Study guide
2.8 Amplifier Classes and Power Amplifiers (Class A, B, C, and AB)
ACMA Amateur Radio Standard Recognition Certificate - Theory Exam
Track your progress as you read
Finished sections get a check mark.
1 Chapter 1: Electrical Principles and Components
1.1 Mathematics, Units and Metric Prefixes 1.2 DC Circuits, Voltage, Current, and Ohm's Law 1.3 Power, Energy, and Resistance Calculations 1.4 Resistor Colour Code and Component Symbol Identification 1.5 Capacitors: Construction, Ratings and Safety 1.6 Inductors: Construction, Factors Affecting Inductance and Combinations 1.7 Capacitor and Inductor Combination Calculations 1.8 AC Circuits, Frequency, Wavelength, and RMS Values 1.9 Reactance and Impedance in AC Circuits 1.10 Resonance, Q Factor and Tuned Circuits 1.11 Transformers: Turns Ratio, Impedance Matching and Eddy Currents 1.12 Rectifiers, PIV and Power Supply Smoothing 1.13 Active Components: Semiconductors, Diodes, and Transistors 1.14 Transistor and FET Symbols, Configurations and External Behaviour
2 Chapter 2: Transmitters and Receivers
2.1 Superheterodyne Receiver Architecture and Block Diagrams 2.2 Frequency Converters, IF Amplifiers and Transceiver Sharing 2.3 Receiver Performance: Sensitivity, Selectivity, and AGC 2.4 Signal-to-Noise Ratio and Receiver Noise Performance 2.5 Transmitter Architecture, Oscillators and Frequency Synthesisers 2.6 Transmission Quality: Frequency Drift, Parasitic Oscillation and Over-Modulation 2.7 Modulation Techniques: AM, FM, and SSB 2.8 Amplifier Classes and Power Amplifiers (Class A, B, C, and AB) 2.9 Modulation, Duty Cycle and Rated Transmitter Power 2.10 Transceiver Control: ALC, Squelch, and Digital Modes 2.11 Digital Transmission Modes and Bandwidth
3 Chapter 3: Transmission Lines and Antennas
3.1 Transmission Lines: Coaxial, Twin-Lead, and Characteristic Impedance 3.2 Standing Wave Ratio (SWR) and Reflection Loss 3.3 Impedance Matching, Baluns, and Antenna Tuning Units (ATUs) 3.4 Resonant Antennas: Dipoles, Verticals, and End-Fed Antennas 3.5 Angle of Radiation, Antenna Height and Current Distribution 3.6 Directional Antennas: Yagis, Gain and Radiation Patterns 3.7 Effective Radiated Power: System Gains and Losses
4 Chapter 4: Radio Wave Propagation
4.1 Electromagnetic Waves: Velocity, Wavelength and Polarisation 4.2 Ground Wave, Direct Wave and Line-of-Sight Propagation 4.3 Ionospheric Layers (D, E, F1, F2) and Skywave Propagation 4.4 Critical Frequency, Maximum Usable Frequency (MUF), and Fading 4.5 Solar Activity, Sunspot Cycles, and VHF Anomalous Propagation 4.6 Seasonal and Diurnal Effects on Band Selection
5 Chapter 5: Electrical and RF Measurements
5.1 Multimeter Operation: Voltage, Current, and Resistance Measurements 5.2 RF Power and SWR Measurement (PEP vs. Average Power) 5.3 Frequency Counters, Dummy Loads, and Test Equipment 5.4 Frequency Standards, Crystal Calibrators and Standard-Frequency Transmissions
6 Chapter 6: Interference and Electromagnetic Compatibility (EMC)
6.1 Harmonic Emissions, Spurious Signals, and Transmitter Filtering 6.2 How Interference Enters Television and Broadcast Receivers 6.3 Masthead Amplifier Overload and Audio-Stage Demodulation 6.4 Electromagnetic Interference (EMI), Breakthrough, Ferrites, and EMC 6.5 Filter Types, Response Curves and EMC Filter Selection 6.6 EMC Good Practice: Minimum Power, Balanced Antennas and Feedline Routing 6.7 Vehicle EMC and Resolving Community Interference Complaints
7 Chapter 7: Safety and Environmental Hazards
7.1 Mains Voltage Safety, Earthing, and High-Voltage Capacitors 7.2 Equipment Approval, Mains Wiring and Fuse Practice 7.3 Station Layout, Cabling and Emergency Shut-Off 7.4 Lightning Protection and Station Security 7.5 Electric Shock: Prevention, Rescue and Emergency Response 7.6 Battery, Chemical and Hearing Safety in the Amateur Station 7.7 RF Exposure: EMR Hazards and Safe Distance 7.8 Antenna Erection, Siting and Power Line Safety
2.8 Amplifier Classes and Power Amplifiers (Class A, B, C, and AB)
Key Takeaways Explain This Topic AI
Amplifier class is defined by conduction angle: portion of 360° input cycle during which active devices conduct current.
