RF power amplifiers shape signal strength, efficiency, and clarity in communication systems. The class of operation, which determines how the active device conducts current during the signal cycle, has a huge impact on performance.
Class A, B, AB, and C amplifiers all balance efficiency and linearity a bit differently. Picking the right one is crucial to meet your system’s requirements.
To really understand these classes, you need to know how conduction angle, biasing, and waveform shape affect both power output and signal distortion.
Class A delivers the highest linearity, but its efficiency is pretty low. Class B and C boost efficiency, but you’ll see more distortion. Class AB lands somewhere in the middle, so it’s often the go-to when you want both performance and power savings.
When you line up these amplifier classes side by side, you start to see where each one fits best. Some are great for high-fidelity signal paths, others for energy-efficient transmitters. Understanding these differences lets engineers design amplifiers that meet their system’s demands without making unnecessary sacrifices.
Fundamentals of Amplifier Classes
Amplifier classes define how a power amplifier’s active device conducts current during the input signal cycle.
They directly influence efficiency, linearity, and what kind of RF power applications the amplifier suits.
Overview of Amplifier Classes
You’ll usually see amplifier classes labeled as A, B, AB, and C.
These letters describe the conduction angle—basically, how much of the input waveform the output device conducts current for.
- Class A: 360° conduction (full cycle)
- Class B: 180° conduction (half cycle)
- Class AB: Between 180° and 360°
- Class C: Less than 180°
Class A gives you the highest linearity, but efficiency takes a hit.
Class C is the king of efficiency, but it introduces a lot of distortion unless you’re working with constant-envelope signals.
Classes B and AB try to balance these trade-offs.
Key Differences Between Class A, B, AB, and C
The big differences come down to biasing, efficiency, and signal distortion.
Biasing sets the transistor’s operating point on its current-voltage curve, and that affects how much of the waveform gets reproduced.
| Class | Conduction Angle | Efficiency (Typical) | Linearity |
|---|---|---|---|
| A | 360° | 25–50% | High |
| B | 180° | ~78% max | Low |
| AB | 180–360° | 35–55% | Medium |
| C | <180° | Up to ~80% | Very Low |
Class A draws DC current all the time, even when there’s no input.
Class B and C barely draw any current without a signal, so they’re more efficient but less linear.
Class AB sits in between, cutting down on the crossover distortion you get in Class B and improving efficiency over Class A.
Role of Amplifier Classes in RF Systems
RF power applications depend on the amplifier class you pick. Modulation type, efficiency needs, and linearity requirements all matter.
Class A pops up in small-signal RF stages where you can’t tolerate distortion.
Class AB is everywhere in cellular base stations and broadband transmitters because it balances performance well.
You’ll see Class B and C in high-power, narrowband RF transmitters. If your modulation can handle distortion (like FM or some digital modes), you can take advantage of their efficiency.
Engineers often use matching networks and filtering to clean up harmonics and make sure the output meets RF standards.
Class A Amplifier Operation
A Class A amplifier uses biasing that keeps the active device conducting for the whole cycle of the input signal.
You get a clean, undistorted output, but you also get high DC power consumption and pretty lousy efficiency compared to other classes.
People usually pick Class A when they care most about signal fidelity.
Principle of Operation
In Class A operation, the transistor or active device stays in the active region for the full 360° of the input waveform.
The bias point usually sits right in the middle of the device’s current-voltage curve. That way, the output can swing equally in both directions without clipping.
You can use a resistive or inductive load. Resistive biasing limits your output swing to the supply voltage, but inductive biasing lets you swing wider, which helps efficiency a bit.
Because current flows all the time, even with no input, the device gets hot. That constant conduction is why efficiency stays low.
Linearity and Fidelity
Class A amplifiers are prized for their high linearity. The output mirrors the input signal’s shape with very little harmonic distortion.
That’s why people use them where amplitude and phase must stay true—think high-quality audio stages or linear RF amplification for amplitude-sensitive modulation.
The linear transfer characteristic comes from staying well within the active region of the device. If you avoid cutoff and saturation, distortion stays low across the whole signal range.
But this linearity isn’t free. The amp always draws current, no matter the input level. That trade-off keeps them out of power-sensitive systems.
Efficiency and Power Consumption
Amplifier efficiency in Class A stages is low because DC power flows constantly.
Here’s what you can expect:
| Biasing Method | Theoretical Max Efficiency |
|---|---|
| Resistive Load | ~25% |
| Inductive Load | ~50% |
Even with small signals, the device draws almost as much DC current as it does at higher levels.
That steady DC power consumption means you need bigger heat sinks and deal with more heat. Sure, efficiency is poor, but at least the load and impedance stay predictable, which simplifies design and adds stability—especially in broadband and high-frequency RF work.
