G7B: Amplifiers and Oscillators – Ham Radio General License Study Guide
G7B covers amplifier classes, RF amplifier efficiency, neutralization, oscillator design, and digital logic fundamentals including gates, counters, and shift registers. The eleven questions in this group test your ability to select the correct class for a given application, calculate or interpret RF efficiency, and identify the behavior of basic digital circuits.
Topics include what neutralizing an amplifier accomplishes, which amplifier class has the highest efficiency, how long a Class A device conducts per cycle, what defines a linear amplifier, which modes are appropriate for Class C amplification, how to measure RF power amplifier efficiency, what determines the frequency of an LC oscillator, how a sine wave oscillator is built, what an AND gate does, how many states a 3-bit binary counter has, and what a shift register is.
Amplifier Classes
Amplifier class describes how much of the AC input cycle the active device (transistor or tube) conducts current. It directly determines efficiency and linearity:
| Class | Conduction Angle | Linearity | Efficiency | Typical Use |
|---|---|---|---|---|
| Class A | 100% (full cycle) | Best | Lowest (~25–50%) | Audio preamps, receive preamplifiers |
| Class B | ~50% | Good (push-pull) | ~78% | Push-pull audio stages |
| Class AB | Between 50% and 100% | Good | Moderate | Linear HF power amplifiers (SSB, AM) |
| Class C | Less than 50% | Poorest (non-linear) | Highest (>70%, often >90%) | FM transmitters, CW, frequency multipliers |
The exam asks two specific questions about class:
- Class A conduction: 100% of the time — the device never cuts off
- Highest efficiency: Class C — it conducts for less than half the cycle, dissipating the least heat
A linear amplifier is one in which the output waveform preserves the shape of the input waveform. This is essential for SSB and AM, where the amplitude variation carries intelligence. A Class C amplifier clips the waveform and is therefore non-linear. It is appropriate for FM because FM carries information in frequency variation rather than amplitude — a constant envelope — so non-linearity does not distort the audio.
RF Efficiency and Neutralization
RF amplifier efficiency measures how effectively DC input power is converted to useful RF output power:
Efficiency = RF output power ÷ DC input power
Example: 100 W RF output from 200 W DC input → 50% efficient
The other quantities (RF input power, DC output power) are not used in the efficiency calculation — the exam distractors include those to test whether you know the correct ratio.
Neutralization is a technique used to prevent an amplifier from oscillating unintentionally. At RF frequencies, stray capacitance inside the active device can feed some of the output signal back to the input in phase with the original signal. If this positive feedback is large enough, the amplifier will break into self-oscillation rather than amplify the input signal. Neutralization cancels this feedback by injecting an equal and opposite (out-of-phase) signal, eliminating the self-oscillations.
Oscillators
An oscillator generates a continuous AC signal without an external input. Two oscillator types appear in G7B:
Sine Wave Oscillator
A sine wave oscillator is built from a filter and an amplifier operating in a feedback loop. The amplifier provides gain to sustain oscillation; the filter selects the desired frequency by allowing only one frequency to be fed back with the correct phase. All other frequencies are attenuated to zero by the filter. The combination sustains a clean sine wave at the filter's resonant frequency.
LC Oscillator
An LC oscillator uses a tank circuit — a parallel combination of an inductor (L) and a capacitor (C) — as the frequency-determining filter. Energy oscillates back and forth between the magnetic field of the inductor and the electric field of the capacitor. The resonant frequency is determined by the inductance and capacitance values in the tank circuit. Varying either value changes the oscillation frequency.
Digital Logic Fundamentals
G7B includes three questions on basic digital circuits commonly found in amateur radio transceivers and accessories.
AND Gate
A two-input AND gate has one rule: the output is high only when both inputs are high. If either input is low, the output is low. This is the strictest of the basic gates — it requires agreement from all inputs to produce a high output.
| Input A | Input B | AND Output |
|---|---|---|
| 0 | 0 | 0 |
| 0 | 1 | 0 |
| 1 | 0 | 0 |
| 1 | 1 | 1 |
Binary Counter
A binary counter counts upward in binary. The number of distinct states equals 2 raised to the power of the number of bits. A 3-bit counter has 2³ = 8 states (000 through 111, or 0–7 in decimal). Binary counters are used in frequency dividers, timers, and digital frequency displays.
Shift Register
A shift register is a clocked array of circuits that passes data in steps along the array. On each clock pulse, the data stored in each stage shifts one position to the next. Shift registers are used to convert parallel data to serial data (and vice versa), to create time delays, and to buffer data streams in digital signal processing.
G7B Practice Questions
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