conducting 1 amp of current, is actually about 1.4 volts. This would
be important to know when designing a power supply
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The CTR-201 Curve Tracer
Early examples of the curve tracer instrument generally included a cathode ray display, much like an oscilloscope.
AC line-operated power supplies swept the device voltages and currents, typically at a rate of 60 or 120Hz. The
instrument had many manual controls and extensive analog circuitry. These instruments were excellent for their
time, but they were large, heavy and expensive.
The CTR-201 system uses a completely different approach. The test hardware connects to a personal computer
via USB, and the computer runs a control program to operate this hardware. The hardware unit contains various
controlable voltage and current sources that actuate the device under test while measuring the voltages and currents
in the device. The measurement results are then handed back to the host PC for display, manipulation, or storage.
There are many advantages to this approach:
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The hardware can be relatively simple, which reduces its size and cost. Where an electronics lab could
perhaps have one curve tracer that was shared by all staff or students, it is now feasible for each work
station to do its own dedicated measurements.
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The measurement algorithms are defined in software, so the capabilities of the instrument can be modified
and extended.
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The curve tracer measures the characteristics of a specific device. For a production design where many units are to be produced, one
should use the worst case parameters of a semiconductor in the design. So you should in general not read the measurements from a curve
tracer of one particular device and use those results directly in a design. But you could use the curve tracer to ensure that a given device meets
or exceeds its datasheet specifications.
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