Syscomp CTR-201, Руководство пользователя

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1 Introduction
Electronic devices are useful because they cause a specific relationship between voltage and current. A curve
tracer displays the VI characteristic of these devices, leading to better understanding of their operation.
(a) Resistor, 1k Ω (b) Diode, MR851
Figure 1: VI Characteristics
As a very simple example, it’s possible to plot the VI characteristic of a resistor on a curve tracer. A resistor
plot appears as a diagonal straight line (figure 1a), the slope inversely proportional to the resistance. There are
simpler and less expensive ways of measuring resistance, but it’s useful to keep that example in mind when
viewing a VI plot.
For example, a diode allows current to flow in the forward direction, and blocks it in the reverse direction.
As well, there is a small voltage drop across the diode when it is conducting. This voltage drop increases with
current, so that the voltage is a non-linear function of the current. We refer to the graph of this function as the
VI Characteristic for the forward-biased diode. Different diodes have different shaped curves. For example, the
voltage drop across a germanium diode is about 0.3 volts. Across an LED, it is 2 or 3 volts. Figure 1b shows the
VI characteristic of a 3 amp silicon power diode.
The curve tracer can also plot a family of VI Curves for NPN and PNP transistors, MOSFETs, JFETs and
many other semiconductor devices.
Applications of a curve tracer include the following:
Education: Viewing Device Characteristics A lab exercise to measure device characteristics - such as the col-
lector curves of a transistor - helps students to retain important information about devices. It also teaches
students how to interpret the curves to determine device parameters and models.
Modelling a Device Detailed knowledge of the characteristics of a device allows one to model it for a simulation
program. For example, the characteristic of the diode in figure 1(b) can be modelled by an ideal diode in
series with a pure voltage source and resistor. The voltage source models the threshold voltage of the diode.
The resistor models the linear portion of the curve at higher currents.
Measuring the Range of a Parameter A device data sheet generally specifies the behaviour of the device under
certain conditions. The curve tracer can determine the behaviour under a range of other conditions. For
example, the datasheet of a transistor may not show its incremental collector resistance. This is readily
determined from a curve tracer plot.
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