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Starting electronics
can calculate each voltage as being a function of the battery
voltage, given by:
and
Now, we know that the voltage across the diode can only
vary between about 0 V and 0V8 (given by the characteristic
curve), but there’s nothing to stop us hypothesising about
larger voltages than this, and drawing up a table of voltages
which would thus occur across the resistor. Table 7.1 is such
a table, but it takes the process one stage further by calculating
the hypothetical current through the resistor at these
hypothetical voltages.
From Table 7.1 we can now plot a second curve on the diode
characteristic curve, of diode voltage against resistor current.
Figure 7.6 shows the completed characteristic curve (labelled
Load line R=60R). The curve is actually a straight line — fairly
obvious, if you think about it, because all we’ve done is plot a
voltage and a current for a resistor, and resistors are ohmic
and linear. Because in such a circuit as that of Figure 7.4 the
resistor is known as a load i.e. it absorbs electrical power, the
line on the characteristic curve representing diode voltage
and resistor current is called the load line.
Where the load line and the characteristic curve cross is
the operating point. As its name implies, this is the point
representing the current through and voltage across the
components in the circuit. In this example the diode voltage
is thus 1 V and the diode current is 33 mA at the operating
point.
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