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G7B

PRACTICAL CIRCUITS

- Digital circuits; amplifiers and oscillators

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G7B011 of 11

What is the purpose of neutralizing an amplifier?

Why In an RF power amplifier, some output signal leaks back to the input through the device's internal capacitance (plate-to-grid in a tube, drain-to-gate in a FET). If that feedback arrives in phase with the input, the stage becomes an oscillator instead of an amplifier. Neutralizing means deliberately feeding back a small, equal-amplitude, out-of-phase sample from the output to cancel that internal feedback, so the stage amplifies only what you drive into it.
Watch out Keeping the carrier on frequency is the job of the oscillator/VFO stability and any PLL or crystal reference, not of the amplifier's neutralization network, though an unneutralized amplifier can certainly produce spurious signals.
Neutralize = cancel the sneak path. Amplifier that oscillates by itself needs out-of-phase feedback to shut it up.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G7B022 of 11

Which of these classes of amplifiers has the highest efficiency?

Why Amplifier class is set by how much of the input cycle the device conducts, and less conduction time means less time dissipating heat. Class A conducts the full 360 degrees (about 25-30% efficient), Class B about 180 degrees (up to ~60%), Class AB somewhere between, and Class C conducts for less than 180 degrees, giving efficiencies of roughly 70-80%. The tradeoff is distortion: Class C is very nonlinear, so it is used for CW, FM, and frequency multipliers where a tuned output circuit restores the waveform, not for SSB.
Watch out Class A is the tempting pick because it is the best amplifier in terms of linearity and low distortion, but it is the worst in efficiency since the device draws current the entire cycle, even with no signal.
Less conduction angle = more efficiency: A (360) < AB < B (180) < C (under 180). C is the efficiency champ, linearity loser.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G7B033 of 11

Which of the following describes the function of a two-input AND gate?

Why An AND gate is the logical equivalent of two switches in series: current only gets through when both are closed. So the output goes high (logic 1) only in the single case where both inputs are high; any low input forces the output low. Out of the four possible input combinations for a two-input AND gate, only one produces a high output.
Watch out The choice saying the output is low only when both inputs are high describes a NAND gate, the inverted AND, and the one saying the output is low when either or both inputs are high describes a NOR.
AND = 'All iNputs up, Done.' Only one of four input combinations gives a high output.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G7B044 of 11

In a Class A amplifier, what percentage of the time does the amplifying device conduct?

Why Amplifier classes are defined by conduction angle, the fraction of the input cycle during which the active device draws current. Class A biases the device in the middle of its linear region so it never cuts off, conducting for the full 360 degrees, which is 100% of the cycle. That gives the best linearity but the worst efficiency (theoretically 50% or less), since the device dissipates power even with no signal.
Watch out The choice saying more than 50% but less than 100% describes Class AB (conduction angle between 180 and 360 degrees), 50% is Class B (180 degrees), and less than 50% is Class C.
A = All the time (360 deg), AB = in between, B = half (180 deg), C = less than half.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G7B055 of 11

How many states does a 3-bit binary counter have?

Why Each bit is a binary digit with two possible values, so n bits give 2^n distinct combinations. With 3 bits that is 2 x 2 x 2 = 8, counting from 000 through 111 (0 to 7 in decimal). So a 3-bit counter cycles through 8 states before rolling over.
Watch out The choice of 16 is 2^4, the count for a 4-bit counter; 6 comes from multiplying 3 by 2 instead of raising 2 to the third power.
States = 2^bits, not bits x 2. 3 bits = 8, 4 bits = 16, 8 bits = 256.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G7B066 of 11

What is a shift register?

Why A shift register is a chain of flip-flops wired so the output of each stage feeds the input of the next. All stages share a common clock, so on every clock pulse the stored bits move one position down the line. That makes shift registers useful for serial-to-parallel conversion, delays, and moving data one bit at a time.
Watch out The op-amp answer confuses digital logic with analog circuitry, and the mixer choices describe circuits that combine signals rather than store and move bits; no mixing takes place in a shift register.
Shift register = flip-flops in a line, clock ticks, bits march one step.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G7B077 of 11

Which of the following are basic components of a sine wave oscillator?

