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Subelement E6
CIRCUIT COMPONENTS
Section E6C
Digital ICs: Families of digital ICs; gates; Programmable Logic Devices (PLDs)
What is the function of hysteresis in a comparator?
  • To prevent input noise from causing unstable output signals
  • To allow the comparator to be used with AC input signal
  • To cause the output to change states continually
  • To increase the sensitivity

Put simply, hysteresis means that the voltage needed to turn the circuit on is higher than the voltage below which it will turn off. This creates a "dead band" where it will not change until the input swings further, much like a thermostat controlling a furnace will not turn off until the temperature rises another degree past the set point. This improves noise immunity because the magnitude of the noise is very small and cannot push it far enough to turn the output on and off.

Hysteresis=hysterical? unstable?

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What happens when the level of a comparator's input signal crosses the threshold?
  • The IC input can be damaged
  • The comparator changes its output state
  • The comparator enters latch-up
  • The feedback loop becomes unstable

A comparator does a "greater-than/less-than" test on two voltages, one of which is often a fixed-voltage reference. When the input signal crosses over the threshold (the fixed reference voltage) the output goes high; when it crosses below the threshold the output goes low.

Thus the the comparator changes its output state.

The responses about damage and latch-up are both faults related to exceeding the supply voltage, not the threshold, and since they are so similar they can't both be right. The response about feedback is incorrect because comparators don't use negative feedback and thus can't oscillate.

-gxti

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What is tri-state logic?
  • Logic devices with 0, 1, and high impedance output states
  • Logic devices that utilize ternary math
  • Low power logic devices designed to operate at 3 volts
  • Proprietary logic devices manufactured by Tri-State Devices

In digital electronics three-state, tri-state, or 3-state logic allows an output port to assume a high impedance state in addition to the 0 and 1 logic levels, effectively removing the output from the circuit.

Ref: https://en.wikipedia.org/wiki/Three-state_logic

kb1cxd

I like to think of it as 0, 1 and Null (High Impedance)

-KE0IPR

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What is the primary advantage of tri-state logic?
  • Low power consumption
  • Ability to connect many device outputs to a common bus
  • High speed operation
  • More efficient arithmetic operations

In digital electronics three-state, tri-state, or 3-state logic allows an output port to assume a high impedance state in addition to the 0 and 1 logic levels, effectively removing the output from the circuit.

This allows multiple circuits to share the same output line or lines (such as a bus which cannot listen to more than one device at a time).

http://en.wikipedia.org/wiki/Three-state_logic

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What is an advantage of CMOS logic devices over TTL devices?
  • Differential output capability
  • Lower distortion
  • Immune to damage from static discharge
  • Lower power consumption

CMOS uses pairs of MOSFET transistors that draw essentially no supply current when they are not actively switching. This means that they consume much less power than TTL, which requires current to flow just to hold state.

The sensitive MOSFET gates actually make CMOS more sensitive to static than TTL, although all modern chips have builtin protection. Differential output is a feature of the chip design that can be done with both TTL and CMOS, and distortion is an analog quantity that has nothing to do with digital logic.

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Why do CMOS digital integrated circuits have high immunity to noise on the input signal or power supply?
  • Larger bypass capacitors are used in CMOS circuit design
  • The input switching threshold is about two times the power supply voltage
  • The input switching threshold is about one-half the power supply voltage
  • Input signals are stronger

CMOS is arguably the most common type of digital IC. An advantage of CMOS logic devices over TTL devices is that they have lower power consumption. (E6C05) CMOS digital integrated circuits also have high immunity to noise on the input signal or power supply because the input switching threshold is about one-half the power supply voltage. (E6C06)

http://www.kb6nu.com/extra-class-question-of-the-day-digital-integrated-circuits/

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What best describes a pull-up or pull-down resistor?
  • A resistor in a keying circuit used to reduce key clicks
  • 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 resistor that insures that an oscillator frequency does not drive lower over time
  • A resistor connected to an op-amp output that only functions when the logic output is false

Digital inputs "float" unless they are driven by something. If not addressed, the sensitive input circuitry might be in an indeterminate state or even oscillate. To keep this from happening a weak resistor "pulls up" or "pulls down" the input to one of the supply rails when nothing is connected, giving it a definitive logic high or low state. Once a driver is connected to the circuit its stronger output takes over.

