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Transistors
These two operational modes of transistors have been given
names. The first, as it switches between two states, one where
the collector current is on or high, the other where it is off or
low, is called digital. Any circuit which uses transistors operating
in digital mode is therefore called a digital circuit.
The other mode, where transistors control, is known as the
analogue mode, because the collector current of the transistor
is simply an analogue of the base current. Any circuit which
uses transistors operating in the analogue mode is known as
an analogue circuit.
Take note — Take note — Take note — Take note
Sometimes analogue circuits are mistakenly called
linear circuits, However, this is wrong, because
although it might appear that a linear law is followed,
this is not so. If transistors were properly
linear they would follow Ohm’s law, but (like diodes)
— remember, they do not follow Ohm’s law and so are
non-linear devices. In an analogue circuit, however,
transistors are operated over a part of their
characteristic curve (remember what a characteristic
curve is from Chapters 6 & 7?) which approximates
a straight line — hence the mistaken name linear. Of
course, you won’t ever make the mistake of calling
an analogue circuit linear, will you?
Yes, like diodes, transistors have characteristic curves too,
but as they have three terminals they have correspondingly
more curves.
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