Class B Amplifier Characteristics
A Class B amplifier conducts current for just half the input signal cycle. This boosts efficiency compared to Class A.
The transistor gets biased near cutoff, so idle current drops. But you’ll also see distortion at low signal levels.
Working Principle
In a Class B amp, each output device conducts for 180° of the input waveform.
The transistor sits idle with almost zero collector current when there’s no signal.
A positive half-cycle turns one transistor on to deliver the output.
When the negative half-cycle comes along, that transistor shuts off, and its partner takes over.
This approach slashes DC power consumption since no quiescent current flows without a signal.
But when devices switch over at zero crossing, you can get non-linearities.
People often pick Class B amplifiers when high efficiency matters more than perfect linearity.
The maximum theoretical efficiency is about 78.5%, so these amps work well in RF transmitters with constant envelope modulation.
Push-Pull Configuration
Most Class B amps use a push-pull setup to get the whole waveform.
Two transistors (or tubes) team up: one handles the positive half, the other the negative.
This cancels out even-order harmonics, which helps clean up the signal compared to a single-ended design.
It also lets you double the voltage swing across the load.
You’ll see complementary NPN and PNP devices in bipolar designs, or N-channel and P-channel in FET setups.
A phase splitter circuit or transformer drives each transistor with the right phase.
Sharing the load between two devices spreads out heat and lets the amp push higher output power without overheating.
Crossover Distortion
Crossover distortion pops up when neither transistor conducts right around zero volts.
Each device needs a small base-emitter or gate-source voltage before it turns on, so there’s this tiny gap where the output isn’t accurate.
In RF systems, that can cause unwanted harmonics and mess with modulation quality.
To fix this, engineers bias both transistors just above cutoff, nudging the amp into Class AB territory.
A small quiescent current flows, shrinking the dead zone and boosting linearity while still keeping efficiency higher than Class A.
Class AB Amplifier Insights
A Class AB amplifier tries to get the best of both worlds: the linearity of Class A and the efficiency of Class B.
It sets the bias somewhere between full conduction and cutoff, trimming idle current but keeping distortion low during crossover.
That’s why it’s a favorite in RF systems where you need both efficiency and signal integrity.
Hybrid Operation and Biasing
Class AB operation borrows from both Class A and B. Each output device conducts for a bit more than half the input cycle, which cuts down on crossover distortion compared to pure Class B.
The bias point is set so a small DC bias current flows even without any RF input. That keeps transistors active near the zero-crossing point.
Engineers use resistor networks, diodes, or active bias circuits to set this up. The trick is to keep quiescent current stable, even as temperature or device characteristics change.
Good bias control stops idle power from getting out of hand, and it ensures a smooth handoff between devices in a push-pull stage. This matters even more at high frequencies, where switching imperfections can create nasty harmonics.
Efficiency Versus Linearity
A Class AB power amp usually hits 35–55% efficiency under good load conditions.
That’s better than Class A’s 25–50%, but not quite up there with Class C’s 70–80%.
Linearity stays close to what you get in Class A—especially at moderate output.
But efficiency drops if your signal has a high peak-to-average power ratio (PAPR), since bias current still flows during quiet times.
Designers sometimes trade a bit of linearity for better efficiency by setting the bias closer to Class B. That increases conduction angle efficiency, but if you’re not careful, crossover distortion can sneak back in—unless you use feedback or compensation.
The dynamic loadline in RF operation might not match DC conditions because of parasitics, so you’ve got to measure and tweak things carefully to get both efficiency and low distortion.
Common Applications
Class AB amplifiers show up in cellular base stations, two-way radios, and audio-frequency RF drivers where you need moderate efficiency and low distortion.
They’re also used in push-pull setups for broadband RF transmitters. That balanced design cancels even-order harmonics and helps keep the spectrum clean.
In satellite communications and broadcast transmitters, Class AB stages often drive final stages that are more efficient, handing off a clean signal for further amplification.
You won’t see this class much in constant-envelope modulation schemes—Class C or D do better there. But for amplitude-varying formats like QAM and PSK, where linearity really matters, Class AB is hard to beat.
Class C Amplifier Performance in RF Systems
Class C amplifiers run with a short conduction period and aim for high efficiency in generating RF power.
They sacrifice linearity for efficiency, so they fit best in transmitters where you can fix distortion or just don’t care much about it.
Operating Principle
A Class C amplifier conducts for less than half the input signal cycle, usually well below 180°.
The transistor or active device gets biased so it stays off most of the time, turning on only for a small chunk near the signal peak.
This biasing puts the quiescent point below cutoff, which cuts down average current flow.
Designers usually use a tuned LC circuit as the output network. This circuit rebuilds the sinusoidal waveform from the narrow current pulses and filters out unwanted harmonics.
If you leave out this tuned load, the output is full of distortion and pretty much useless for most RF jobs.