Also, on an overloaded output stage the levels may not rise or fall to the proper voltage required, so pull-up or pull-down resistors assist the output stage.

WA5OFZ

HINT: The terms 'pull-up' and 'pull-down' could be thought to infer 'negative' and 'positive.' The correct answer is the only one that includes those terms.

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In Figure E6-5, what is the schematic symbol for a NAND gate?
  • 1
  • 2
  • 3
  • 4

A list of symbols can be found here: https://en.wikipedia.org/wiki/Electronic_symbol

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What is a Programmable Logic Device (PLD)?
  • A device to control industrial equipment
  • A programmable collection of logic gates and circuits in a single integrated circuit
  • Programmable equipment used for testing digital logic integrated circuits
  • An algorithm for simulating logic functions during circuit design

A PLD is a component which can be used to create digital logic circuits and can be reconfigured. There are multiple types of Programmable Logic Devices, such as CPLDs, FPGAs, PLAs, PALs, and others.

What all of them have in common is that they allow some number of logic gates and circuits which can be programmed by the person using them -- that is, rather than being custom designed for a specific purpose they can be reconfigured to solve a problem by the circuit designer.

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In Figure E6-5, what is the schematic symbol for a NOR gate?
  • 1
  • 2
  • 3
  • 4

1 - AND

2 - NAND

3 - OR

4 - NOR

5 - NOT

Electronic Symbol Reference: https://en.wikipedia.org/wiki/Logic_gate#Symbols

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In Figure E6-5, what is the schematic symbol for the NOT operation (inverter)?
  • 2
  • 4
  • 5
  • 6

1 - AND gate
2 - NAND gate
3 - OR gate
4 - NOR gate
5 - NOT gate
6 - Figure 6 is most similar to an Amplifier

The NOT gate is the only one that has one input and one output. The NOT gate inverses the input.

chevdor

-KE0IPR

See https://en.wikipedia.org/wiki/Logic_gate#Symbols

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What is BiCMOS logic?
  • A logic device with two CMOS circuits per package
  • A FET logic family based on bimetallic semiconductors
  • A logic family based on bismuth CMOS devices
  • An integrated circuit logic family using both bipolar and CMOS transistors

From Wikipedia:

"BiCMOS is an evolved 'semiconductor technology that integrates two formerly separate semiconductor technologies, those of the bipolar junction transistor and the CMOS transistor, in a single integrated circuit device"


Hint: Bi in BiCMOS implies bipolar, which is in the correct answer.

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Which of the following is an advantage of BiCMOS logic?
  • Its simplicity results in much less expensive devices than standard CMOS
  • It is totally immune to electrostatic damage
  • It has the high input impedance of CMOS and the low output impedance of bipolar transistors
  • All of these choices are correct

Just remember BiCMOS combines the qualities of Bipolar transistors and CMOS!

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What is the primary advantage of using a Programmable Gate Array (PGA) in a logic circuit?
  • Many similar gates are less expensive than a mixture of gate types
  • Complex logic functions can be created in a single integrated circuit
  • A PGA contains its own internal power supply
  • All of these choices are correct

Unlike microprocessors, which perform logical operations sequentially, a Programmable Gate Array (PGA) contains a very large number of logic blocks which can all function in parallel. A developer can configure and interconnect these logic blocks to create complex logic functions which are very fast.

PGAs combine the speed of dedicated hardware logic circuits with the ease and flexibility of defining and modifying logic functions using a high-level abstraction called a Hardware Description Language (HDL).

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