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Subelement E7
PRACTICAL CIRCUITS
Section E7B
Amplifiers: Class of operation; vacuum tube and solid-state circuits; distortion and intermodulation; spurious and parasitic suppression; microwave amplifiers; switching-type amplifiers
For what portion of a signal cycle does a Class AB amplifier operate?
  • Correct Answer
    More than 180 degrees but less than 360 degrees
  • Exactly 180 degrees
  • The entire cycle
  • Less than 180 degrees

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.

CrossoverDistortion

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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What is a Class D amplifier?
  • Correct Answer
    A type of amplifier that uses switching technology to achieve high efficiency
  • A low power amplifier that uses a differential amplifier for improved linearity
  • An amplifier that uses drift-mode FETs for high efficiency
  • A frequency doubling amplifier

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.

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Which of the following components form the output of a class D amplifier circuit?
  • Correct Answer
    A low-pass filter to remove switching signal components
  • A high-pass filter to compensate for low gain at low frequencies
  • A matched load resistor to prevent damage by switching transients
  • A temperature compensating load resistor to improve linearity

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. -N5MAJ

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

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Where on the load line of a Class A common emitter amplifier would bias normally be set?
  • Correct Answer
    Approximately half-way between saturation and cutoff
  • Where the load line intersects the voltage axis
  • At a point where the bias resistor equals the load resistor
  • At a point where the load line intersects the zero bias current curve

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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What can be done to prevent unwanted oscillations in an RF power amplifier?
  • Tune the stage for maximum SWR
  • Tune both the input and output for maximum power
  • Correct Answer
    Install parasitic suppressors and/or neutralize the stage
  • Use a phase inverter in the output filter

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

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Which of the following amplifier types reduces or eliminates even order harmonics?
  • Push-push
  • Correct Answer
    Push-pull
  • Class C
  • Class AB

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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Which of the following is a likely result when a Class C amplifier is used to amplify a single-sideband phone signal?
  • Reduced intermodulation products
  • Increased overall intelligibility
  • Signal inversion
  • Correct Answer
    Signal distortion and excessive bandwidth

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.

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How can an RF power amplifier be neutralized?
  • By increasing the driving power
  • By reducing the driving power
  • Correct Answer
    By feeding a 180-degree out-of-phase portion of the output back to the input
  • By feeding an in-phase component of the output back to the input

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.

-KE0IPR

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Which of the following describes how the loading and tuning capacitors are to be adjusted when tuning a vacuum tube RF power amplifier that employs a Pi-network output circuit?
  • The loading capacitor is set to maximum capacitance and the tuning capacitor is adjusted for minimum allowable plate current
  • The tuning capacitor is set to maximum capacitance and the loading capacitor is adjusted for minimum plate permissible current
  • The loading capacitor is adjusted to minimum plate current while alternately adjusting the tuning capacitor for maximum allowable plate current
  • Correct Answer
    The tuning capacitor is adjusted for minimum plate current, and the loading capacitor is adjusted for maximum permissible plate current

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.

K4AGO KK4DkZ

Hint: There are only two choices with the word permissible. The wrong answer should be obvious because it doesn't make any sense. 73 KD8YFM

Hint2: Be careful of maximum loading capacities

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In Figure E7-1, what is the purpose of R1 and R2?
  • Load resistors
  • Correct Answer
    Fixed bias
  • Self bias
  • Feedback

R1 and R2 perform a simple voltage divider circuit. This allows the base of the bipolar junction transistor (BJT) to be fixed at a specific DC voltage.

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In Figure E7-1, what is the purpose of R3?
  • Fixed bias
  • Emitter bypass
  • Output load resistor
  • Correct Answer
    Self bias

Self-Biasing A.K.A. Transistor Biasing is the process of configuring a transistor's DC operating voltage/current conditions to the correct level so that any Alternating Current (AC) input signal can be amplified appropriately by the transistor.

Also, this assists with elimination of clipping.

-KE0IPR

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What type of amplifier circuit is shown in Figure E7-1?
  • Common base
  • Common collector
  • Correct Answer
    Common emitter
  • Emitter follower

The emitter is held very close to ground due to R3 being fairly low impedance and C3 shunting the desired signal frequencies to ground. Some similar designs eliminate R3 and C3 altogether.

Since the input and output signals are ground referenced, the emitter and signals on the emitter are common (aka a ground or circuit return path) to both the input and the output.

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In Figure E7-2, what is the purpose of R?
  • Correct Answer
    Emitter load
  • Fixed bias
  • Collector load
  • Voltage regulation

This circuit is an Emitter Follower (also known as a Common Collector Amplifier).

It does not produce a voltage gain, but does offer significant current gain.

R provides a load for the emitter, and in so doing, also "lifts" the emitter far enough above ground so that a changing voltage that "follows" the base voltage can appear across it.

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Why are switching amplifiers more efficient than linear amplifiers?
  • Switching amplifiers operate at higher voltages
  • Correct Answer
    The power transistor is at saturation or cut off most of the time, resulting in low power dissipation
  • Linear amplifiers have high gain resulting in higher harmonic content
  • Switching amplifiers use push-pull circuits

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.

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What is one way to prevent thermal runaway in a bipolar transistor amplifier?
  • Neutralization
  • Select transistors with high beta
  • Correct Answer
    Use a resistor in series with the emitter
  • All of these choices are correct

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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What is the effect of intermodulation products in a linear power amplifier?
  • Correct Answer
    Transmission of spurious signals
  • Creation of parasitic oscillations
  • Low efficiency
  • All of these choices are correct

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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Why are odd-order rather than even-order intermodulation distortion products of concern in linear power amplifiers?
  • Correct Answer
    Because they are relatively close in frequency to the desired signal
  • Because they are relatively far in frequency from the desired signal
  • Because they invert the sidebands causing distortion
  • Because they maintain the sidebands, thus causing multiple duplicate signals

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.

Reference: WB6NOA - Gordon Wests, Extra Class License Preparations

Third-order intermodulation can be big problem with power amplifiers creating a small amount of in-band spurious emissions close to your operating frequency. The third-order IMD will elicit ham operators claiming your signal is wide or splattering.

-KE0IPR

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What is a characteristic of a grounded-grid amplifier?
  • High power gain
  • High filament voltage
  • Correct Answer
    Low input impedance
  • Low bandwidth

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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