Class A conducts for full 360°, providing maximum linearity but low theoretical efficiency (25% to 50%).
Class B conducts for exactly 180° at cutoff, offering 78.5% efficiency; requires push-pull configuration to prevent severe distortion.
Class AB conducts between 180° and 360° to eliminate crossover distortion while achieving 50-70% efficiency, and is the class used in practice for linear SSB amplification.
Class C conducts for less than 180°, achieving 75-85% efficiency; highly non-linear, suitable strictly for constant-envelope signals (CW, FM).
Last updated: July 2026
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).
1. Conduction Angle and Operating Classes
The fundamental metric distinguishing amplifier classes is the Conduction Angle ( θ \theta θ ) —the portion of the 360-degree input AC signal cycle during which current flows through the amplifying device.
Comprehensive Amplifier Class Comparison Table
Class Conduction Angle ( θ \theta θ ) DC Bias Point Max Theoretical Efficiency Operating Linearity Common Amateur RF Applications
Class A 360 ∘ 360^\circ 36 0 ∘ (Full cycle) Centre of linear region 25% (Resistive); 50% (Inductive) Highest / Perfect Linearity Receiver LNAs, low-level driver stages
Class B 180 ∘ 180^\circ 18 0 ∘ (Half cycle) Set exactly at cutoff 78.5% ( π 4 \frac{\pi}{4} 4 π ) Moderate (Push-Pull required) Audio PAs, RF push-pull drivers
Class AB 180 ∘ < θ < 360 ∘ 180^\circ < \theta < 360^\circ 18 0 ∘ < θ < 36 0 ∘ Slightly above cutoff 50% to 70% High Linearity SSB voice, AM, digital linear HF PAs
Class C < 180 ∘ < 180^\circ < 18 0 ∘ (Short pulses) Deeply below cutoff 75% to 85%+ Non-linear FM voice, CW transmitters only
- Class D/E Switching (ON/OFF) Switched mode 90% to 98% Non-linear (Pulse) Modern QRP, digital transmitters
- 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 ∘ 360^\circ 36 0 ∘ 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 b e ≈ 0.7 V V_{be} \approx 0.7\text{V} V b e ≈ 0.7 V for silicon BJT). Current conducts for exactly 180 ∘ 180^\circ 18 0 ∘ (half of the AC cycle).
Efficiency: Achieves a maximum theoretical efficiency of π 4 ≈ 78.5 % \frac{\pi}{4} \approx 78.5\% 4 π ≈ 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 ∘ 180^\circ 18 0 ∘ (typically 190 ∘ 190^\circ 19 0 ∘ to 210 ∘ 210^\circ 21 0 ∘ ).
Elimination of Crossover Distortion: The small forward bias eliminates crossover distortion while maintaining high collector/drain efficiency ( 50 % 50\% 50% to 70 % 70\% 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 ∘ 180^\circ 18 0 ∘ of the input cycle (typically 90 ∘ 90^\circ 9 0 ∘ to 120 ∘ 120^\circ 12 0 ∘ ).
High Efficiency: Because the device is turned OFF for most of the cycle, efficiency is exceptionally high ( 75 % 75\% 75% to 85 % + 85\%+ 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 Q 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 ⋅ I P = V \cdot I P = V ⋅ I ) drops virtually to zero, yielding theoretical efficiencies near 100 % 100\% 100% .
- 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 g p C_{gp} C g p in vacuum tubes, or gate-to-drain capacitance C g d C_{gd} C g d 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 C_n C n ) . This out-of-phase signal exactly cancels the internal positive feedback, stabilising the amplifier against parasitic self-oscillation.