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.

Last edited by orochimaru. Register to edit

Tags: arrl chapter 6 arrl module 6c

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.

Last edited by phongn. Register to edit

Tags: arrl chapter 6 arrl module 6c

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.

Last edited by kd7bbc. Register to edit

Tags: arrl chapter 6 arrl module 6c

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

Last edited by orochimaru. Register to edit

Tags: arrl chapter 6 arrl module 6c

Unwanted oscillations are also called parasitic oscillations. Naturally, a parasitic suppressor suppresses parasitic oscillations.

SWR, power, and phase are all more or less unrelated to oscillation.

Silly Hint: Parasites are unwanted

Last edited by sympathique. Register to edit

Tags: arrl chapter 6 arrl module 6c

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

Last edited by chevdor. Register to edit

Tags: arrl chapter 6 arrl module 6c

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).

Last edited by jaeger21. Register to edit

Tags: arrl chapter 6 arrl module 6c

With premise that combining two signals together out of phase by 180 degrees causes them to cancel AND that the correct answer here has 180 degree out-of-phase in it we can conclude "By feeding a 180-degree out-of-phase portion of the output back to the input" is the correct answer.

Last edited by kd7bbc. Register to edit

Tags: arrl chapter 6 arrl module 6c

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

Last edited by kd7bbc. Register to edit

Tags: arrl chapter 6 arrl module 6i

An emitter-follower is a single BJT transistor configuration with a voltage tap at the emitter, hence the "follower" in the name. The collector is connected to the energy source (which is why this is sometimes called a "common-collector"), and input signals at the base modulate the current flowing to the emitter, where a resistance to ground produces a voltage output. By nature, BJTs have a low input impedance.

TEST TIP: "Emitter-FOLLOWER" = "...output that FOLLOWS."

Last edited by w9mpx. Register to edit

Tags: none

Linear amplifiers (such as Class A amps) bias transistors in their linear region. Whenever these transistors are conducting they are never fully on nor off, but have some instantaneous resistance which generates heat.

An ideal switch when closed has 0 ohms of resistance, and when open has infinite resistance. In either case the switch does not generate any heat.

Transistors in switching amplifiers act much like switches--they are either on or off most of the time. Because switched transistors are seldom partially on they don't generate much heat, resulting in low power dissipation compared to transistors operating in their linear region.

Hint: 'linear' descriptor of time, only answer with 'time'

Hint: The only one that doesn’t say amplifier

Last edited by antoszewski. Register to edit

Tags: arrl chapter 6 arrl module 6c

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.

Last edited by kv0a. Register to edit

Tags: arrl chapter 6 arrl module 6b

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

Last edited by zapdawg. Register to edit

Tags: arrl chapter 6 arrl module 6c

Odd-order, rather than even-order, intermodulation distortion products are of concern in linear power amplifiers because they are relatively close in frequency to the desired signal.

Hint: Odds attract, are close

Last edited by kd7bbc. Register to edit

Tags: arrl chapter 6 arrl module 6c

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?

Last edited by wileyj2956. Register to edit

Tags: arrl chapter 6 arrl module 6b