Why Any oscillator needs two things: gain to make up for circuit losses, and a frequency-selective feedback path that returns part of the output back to the input in phase. That is the Barkhausen condition: loop gain of at least 1 with 0 (or 360) degrees phase shift around the loop. The filter, usually an LC tank, crystal, or RC network, decides which single frequency satisfies that condition, so the amplifier sustains a clean sine wave there.
Watch out A frequency multiplier and mixer are things you add after an oscillator to move or multiply its output, not what generates the oscillation; and note the correct answer says feedback, while the feed-forward choice describes a path that never returns energy to the input.
Oscillator = amplifier + filter + feedback. Gain replaces losses, the filter picks the frequency.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G7B088 of 11

How is the efficiency of an RF power amplifier determined?

Why Efficiency is always useful power out divided by power in. For an RF amplifier the useful output is the RF signal power delivered to the load, and the power in is the DC power drawn from the supply (volts times amps at the collector or plate). So efficiency = RF output power / DC input power, usually multiplied by 100 to express it as a percent; the rest of the DC power becomes heat. Typical figures are roughly 25-35 percent for Class A, 50-60 percent for Class AB, and up to 80 percent or more for Class C.
Watch out Dividing DC input by DC output is meaningless here because the amplifier's output is RF, not DC, and that ratio would be upside down anyway. Any answer that adds powers or uses reciprocals is not a ratio of output to input at all.
Efficiency = what you get out over what you pay for: RF out / DC in. Output on top, always.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G7B099 of 11

What determines the frequency of an LC oscillator?

Why An LC oscillator sets its frequency with a resonant tank circuit, a coil and capacitor whose resonant frequency is f = 1 / (2*pi*sqrt(LC)). The amplifier stage supplies gain and feedback in phase, but the tank decides which frequency survives, because that is where the circuit's impedance peaks and losses are lowest. Change either the inductance or the capacitance and the output frequency moves.
Watch out Counters and dividers belong to digital frequency synthesis, where a fixed reference is divided down, not to a free-running LC oscillator; time delay in a lag circuit describes an RC phase-shift or multivibrator type of oscillator.
LC oscillator = L and C. Frequency lives in the tank: f = 1/(2*pi*sqrt(LC)).
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G7B1010 of 11

Which of the following describes a linear amplifier?

Why "Linear" means the output is a scaled copy of the input: doubling the input voltage doubles the output voltage, so the waveform shape is preserved. This matters for modes whose amplitude carries information, such as SSB and AM, since any distortion creates harmonics and intermodulation products that splatter into adjacent frequencies. Class A, AB and B amplifiers are used for linear service; Class C is deliberately nonlinear.
Watch out The Class C choice is the opposite case: it conducts for less than half the cycle, is very efficient but badly distorts the envelope, so it suits only constant-amplitude modes like FM and CW, and its rich harmonic output is exactly what makes it useful as a frequency multiplier.
Linear = faithful copy, just bigger. Class C is efficient but ugly, fine for FM/CW, never for SSB.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G7B1111 of 11

For which of the following modes is a Class C power stage appropriate for amplifying a modulated signal?

Why A Class C stage conducts for less than half of each RF cycle, so its output is a series of current pulses that a tuned tank reshapes into a sine wave. That process is highly nonlinear and destroys any amplitude information in the drive signal, but it is very efficient. FM carries its information in frequency alone at constant amplitude, so nothing is lost and Class C works fine.
Watch out SSB and AM both vary the envelope amplitude, so a Class C amplifier would clip and distort them, generating splatter. Those modes need a linear stage, typically Class A or Class AB.
Class C kills amplitude. Constant-envelope modes only: FM and CW yes, SSB and AM no.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
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