Hint: to neutralize is to eliminate self-oscillations

The reason for neutralizing the final amplifier stage of a transmitter is to eliminate self-oscillations.

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This neutralization is done by returning a portion of the amplifier output back into the input in such a way that the signal is 180 degrees out of phase. This cancels the positive feedback loop which if not corrected can cause self-oscillations which may be harmful.

For more info see Wikipedia: Electronic Amplifier

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Hint: [C]lass C

(D). Class C amplifiers have the highest efficiency. This class of amplifier has high efficiency, but because they only conduct less than half of the input signal, they have high distortion. This makes them unsuitable for most phone transmissions, but effective for CW operation.

For more info see Wikipedia: Class C amplifiers

Quick and dirty mnemonic: "Efficiency" is spelled with a "C" as in Class "C"

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Hint: if they go high, we BOTH go HIGH

(B). In a two input AND gate output is high only when both inputs are high.

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The AND gate uses a binary system of Boolean or digital logic to express the "AND" or conjunction of two states. The AND gate follows the truth table shown below.

So notice on the table that Output is only high "1" when input A is high "1" AND input B is high "1" as well. If either or both of the inputs is low "0", then the output must also be low "0".

For more info see Wikipedia: AND gate

AND Gate

\(A\)	\(B\)	\(A {\tiny \text{ AND }} B\)
\(0\)	\(0\)	\(0\)
\(0\)	\(1\)	\(0\)
\(1\)	\(0\)	\(0\)
\(1\)	\(1\)	\(1\)

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Hint: If they go low then one OR (nOR) EITHER of us goes HIGH

In a two input NOR gate, output is low when either one or both inputs are high.

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The NOR gate follows a binary system of Boolean or digital logic to express the idea of "not or". This table has the inverse output of an OR gate. The NOR gate follows the logic table shown below.

So if either A OR B OR both are high "1" then the OR function would also be "1", but now take the inverse, because it is NOT-OR and the output is then "0" or low.

For more info see Wikipedia: NOR gate

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Here's a table showing the logic values for OR and NOR Gates:

\(A\)	\(B\)	\(A\ \raise0.25ex {\tiny OR}\ B\)	\(A\ \raise0.25ex {\tiny NOR}\ B\)
\(0\)	\(0\)	\(0\)	\(1\)
\(0\)	\(1\)	\(1\)	\(0\)
\(1\)	\(0\)	\(1\)	\(0\)
\(1\)	\(1\)	\(1\)	\(0\)

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Hint: a 3-bit = 2³ (3 bits of 2)

A 3-bit binary counter has 8 states.

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The number of states increases exponentially with each bit increase. The equation for finding the number of states in a binary counter is simply \(2^N\). 2 being the numeric value of the number of states in a single bit (only 2 states possible, 1 or 0) and \(N\) is a power equal to the number of bits. So it follows:

# of bits (\(N\))	# of states (powers of 2)	# of states (as a whole #)
2-bits	\(2^2\)	\(4\)
3-bits	\(2^3\)	\(8\)
4-bits	\(2^4\)	\(16\)
5-bits	\(2^5\)	\(32\)
\(\ldots\)	\(\ldots\)	\(\ldots\)
\(N\)-bits	\(2^N\)

For the visual folks, actual states for a 3 bit counter are:

\[ 000\\ 001\\ 010\\ 011\\ 100\\ 101\\ 110\\ 111\\ \]

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Hint: An Employee’s Shift ends when CLOCKS out

(A). A Shift register is a clocked array of circuits that passes data in steps along the array. The shift register is a series of "Flip Flops" using a common clock input. These "Flip flops" produce a binary array of 0 or 1 values, with the output of one flip flop becoming the input signal for the next flip flop in the sequence.

HINT: for your "shift" at work, you "clock" in and out!

For more info see Wikipedia: Shift register

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Key word: Feedback

Oscillator is a ground loop so what all is wrong comes around (feedback, not forward)

The basic components of virtually all sine wave oscillators are filter and an amplifier operating in a feedback loop**. The sine wave oscillator uses an amplifier with a positive feedback loop in which the output from the amplifier is filtered for desired frequencies and then fed back into the input of the amplifier. The cycle repeats, taking the signal from a state of noise to a clear sine wave at the desired frequency.

For more info see Wikipedia: Electronic oscillator

Silly Hint: “Sine” in Latin means “without”. So without any waves in the beach, makes the water recede, or ‘feedback”. You may have also seen it as a legal term “sine qua non”. Without it, there’s No Deal!”

SILLY HINT2: you "filter" the signal and "amplify" it on the display.

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The efficiency of an RF power amplifier can be determined by dividing the RF output power by the DC input power.

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An RF amplifier is used to take low-power radio frequency signals and convert them to higher power signals.

Efficiency is a measure of how much of the input power actually gets converted to output power.

This is sometimes expressed as a percentage, other times as a ratio (power out to power in).

\[\text{Efficiency in %} = \frac{\text{Power out}_{RF}}{\text{Power in}_{DC}} \times 100\]

If you have trouble remembering how to compute the efficiency, here are some tips that can help eliminate distractors:

It's a ratio. This means dividing a number by another. That eliminates the distractors that talk about adding or multiplying.

Amplifiers amplify. However, they are not completely efficient, so the power going into the amplifier will be than than the output power when the input signal, such as a microphone or antenna, is relatively small.

For more info see Wikipedia: RF power amplifier

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Silly memory trick: The [R]oman [F]leet” can only be defeated if you [D]ivide & [C]onquer” them (RF / DC)

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Hint: Linear (inductor) C (capacitor) = LC The inductance and capacitance in the tank circuit determine the frequency of an LC oscillator. An oscillator uses an amplifier in which the signal output is filtered for the desired frequency and then sent in a feedback loop back into the input of the amplifier. The LC oscillator uses an LC circuit consisting of an inductor (L) and capacitor (C) in parallel for filtering the RF signal in the feedback loop.

For more info see Wikipedia: Electronic Oscillator, LC circuit

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Silly Hint: Think of a linear descendant PRESERVES the family line.

(B). A linear amplifier is an amplifier in which the output preserves the input waveform. The linear amplifier simply increases the power (amplitude) of the signal. The form of the wave is not changed - what goes in is what comes out!

For more info see Wikipedia: Linear amplifier

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Hint: FM CLASS(i)C rock

FM operation is an appropriate mode for using a Class C power state for amplifying the modulated signal. Class C amplifiers conduct only a fraction of the input signal (usually less than 50%), so distortion is high, but efficiency is also high. Because of this high distortion, the class C amplifiers are not good for phone signals such as AM or SSB, as these require more complete signal. FM however, works well with the Class C amplifier.

(Note that in the 2015-2019 General pool the correct answer to this question was "CW", but it was changed in the 2019 pool release)

For more info see: Electronic Amplifiers - 4.4 Class C amplifier

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