Class AB vs Class C Amplifiers: Efficiency vs Linearity

When you sit down to design or select a final‑stage power amp for a CB or amateur rig, the first question that surfaces is “Do I need raw efficiency or pristine linearity?” The answer hinges on the operating class of the device, the intended mode of transmission, and the practical constraints of your installation. Below we dissect the core physics of Class AB and Class C amplifiers, map those characteristics to real‑world radio modes, and outline the decision points that seasoned builders weigh before committing to a design.

CLASS AB: THE BALANCE OF LINEARITY AND EFFICIENCY

Class AB occupies the middle ground between the fully linear Class A and the highly efficient Class B. In a typical push‑pull configuration each transistor conducts for slightly more than half of the RF cycle—usually around 180° ± 30°. This overlap eliminates the crossover distortion that plagues pure Class B while preserving a respectable efficiency boost over Class A.

Why Linearity Matters

Linear operation means the output voltage waveform faithfully follows the input envelope. For amplitude‑modulated modes such as SSB, the audio information is encoded directly in the amplitude of the carrier. Any distortion introduced by the final stage translates into audible artifacts, splatter, and, more importantly, regulatory non‑compliance. Because the FCC limits CB SSB to 12 W PEP, most CB builders stay well within the linear region, but the same principle applies to amateur SSB where the legal ceiling can reach 1500 W PEP.

Efficiency Considerations

Class AB typically delivers 50‑70 % efficiency at rated output. That range is sufficient for most portable or mobile installations where heat dissipation must be managed but where a modest power supply is acceptable. The trade‑off is a slightly higher quiescent current, which means a larger bias network and more careful thermal design.

CLASS C: MAXIMUM EFFICIENCY FOR NON‑LINEAR MODES

Class C pushes the conduction angle well below 180°, often down to 90° or less. The device conducts only during the peak of the RF cycle, which means the average power drawn from the supply is dramatically lower while the peak output can be very high. This makes Class C the champion of efficiency, routinely achieving 75‑90 %.

When Efficiency Trumps Linearity

Modes that do not rely on amplitude fidelity—such as CW (continuous wave) and FM (frequency modulation)—can tolerate the severe distortion inherent in Class C operation. In CW, the carrier is simply turned on and off; the envelope shape is irrelevant. FM varies the instantaneous frequency, not the amplitude, so a non‑linear amplifier can be used provided a suitable linearizer or frequency‑stabilizing network follows the final stage.

Practical Implications for CB and Ham

For CB operators limited to 4 W AM or 12 W PEP SSB, the efficiency gains of Class C are rarely worth the loss of linearity, especially since external linear amplifiers are prohibited. However, amateur operators running high‑power CW or FM on bands where 1500 W PEP is permissible often favor Class C to keep power consumption and cooling requirements manageable.

CHOOSING THE RIGHT AMPLIFIER FOR YOUR BUILD

The decision matrix is less about “which class is better” and more about aligning the amplifier’s intrinsic traits with your operating environment.

Mode Compatibility

• SSB / AM / Data modes: Require linearity → Class AB (or Class A for ultra‑low distortion).
• CW / FM / Digital voice (where envelope is not amplitude‑critical): Can accept non‑linear output → Class C, often with a post‑amp linearizer for FM.

Installation Constraints

• Power budget: Mobile rigs with limited battery capacity benefit from Class C’s higher efficiency.
• Cooling capacity: Class AB generates more heat per watt of output; ensure adequate heat sinking or forced air.
• Space: Class C designs can be more compact because lower dissipation allows smaller heatsinks.

Regulatory Landscape

CB operators must remember that external linear amplifiers are not legal under FCC Part 95. Even if a Class AB build could push beyond 12 W PEP, doing so would violate the rules. Amateur operators, on the other hand, are free to use high‑power amplifiers up to the limits of their license class, but the FCC still requires “the minimum power necessary to carry out the communication.” This principle nudges many hams toward modest‑power Class AB builds for everyday use, reserving high‑efficiency Class C rigs for special occasions like contests or field operations.

QUICK SANITY CHECKLIST

• ✓ Verify the intended mode: linearity‑critical (SSB/AM) or not (CW/FM).
• ✓ Confirm legal power limits for your band and license class.
• ✓ Assess power‑supply capacity and cooling provisions.
• ✓ Choose bias network design appropriate to the class (stable bias for AB, robust shutdown for C).
• ✓ Include protection circuits: over‑current, over‑voltage, and thermal shutdown.
• ✓ Plan for proper grounding and shielding to avoid RF feedback.

By matching the amplifier class to your mode, power constraints, and regulatory environment, you can build a reliable, efficient transmitter that respects both the art of radio and the law.

Educational reference only. Always verify all values independently against your specific device datasheet and build documentation. Working on RF and high-voltage equipment carries real risk.

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