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Starting electronics
Every picture tells a story
Interestingly, digital circuits of all types (and we’ll cover a few
over the next few pages) are made up using just this simple
and basic configuration. Even whole computers use this basic
inverter as the heart of all their complex circuits.
To make it easy and convenient to design complex digital
circuits using such transistor switches, we use symbols instead
of having to draw the complete transistor circuit. The
symbol for an inverter is shown in Figure 10.4, and is fairly
self-explanatory. The triangle represents the fact that the
circuit acts as what is called a buffer — that is, connecting
the input to the output of a preceding stage does not affect
or dampen the preceding stage in any way. The dot on the
output tells us that an inverting action has taken place, and
so the output state is the opposite of the input state in terms
of logic level.
Figure 10.4
The symbol of an inverter — the simplest form of digital logic gate
To aid our understanding of digital logic circuits, input and
output states are often drawn up in a table — known as a
truth table, which maps the circuit’s output states as its input
states change. The truth table for an inverter is shown in
Figure 10.5. As you’ll see, when its input state is 1 its output
state is 0. When its input is 0, its output is 1 — exactly as
we’ve already hypothesised.
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