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. Just 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 is used in ordinary physics too. It comes from the Greek meaning "shortcoming" or "to fall short" (of what's needed to make a change)

Memory Trick: When a person is HYSTERICAL they are usually UNSTABLE,

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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.

Hint: When traveling, you change state when you cross the threshold (or border) into another state.

Memory Aid: “Comparator” appears in both the question and the correct answer choice.

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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

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

In this case "high impedance" pretty much just means "disconnected". A common case where this is used is where you have several digital devices connected to a shared data bus. When devices aren't transmitting data to the bus, the outputs should not be 1 or 0, they should be effectively disconnected from the bus so they don't corrupt data from another device. Since they can't easily be physically disconnected, they're "disconnected" by a transistor(s) that provide high impedance to any current flow. In Verilog, high impedance is denoted with Z so you get 1, 0, and Z where Z pretty much means "logically disconnected".

Hint: Only one answer lists the 3 states.

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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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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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CMOS (Complementary metal-oxide-semiconductor) is arguably the most common type of digital IC. An advantage of CMOS logic devices over TTL (Transistor-Transistor Logic) 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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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.

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

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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.

HINT: The only answer with "gates" within the answer.

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

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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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