13.1 RF Amplifier Classes & Efficiency

13.1 RF Amplifier Classes & Efficiency

Quick Answer: Class A = full 360° conduction, highest linearity, lowest efficiency. Class B = 180°. Class AB = >180° and <360°. Class C = biased beyond cutoff, conducts <180°, highest efficiency. Use linear finals (A/AB) for SSB; use Class C for FM/CW. Non-linear SSB amplification → distortion/IMD. Even-order harmonics fall with push-pull; parasitics die with neutralization.

Topic 3-G (Transmitters) is the transmit-side companion to Topic 3-F receivers. GROL holders adjust, repair, and maintain radiotelephone transmitters in aviation, maritime, and international fixed public service—work that starts with knowing which amplifier class is legal and safe for the emission on the air.

Why amplifier class matters on a GROL bench

A transmitter final must do two things at once: deliver the licensed RF power, and preserve the modulation envelope the mode requires. Efficiency (RF out vs DC in) drives heat, power-supply size, and battery life. Linearity (output proportional to input) drives spectral cleanliness. Those goals fight each other. Element 3 therefore defines classes by how much of the input RF cycle the active device conducts—the conduction angle or operating angle.

Memorize the pool wording, not just the table nicknames.

Class A — full-cycle linear workhorse

Class A is distinguished by output throughout the entire signal cycle; the input never goes into the cutoff region. The distinguishing feature: output for the entire 360 degrees of the signal cycle.

Because the device is always on, the transfer stays on the most linear part of the curve. Pool answer to “highest linearity and least distortion?” → Class A.

The cost is continuous quiescent current. Even with no RF drive, Class A draws substantial DC and turns much of it into heat. Class A drivers and low-level linear stages are common; pure Class A high-power finals are rare in modern commercial HF/VHF gear because of heat and supply demand—but the exam still uses Class A as the linearity reference.

Signal-handling metric for Class A: the peak voltage of the input is the most valuable amplitude parameter when evaluating how large a signal the stage can handle without leaving the linear region (clipping into cutoff or saturation). RMS and average hide the peaks that first cause distortion.

Class B and Class AB — half-cycle and overlap

Class B: output present for 180 degrees of the input cycle—biased at cutoff, conducting only on alternate half-cycles. A single-ended Class B RF stage needs a tank (flywheel) to reconstruct the missing half of the RF cycle. In audio or baseband push-pull, two devices share the two half-cycles.

Class AB: operating angle more than 180 degrees but less than 360 degrees when driven by a sine wave. Each device is biased slightly into conduction so the halves overlap near crossover. That overlap kills the crossover notch that pure Class B suffers, while still saving a large fraction of Class A’s idle current. Class AB is the workhorse linear RF final in many commercial SSB exciters and amplifiers GROL techs service.

Pool recap:

DC input estimate (Class AB, unmodulated carrier)

Pool-style power question: approximate DC input power to a Class AB RF power amplifier in an unmodulated carrier transmitter when PEP output is 500 watts → 1000 watts. That implies roughly 50% efficiency as the exam’s working estimate for that scenario (DC in ≈ 2 × RF out for the unmodulated case they give). Use it as a pool anchor, not as a universal datasheet formula for every design.

Class C — efficiency king for constant envelope

Class C is distinguished by bias set well beyond cutoff. Output is present for less than 180 degrees of the input cycle. Short, intense current pulses drive a high-Q tank circuit that rings at the carrier frequency and restores a nearly sinusoidal RF voltage across the load.

Which class provides the highest efficiency? Class C.

Advantage of Class C over Class A? Higher efficiency (not better linearity, not lower distortion, not wider bandwidth).

Because Class C is deeply nonlinear, it cannot faithfully amplify a varying envelope. Amplitude information is destroyed; new spectral products appear. That is fatal for SSB phone and ordinary AM linear service, but acceptable—and desirable—for FM, PM, and CW/FSK where the RF amplitude is essentially constant and a tank or filter cleans the waveform.

Efficiency versus linearity — the exam tradeoff

Think of one axis: conduction angle.

• Longer conduction (toward 360°) → smoother waveform copy → better linearity, more continuous dissipation → worse efficiency.
• Shorter conduction (toward brief pulses) → better efficiency, severe harmonic generation and envelope crushing → worse linearity.

SSB linear vs FM/CW efficiency is the applied form of that axis:

1. SSB peak-to-average speech ratios are high (pool later: ~2.5:1 PEP-to-average for normal voice peaks). The final must track every peak without flat-topping → linear AB/A chain, heat sinks, ALC, and careful drive.
2. FM/CW keep nearly constant RF amplitude → Class C multiplies efficiency; tank/harmonic filters handle spectral cleanup rather than envelope fidelity.

Putting a Class C brick on an SSB exciter output is a classic failure mode: harsh audio, splatter, and intermodulation products that fail type-acceptance and neighboring-channel rules.

Intermodulation distortion and non-linear stages

What happens when a non-linear amplifier is used with a single-sideband phone transmitter? Distortion. (Not “better efficiency that somehow helps intelligibility.”)

Nonlinearity mixes frequency components already present in the multi-tone SSB envelope. Third-order and higher products land in-band (muddy speech) and adjacent-channel (splatter). Two-tone test specs and FCC emission masks exist because of this physics.

Related transmitter-proximity problem (Topic 3-G technology cluster): when two transmitters sit close together, their signals can mix in one or both final amplifiers and generate sum and difference products. That condition is intermodulation interference—the same family of nonlinear mixing, now between stations rather than inside one SSB envelope. Mitigation on the pool includes a terminated circulator or ferrite isolator in the feed line to the transmitter and duplexer so reverse energy cannot re-enter the final as easily.

Harmonics, push-pull, and parasitics

Even-order harmonics (2nd, 4th, …) are reduced or prevented in amplifier design by using a push-pull configuration. Opposite-phase devices cancel even harmonics in the combined output while reinforcing the fundamental (and odd harmonics still need filtering).

Parasitic oscillations (unwanted oscillation at frequencies unrelated to the intended tank) are eliminated by neutralization—intentional feedback that cancels the device’s internal path that was supporting the parasite. “Tune for max power” is not the cure; it often makes parasitics worse.

RF power amplifier job statement

Primary function of an RF power amplifier in a transmitter: increase the power level of the RF signal to the level required for transmission. It does not generate the carrier (oscillator), does not create the intelligence (modulator), and does not replace the harmonic filter—though the PA’s class and matching network heavily influence what the filter must suppress.

Exam-day checklist (3-G amplifiers)

1. Class A → 360°, never cutoff, highest linearity / least distortion.
2. Class B → 180°; Class AB → >180° & <360°; Class C → <180°, bias beyond cutoff, highest efficiency.
3. Class C advantage over A → higher efficiency; use C for FM/CW, linear for SSB.
4. Class A signal-handling metric → peak voltage.
5. Class AB unmodulated example → ~1000 W DC in for 500 W PEP out (pool figure).
6. Nonlinear SSB amp → distortion; even harmonics → push-pull; parasitics → neutralization; co-sited mixing → intermodulation interference, isolate with circulator/isolator.

Next section covers oscillators, modulators, and resonance matching networks that feed and terminate these amplifier stages.