(a) Control Settings
(b) Result
Figure 14: 2N4403 PNP BJT
4
How It Works
A block diagram of the curve tracer is shown in figure 20. It consists of two programmable voltage sources and
one programmable bidirectional current source. These sources are controlled by 12 bit D/A converters.
The measured voltages and currents are shown conceptually on the diagram as voltmeters and ammeter. These
are actually A/D converter inputs that are 12 or 16 bit resolution.
By convention, the three terminals are labelled
Collector
,
Base
and
Emitter
, which refers to the con-
nections for a BJT (transistor). For a MOSFET, the corresponding terminals are
Drain
,
Gate
and
Source
.
The curve tracer can perform measurements on a wide variety of semiconductors besides these two devices.
Notice that a ground terminal is not available to the outside world. Measurements are performed between the
three terminals.
The base current generator actually consists of two current sources.
I
source
supplies current out the Base
terminal.
I
sink
accepts current into the Base terminal. At any given time, only one of these sources is active. The
compliance of the current source is equal to the full voltage range of measurements, about 42 volts.
In addition to the circuitry shown in figure 20, the unit includes:
•
Case temperature sensor, to monitor the temperature of the metal case and shut down supplies if the tem-
perature becomes excessive.
–
The temperature warning threshold is indicated by the temperature readout changing to an orange
colour at
45
◦
C.
–
The software critical temperature is
50
◦
C. Above that temperature the software will not run.
–
The firmware critical temperature is
52
◦
C. Above that temperature the firmware will disable the out-
puts.
•
Voltage monitors for the 5 VDC USB supply and 48 VDC main supply.
16