need to
understand is how gates are used. However, it is interesting to see that transistors can
be used to create a gate. Figure 2.6 provides an example by showing the internal struc-
ture of a nor gate composed of transistors, resistors, and components known as diodes.
The diagram reveals the underlying complexity: six transistors, five resistors, and three
diodes are needed to form a single nor gate.
input 1
input 2
0 volts
output
5 volts
4 k
4 k
1.6 k
1 k
130
diode
Figure 2.6
The internal structure of a nor gate formed from transistors and
other components. A solid dot indicates an electrical connection
between two wires. Resistors are labeled with a value in ohms,
with k indicating multiplication by 1000.
The drawing in the figure is known as a schematic diagram. Each line on a
schematic corresponds to a wire that connects one component to another. In addition,
the schematic shows wires that correspond to two inputs, an output, power (five volts),
and ground (zero volts).
The diagram in Figure 2.6 uses a common convention: two lines that cross do not
indicate an electrical connection unless a solid dot appears. That is, two lines that cross
without a dot correspond to a situation in which there is no physical connection; we can
imagine that the wires are positioned so an air gap exists between them (i.e., the wires
do not touch).