Because conduction time is so short, the device dissipates less average power than in Class A or B. But peak currents during conduction get high, so you need to pick components that can handle the voltage and current stress.
Efficiency and Distortion
You can push Class C amplifiers to drain efficiencies of about 70–85% in real-world designs, and the theory says you could almost hit 100%. That happens because the device conducts for only a short time, so voltage and current overlap a lot less.
But here’s the catch: the output gets pretty distorted. The device spits out a string of pulses, not a smooth waveform, so there’s a lot of harmonic content.
A resonant load network steps in to filter most of those harmonics, and you end up with a clean RF signal at your target frequency. In narrowband setups, this works well, but in broadband systems, you’ll see too much distortion unless you add more linearization tricks.
Since these amplifiers run so efficiently, they don’t get as hot. You can use smaller heat sinks and shrink down your transmitter designs. That’s a big reason why people like them for high-power RF stages.
Typical RF Applications
You’ll find Class C amplifiers all over RF transmitters where the signal’s envelope stays constant, like in FM, CW, or some digital modulation schemes that don’t care about amplitude accuracy.
They’re a staple in high-power AM transmitter stages, as long as you have linear drivers handling the modulation first.
Some typical applications are:
- Broadcast transmitters for FM and AM
- Two-way radios and marine comms
- RF power stages in radar transmitters
- Industrial RF heating and plasma generators
You almost never see them for audio or broadband modulation. The distortion is just too much to fix without getting overly complicated. In RF, though, the efficiency usually outweighs that flaw if your modulation type allows it.
Comparative Analysis and Design Considerations
Every amplifier class strikes its own balance between efficiency, linearity, and how tricky it is to design. You have to pick the right biasing and loadline for each, and that choice shapes how they perform in wireless systems or with different modulation schemes.
Efficiency Comparison of Amplifier Classes
Efficiency basically tells you how much of your DC input power turns into RF output.
Typical maximum efficiencies under ideal conditions:
| Class | Resistive Load | Inductive Load |
|---|---|---|
| A | ~25% | ~50% |
| B | ~78% | ~78% |
| AB | 50–70% | 50–70% |
| C | 70–85% | 70–85% |
Class A runs for the whole input cycle, so it wastes a lot of DC power if the signal’s weak. Class B only conducts for half the cycle, which cuts down idle current. Class AB lands somewhere in between—better than Class A for efficiency, but not quite up there with Class C.
Class C takes the crown for efficiency by only conducting for less than half the cycle, though the signal gets pretty mangled. That’s why people use it for constant-envelope signals.
Linearity and Distortion Trade-Offs
Linearity is all about how closely the output tracks the input, without adding junk frequencies.
Class A nails linearity, since the transistor stays active the whole time. You get minimal distortion, so it’s perfect for amplitude-sensitive stuff like QAM or AM.
Class B, on the other hand, struggles with crossover distortion when the device switches between the two halves of the waveform. Class AB helps by biasing the transistor just above cutoff, so the signal comes out cleaner.
Class C distorts the amplitude a lot but keeps phase and frequency info intact. That’s fine for FM, PM, or other constant-envelope modes. You can filter out harmonics, but for signals that change amplitude, the in-band distortion won’t go away.
Biasing and Loadline Implications
Biasing sets your transistor’s resting spot on the DC loadline—basically, it defines how voltage and current behave when nothing’s happening.
With Class A, you put the quiescent point right in the middle, so you get the widest voltage swing but also the most idle current. That kills efficiency.
Class B biasing drops the quiescent point to cutoff, which saves on idle current but brings in crossover distortion. Class AB sits just above cutoff, giving you a middle ground on both distortion and efficiency.
Inductive loads let you swing the voltage wider than resistive ones, which helps Class A amps run more efficiently. But at high frequencies, parasitic capacitances and device delays can shift the AC loadline, so you have to get the matching network just right.
Selecting Amplifier Classes for Wireless Communication
You’ll find that the best amplifier class really depends on your modulation type, how much efficiency you need, and just how much distortion you’re willing to put up with.
- Class A: People usually pick this for small-signal amplifiers or when they need high linearity in wideband systems.
- Class AB: You’ll see this a lot in cellular base stations, where folks want a balance—moderate efficiency and low distortion.
- Class B: Some push-pull designs still use this, but honestly, it’s not that common in modern RF because of the distortion.
- Class C: If you’re after high efficiency and you’re working with constant-envelope modulation like FM or GMSK, this is the go-to.
Designers really need to think about impedance stability too. With Class A, you get nearly constant input and output impedance, no matter the signal level, which makes broadband design a lot easier.
On the other hand, Classes B, AB, and C have impedance that changes with input level. That can bump up the risk of oscillation and makes them tougher to use at very high frequencies.