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2020 Extra Class study guide: E6C - Digital ICs: Families of digital ICs;
gates; Programmable Logic Devices (PLDs)

Posted: 11 Feb 2020 12:45 PM PST
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Integrated circuits (ICs) are now an integral part (pun intended) of
amateur radio electronics. There are several different technologies used to
manufacture ICs including transistor-transistor logic, or TTL;
complementary metal-oxide semiconductor, or CMOS; and BiCMOS, which uses a
combination of bipolar and CMOS transistors.

CMOS is arguably the most common type of digital IC. One of the reasons for
this is that CMOS logic devices consume less power than TTL devices. They
also are more immune to power supply noise and noise on the inputs because
the input switching threshold is about one-half the power supply voltage.
QUESTION: What is an advantage of CMOS logic devices over TTL devices?
(E6C05)

ANSWER: Lower power consumption
QUESTION: Why do CMOS digital integrated circuits have high immunity to
noise on the input signal or power supply? (E6C06)

ANSWER: The input switching threshold is about one-half the power supply
voltage

BiCMOS logic is an integrated circuit logic family that uses both bipolar
and CMOS transistors. An advantage of BiCMOS logic is that it has the high
input impedance of CMOS and the low output impedance of bipolar transistors.
QUESTION: Which of the following is an advantage of BiCMOS logic? (E6C04)

ANSWER: It has the high input impedance of CMOS and the low output
impedance of bipolar transistors

Some devices have three different output states: 0, 1, and a high-impedance
state. We call these devices tri-state logic devices. The high-impedance
state allows you to connect many of them to a common bus. When a device’s
outputs are in the high-impedance state, they act as if they are
disconnected.
QUESTION: What is tri-state logic? (E6C03)

ANSWER: Logic devices with 0, 1, and high-impedance output states

When working with digital ICs, it is important to recognize the various
symbols for the different types of logic gates. Figure E6-3 shows the
schematic symbols for the most common logic gates. It includes the
following:

AND gate
NAND gate
OR gate
NOR gate
NOT (inverter)
Not a valid logic gate symbol



QUESTION: In Figure E6-3, what is the schematic symbol for a NAND gate?
(E6C08)

ANSWER: 2
QUESTION: In Figure E6-3, what is the schematic symbol for a NOR gate?
(E6C10)

ANSWER: 4
QUESTION: In Figure E6-3, what is the schematic symbol for the NOT
operation (inverter)? (E6C11)

ANSWER: 5

When designing circuits with digital ICs, you may not use all of the inputs
of the gates in that IC. To set that input to a digital 1 or 0, you might
use a pull-up resistor or a pull-down resistor. A pull-up would be
connected to the positive supply line while a pull-down would be normally
connected to ground. Pull-up resistors are also often used on the output of
a digital circuit to prevent the output from floating.
QUESTION: What best describes a pull-up or pull-down resistor? (E6C07)

ANSWER: A resistor connected to the positive or negative supply line used
to establish a voltage when an input or output is an open circuit

A particular type of IC is called a comparator. Comparators compare an
input voltage to a threshold voltage, and when the level of the input
signal crosses the threshold, the comparator changes its output state.

Comparators have a property called hysteresis. Basically, what this means
is that the threshold voltage is lower when the input voltage is decreasing
than the threshold voltage when the input voltage is increasing. The
function of hysteresis in a comparator is to prevent input noise from
causing unstable output signals. If the threshold voltage was the same for
both increasing and decreasing input voltages, and the input voltage was
right at the threshold voltage, then noise could cause that input voltage
to go above and below the threshold randomly. If the comparator input did
not have hysteresis, then its output would switch randomly.
QUESTION: What happens when the level of a comparators input signal crosses
the threshold? (E6C02)

ANSWER: The comparator changes its output state
QUESTION: What is the function of hysteresis in a comparator? (E6C01)

ANSWER: To prevent input noise from causing unstable output signals



Many modern electronic devices now use programmable logic devices instead
of cobbling together a digital circuit with a collection of ICs with simple
gates. A Programmable Logic Device (PLD) is an integrated circuit that has
a collection of logic gates and circuits that you can connect in different
ways. Programmable logic devices can have thousands or even millions of
gates in a single IC. To design digital circuits with PLDs, designers use
computer-aided design software to connect and configure the logic gates.
QUESTION: What is a Programmable Logic Device (PLD)? (E6C09)

ANSWER: A programmable collection of logic gates and circuits in a single
integrated circuit

The post 2020 Extra Class study guide: E6C – Digital ICs: Families of
digital ICs; gates; Programmable Logic Devices (PLDs) appeared first on
KB6NUs Ham Radio Blog.