2.8 Amplifier Classes and Power Amplifiers (Class A, B, C, and AB)

2.8 Amplifier Classes and Power Amplifiers (Class A, B, C, and AB)

RF Power Amplifiers (PAs) take low-level RF energy generated by synthesisers and modulators and boost it to the power levels required for antenna radiation (up to 100 W pX (peak envelope power) for emission modes J3E and R3E, or 30 W pY (mean power) in any other mode, for an amateur standard station under the Radiocommunications (Amateur Stations) Class Licence 2023). The operating characteristics of an RF power amplifier are determined primarily by its operating class, which is set by the DC bias voltage applied to the active device (bipolar transistor, MOSFET, or vacuum tube).

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1. Conduction Angle and Operating Classes

The fundamental metric distinguishing amplifier classes is the Conduction Angle ($\theta$)—the portion of the 360-degree input AC signal cycle during which current flows through the amplifying device.

Comprehensive Amplifier Class Comparison Table

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2. Detailed Breakdown of Amplifier Operating Classes

Class A Amplifiers

In a Class A amplifier, DC bias is set precisely in the middle of the linear operating range. Current flows through the active device for the entire $360^\circ$ of the input sinusoidal cycle.

• Advantages: Superior linearity and lowest harmonic distortion. It amplifies weak signals without changing signal shape.
• Disadvantages: Highly inefficient. The device draws full DC bias current constantly, even when zero input RF signal is present. Heavy heat generation limits its use to low-power stages (preamps, buffer stages).

Class B Amplifiers

In Class B, the device is biased exactly at cutoff ($V_{be} \approx 0.7\text{V}$ for silicon BJT). Current conducts for exactly $180^\circ$ (half of the AC cycle).

• Efficiency: Achieves a maximum theoretical efficiency of $\frac{\pi}{4} \approx 78.5%$.
• Push-Pull Configuration: A single transistor in Class B would clip off the negative half of the input wave. To reconstruct a clean sine wave, two transistors are arranged in Push-Pull: one transistor amplifies the positive half-cycle, and the second amplifies the negative half-cycle.
• Crossover Distortion: Slight non-linearities near the zero-crossing point where one transistor turns off and the other turns on create crossover distortion.

Class AB Amplifiers (Linear PAs)

Class AB is the workhorse of amateur radio HF linear amplifiers. By biasing the active devices slightly above cutoff, a small quiescent idle current flows. Conduction occurs for slightly more than $180^\circ$ (typically $190^\circ$ to $210^\circ$).

• Elimination of Crossover Distortion: The small forward bias eliminates crossover distortion while maintaining high collector/drain efficiency ($50%$ to $70%$).
• Essential for SSB: Single Sideband (SSB) signals vary constantly in amplitude. Passing an SSB signal through a non-linear amplifier destroys the speech envelope, creating severe splatter. Therefore, A linear amplifier is required for SSB - Class A is linear but inefficient, so Class AB is used in practice for all HF linear power amplifiers.

Class C Amplifiers

In Class C, the amplifier is biased deeply below cutoff. Current flows in short, high-peak pulses for less than $180^\circ$ of the input cycle (typically $90^\circ$ to $120^\circ$).

• High Efficiency: Because the device is turned OFF for most of the cycle, efficiency is exceptionally high ($75%$ to $85%+$).
• Severe Non-Linearity: Class C operation completely destroys amplitude envelope information. It CANNOT be used for SSB or AM signals.
• Flywheel Effect: When amplifying FM or CW signals, the short current pulses strike a high-$Q$ resonant LC tank circuit in the amplifier output. The tank circuit acts like a mechanical flywheel, ringing smoothly to reconstruct a clean sinusoidal RF output wave.

Switching Amplifiers (Class D, E, and F)

Modern digital transmitters increasingly use switching classes. Rather than operating transistors as variable resistors in linear regions, transistors operate as high-speed electronic switches (either fully ON with zero voltage drop, or fully OFF with zero current flow). Power dissipation ($P = V \cdot I$) drops virtually to zero, yielding theoretical efficiencies near $100%$.

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3. Drive Requirements and Neutralisation

RF Drive Power and Overdriving Risks

Drive Power is the RF input power required to excite a power amplifier stage to its full rated output. Overdriving a linear power amplifier (supplying excess drive power) forces active transistors into saturation. This causes flat-topping (clipping of peak RF amplitudes), generating heavy odd-order intermodulation distortion and causing severe adjacent-channel splatter.

Parasitic Oscillations and Neutralisation

High-power RF amplifiers are prone to self-oscillation at unwanted parasitic frequencies due to internal feedback capacitance within active devices (such as grid-to-plate capacitance $C_{gp}$ in vacuum tubes, or gate-to-drain capacitance $C_{gd}$ in power MOSFETs).

This internal capacitance feeds a portion of the amplified output signal back to the input in phase (positive feedback), transforming the power amplifier into an unintended, uncontrollable oscillator.

Neutralisation is the process of deliberately feeding back a small portion of the output RF voltage to the input circuit 180 degrees out of phase (negative feedback) via an adjustable Neutralising Capacitor ($C_n$). This out-of-phase signal exactly cancels the internal positive feedback, stabilising the amplifier against parasitic self-oscillation.