As the name indicates, class AB amplifiers operate somewhere between class A and class B. Or perhaps the name is short for "Almost B" -- which would be true as well.

A class A amplifier operates at a full 360 degrees. The purity of amplification is terrific, but the efficiency is not.

Class B amplification uses two "finals", each operating for 180 degrees of the wave -- a great improvement in efficiency. However, with the introduction of bipolar transistors as finals, a problem with class B amplification developed. In a pure class-B configuration, there is a period of time where neither the forward-going nor the negative-going final has a forward-biased base-emitter junction. When this type of amplifier is fed a pure sine wave, there is a "hitch" at the zero crossing caused by neither final being forward-biased. This is called crossover distortion, and is a highly undesirable alteration of a pure sine wave.

The simple and elegant solution to this was to bias the finals so that each one operates slightly more than 180 degrees (but less than 360 degrees) to eliminate the problem of crossover distortion. The result is an amplifier with "Almost Class B" efficiency, but with distortion characteristics close to class A. Or, as it is called, Class AB.

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Just remember that the key feature of Class D amplifiers is high efficiency.

Each one of the wrong answers misleads you by putting the letter D in the description: differential, drift-mode, and doubling.

Trick: Class D, D is for Digital -> Technology is the answer.

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A passive low-pass filter removes the unwanted high-frequency components, i.e., smoothes the pulses out and recovers the desired low-frequency signal. To maintain high efficiency, the filter is made with purely reactive components (inductors and capacitors), which store the excess energy until it is needed instead of converting some of it into heat.

Wikipedia.org - Class-D amplifier link: http://en.wikipedia.org/wiki/Class-D_amplifier

Silly memory tip: A “D” is a low pass grade.

Hint: The word "components" in question and only one answer stem.

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Setting the bias half-way between saturation and cutoff gives the maximum possible swing in collector voltage without entering either saturation or cut- off.

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to prevent unwanted oscillations you need to neutralize them.

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Symmetrical push–pull circuits must cancel even order harmonics, like f2, f4, f6 and therefore promote odd order harmonics, like (f1), f3, f5 when driven into the nonlinear range.

https://en.wikipedia.org/wiki/Push–pull_output

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From Wikipedia: "Class-C amplifiers conduct less than 50% of the input signal and the distortion at the output is high". So, the option that mentions distortion is correct.

Silly hint: Class C - correct answer contains words starting with the next letters in the alphabet; signal distortion and excessive bandwidth. (c, d, &e).

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Most high-power amplifiers use vacuum tube technology. These amplifiers require that the operator tune the output circuit. The tuning capacitor is adjusted for minimum plate current and the loading capacitor is adjusted for maximum permissible plate current.

This is the correct procedure for adjusting the loading and tuning capacitors when tuning a vacuum tube RF power amplifier that employs a Pi-network output circuit.

Hint: There are only two choices with the word permissible. The wrong answer should be obvious because it doesn't make any sense.

Hint2: Be careful of maximum loading capacities

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From Wikipedia:

Silicon shows a peculiar profile, in that its electrical resistance increases with temperature up to about 160 °C, then starts decreasing, and drops further when the melting point is reached. This can lead to thermal runaway phenomena within internal regions of the semiconductor junction; the resistance decreases in the regions which become heated above this threshold, allowing more current to flow through the overheated regions, in turn causing yet more heating in comparison with the surrounding regions, which leads to further temperature increase and resistance decrease. This leads to the phenomenon of current crowding and formation of current filaments (similar to current hogging, but within a single device), and is one of the underlying causes of many semiconductor junction failures.

Resistors limit current.

REMEMBER: We want to resist thermal runaway.

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Intermodulation products are third order products (or harmonics) that are caused by nonlinearities in amplification/mixing. As these signals are simply byproducts of amplification and are unintended, these signals are by definition spurious.

http://www.arrl.org/intermodulation

https://en.wikipedia.org/wiki/Intermodulation

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Memory Aid: The ground is under your feet so it is low and won't impede your walk.

Compare to transistor base, collector, emitter. Which common lead type has low impedance? Which lead is like the grid in a vacuum tube?

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A klystron is a specialized linear-beam vacuum tube, invented in 1937 by American electrical engineers Russel and Sigurd Varian, which is used as an amplifier for high radio frequencies, from UHF up into the microwave range.

Source: Wikipedia - Klystron

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Hint: "amplifier" and "amplification" are in the answer

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[FET = Field Effect Transistor]

One advantage of the FET is its high gate to main current resistance, on the order of 100 MΩ or more, thus providing a high degree of isolation between control and flow. Because base current noise will increase with shaping time,[19] a FET typically produces less noise than a bipolar junction transistor (BJT), and is thus found in noise sensitive electronics such as tuners and low-noise amplifiers for VHF and satellite receivers. It is relatively immune to radiation. It exhibits no offset voltage at zero drain current and hence makes an excellent signal chopper. It typically has better thermal stability than a BJT.[3] Because they are controlled by gate charge, once the gate is closed or opened, there is no additional power draw, as there would be with a bipolar junction transistor or with non-latching relays in some states. This allows extremely low-power switching, which in turn allows greater miniaturization of circuits because heat dissipation needs are reduced compared to other types of switches.

per https://en.wikipedia.org/wiki/Field-effect_transistor

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