Download Logic Gates

Logic Gates
Unit 16
Objectives
Upon completion of this unit the student will be able to:
1. Explain how different kinds of logic gates function.
2. Use diodes and transistors to build logic gates.
Recall for unit 11 the integrated circuits (ICs) are made by forming individual
transistors, diodes and resistors on small silicon chips. Most integrated
circuits can be categorized as analog, digital or analog-digital, according to
their function.
Analog (or linear) IC’s produce, amplify or respond to variable voltages.
Analog ICs include many kinds of amplifiers, timers and voltage regulators.
Digital ICs or logic circuits respond to or produce signals having only two
voltage levels. These circuits count, compare or otherwise process
information in the form of on-off electrical pulses. Digital ICs include
microprocessors, microcomputers, memories and many kinds of simpler
chips.
Microprocessors equipped with ROM (Read Only Memory, which stores
constantly used, unchanging computer programs) are used to perform
process-control, testing,, monitoring and diagnostic functions. For example,
they are used in car ignition systems and in car-engine diagnosis.
No matter how complicated, all logic circuits are made from a simple
building blocks called logic gates. Logic gates are like electronically controlled
switches. They are either on or off. A typical logic gate has two inputs and
one output.
The simplest logic gates can be compared to mechanical switch gates.
Notice that the circuit in Figure 16-1 contains a battery, a lamp and two
switches in series. The switches are the gate’s input and the lamp is the
output. Because the two switches are connected in series, the lamp will
light only when both switches A and B are closed. For this reason, the
switch gate is called an AND gate.
AND Switch Gate
The Table in Figure
16-2 summarizes
the operation of an
AND switch gate. It
includes all possible
on/off
combinations of
switches.
The punch press you read about in Unit 15 is an example of AND logic.
For the punch press to operate, both switches must be turned on.
Notice the circuit in Figure 16-3 also contains a battery, a lamp and two
switches. But this time the switches are in parallel. In this case the lamp
will light when Switch A or Switch B or both switches are closed. For
this reason, the switch gate is called an OR gate.
OR Switch Gate
The table in Figure 164 summarizes the
operation of an OR
switch gate. It
includes all possible
on/off combinations
of switches.
You can substitute the digits 0 and 1 for the off and on positions,
respectively of a switch. In this case, the tables in Figures 16-2 and 16-4
can be expressed as shown in Figure 16-5.
A NOT gate reverses (or inverts) the usual action of a switch. For
example, in a circuit containing a battery, a lamp, a switch, and a NOT
gate, the lamp would be lit only when the switch is opened. When the
switch is closed, the lamp would be off. NOT gates, or inverters, make it
possible to create NAND ( not AND) and NOR (not OR) logic gate The
following Figure shows the binary tables of NAND and NOR gates.
Notice that the outputs of these gates are the opposite of the
corresponding AND and OR gates.
The simplest electrically controlled gates use PN junction diodes that
are switched on (forward biased) or off (reverse biased) by an input
signal of several volts (binary 1) or an input near or at ground (binary
0).
In the diode OR gate shown in the following Figure, when the input
voltage at A or B is more positive than ground, the voltage passes
through the forward biased diodes and appears at the output.
Otherwise, the output is at or near ground.
In the diode AND gate shown in the following Figure, when the input
voltage at A and B is more positive than ground, current flows to the
output. IF either A or B is at or near ground, one or both diodes
become forward biased and the current flows away from the output.
Notice that the output never reaches a full 6 volts in either gate. This is
because the diodes require a forward voltage of 0.6 volt, and this
voltage is subtracted from the output voltage.
The voltage drop of diode gates means that amplification is needed in
order to connect a series of gates together. Transistors can provide the
needed amplification and they can act as gates.
In modern electronic computers, transistors are the devices that act as
gates (switches). As required operations of computers have become
more complex, switches were developed that have a variety of ways in
which they can be turned on and off. Today, computer contain
thousands of transistors integrated in a semiconductor chip (IC).
You will now have the opportunity in the Experiments to make logic
gates using diodes and transistors. For most applications, however, an
integrated circuit can be purchased containing the needed gate.
Combinational Logic
Combinational circuits are built of five basic logic gates:

AND gate - output is 1 if BOTH inputs are 1

OR gate - output is 1 if AT LEAST one input is 1

XOR gate - output is 1 if ONLY one input is 1

NAND gate - output is 1 if AT LEAST one input is 0

NOR gate - output is 1 if BOTH inputs are 0
Truth Tables
The descriptions above are adequate to describe the functionality of
single blocks, but there is a more useful tool available: the truth table.
Truth tables are simple plots which explain the output of a circuit in
terms of the possible inputs to that circuit. Here are truth tables
describing the six main elements: