Introduction to Class C Amplifiers

Class C amplifiers are a type of electronic amplifier where the active element (usually a transistor or vacuum tube) conducts for less than 50% of the input signal cycle. They are one of the most efficient types of RF power amplifiers but have higher distortion than Class A or B designs.

Key Characteristics of Class C Amplifiers

• Conduction angle less than 180° (50% duty cycle)
• High efficiency, typically 65-85%
• Significant distortion of the input signal
• Require tuned load for proper operation
• Used in RF transmitters and some switch-mode power supplies

How Class C Amplifiers Work

In a Class C amplifier, the active device is biased so that it only conducts during a small portion of the positive input signal cycle. This is known as the conduction angle. A conduction angle of 180° corresponds to a 50% duty cycle.

Conduction Angle and Efficiency

The table below shows the theoretical maximum efficiency for different conduction angles in a Class C amplifier:

Conduction Angle (degrees)	Maximum Theoretical Efficiency
180	50%
150	65.7%
120	78.5%
90	89.6%
60	97.5%

In practice, efficiencies of 65-85% are typical for real Class C designs. Smaller conduction angles yield higher efficiency but also increase distortion.

Tuned Load and Harmonics

For a Class C amplifier to work properly and achieve high efficiency, the load impedance must be tuned to resonate at the fundamental frequency of the input signal. This tuned load filters out the harmonics generated due to the high distortion.

A parallel resonant tank circuit is commonly used as the tuned load. It presents a high impedance at the resonant frequency and a low impedance at harmonic frequencies.

Applications of Class C Amplifiers

RF Power Amplifiers

The primary application of Class C amplifiers is in radio frequency (RF) power amplifier stages, especially the final power amplifier before the antenna. Their high efficiency makes them well-suited for battery-powered and high power transmitters.

Some examples of where Class C RF amplifiers are used:

• Handheld radios and walkie-talkies
• CB radios
• Aircraft and marine radio transmitters
• Broadcast transmitters
• Radar systems
• Induction heating
• Plasma generators

Comparison to Other Amplifier Classes for RF

Here is a comparison of Class C to other common amplifier classes used for RF power amplifiers:

Amplifier Class	Efficiency	Linearity	Typical Applications
Class A	25-50%	Excellent	Low-level stages, drivers
Class B	50-70%	Good	Driver stages
Class AB	35-60%	Very good	Linear power amplifiers
Class C	65-85%	Poor	High power RF and microwave amplifiers
Class D	80-95%	Poor	Audio power amplifiers
Class E/F	80-95%	Poor	VHF and UHF power amplifiers

Class C gives the highest efficiency of the linear amplifier classes, at the cost of very high distortion. Class E and F amplifiers can achieve even higher efficiencies for certain frequencies by using switching techniques.

Switched-Mode Power Supplies

Another application of the Class C amplifier is in DC-DC converters and switch-mode power supplies. Here, the active device is operated as a switch and the tuned load is replaced by an inductor and capacitor network that smooths the output into DC.

While Class D is more commonly used in modern switched-mode power supplies, Class C can be used in some topologies like:

• Boost converters
• Flyback Converters
• Ringing choke converters

The high efficiency and simple circuitry of Class C makes it attractive for these applications, especially at high frequencies and low power levels.

Designing Class C Amplifiers

Biasing and Drive Level

Proper biasing is critical to the operation of a Class C amplifier. Unlike Class A or B designs that have the transistor conducting with no input signal, Class C requires the transistor to be cut off with no drive.

There are two main ways to achieve Class C bias:

1. Using a negative bias voltage to set the quiescent point below the threshold
2. Using a high input drive level to overcome the cutoff of a transistor biased for Class B

The first method allows more control over the conduction angle. The second is simpler but the conduction angle depends on the drive level.

Tuned Load Design

The tuned load is the other key part of a Class C amplifier. It must present the correct load impedance at the fundamental frequency to achieve high efficiency and filter out harmonics.

Some guidelines for Class C tuned load design:

• Use a parallel resonant circuit with high Q (low resistance)
• Tune the load to resonate at the fundamental frequency
• Use a coupling network to match the load impedance to the transistor
• Provide sufficient harmonic attenuation to meet emission requirements
• Consider transmission line effects for high frequencies

Simulation tools like SPICE are very helpful for optimizing the tuned load design and analyzing the amplifier’s performance.

Transistor Selection

Selecting the right transistor is important for a successful Class C design. Some key parameters to consider:

• Maximum collector/drain voltage and current ratings
• Transition frequency (ft) and maximum frequency of oscillation (fmax)
• Input and output capacitances
• Thermal resistance and power dissipation capability
• Linearity (for amplitude modulation)
• Cost and availability

Bipolar junction transistors (BJTs) and field-effect transistors (FETs) like MOSFETs and GaN HEMTs are commonly used in modern Class C amplifiers. Vacuum tubes are still used for very high power levels.

Limitations and Trade-offs

While Class C amplifiers offer high efficiency, they have some significant limitations and trade-offs that must be considered.

Linearity and Distortion

The high distortion of Class C amplifiers makes them unsuitable for amplifying amplitude-modulated signals without correction. The conduction angle must be greater than 120° to pass any amplitude modulation.

Techniques like feed-forward and predistortion can be used to improve linearity at the cost of added complexity. Class C is better suited for constant-envelope modulation schemes like FSK and FM.

Frequency Range

The maximum operating frequency of Class C amplifiers is limited by the transistor’s ft and fmax. Parasitic reactances and package inductance also become significant at VHF and above.

Careful PCB layout and transistor packaging (chip, SMD, etc.) are needed for high frequency operation. Class E and F amplifiers are preferred to Class C at UHF and microwave frequencies.

Drive Power

Class C amplifiers require a high drive level to achieve a low conduction angle and high efficiency. The required drive can be several times the transistor’s maximum output.

This increases the power consumption and complexity of the preceding driver stage. High drive can also overstress the transistor and cause reliability issues.

Narrow Bandwidth

The tuned load of a Class C amplifier limits the bandwidth over which it can operate efficiently. The bandwidth depends on the Q factor of the resonant circuit.

Wider bandwidth can be achieved with a lower Q, but this reduces efficiency. Techniques like stagger tuning and transmission line transformers can help extend the bandwidth.

Class C Amplifier Variants

Several variants of the Class C amplifier have been developed to improve efficiency, linearity, or bandwidth. Some examples include:

Doherty Amplifier

The Doherty amplifier uses two transistors, a carrier and a peak amplifier, to improve efficiency at low signal levels. The carrier is biased in Class B or AB while the peak is biased in Class C.

At low levels, only the carrier amplifier operates. At higher levels, the peak amplifier turns on and modulates the load impedance seen by the carrier, allowing it to maintain efficiency.

Chireix Outphasing Amplifier

The Chireix outphasing amplifier, also known as LINC (linear amplification with nonlinear components), uses two Class C amplifiers driven with phase-modulated signals. The outputs are combined with a special load network.

By varying the relative phase of the two amplifiers, the output amplitude can be modulated while maintaining high efficiency. This allows linear amplification of AM signals with Class C efficiency.

Envelope Elimination and Restoration (EER)

EER is a technique that separates the amplitude and phase information of a modulated signal. A Class C amplifier is used to efficiently amplify the phase-modulated carrier. The amplitude modulation is restored by varying the supply voltage to the Class C stage.

EER can achieve high efficiency for linear amplification but requires a wide-bandwidth, high-efficiency modulator for the supply. It has been used in some cellular base station transmitters.

FAQ

What is the efficiency of a Class C amplifier?

Class C amplifiers can achieve theoretical efficiencies up to 100% for very low conduction angles. Practical Class C designs have efficiencies in the range of 65% to 85%, higher than other linear amplifier classes.

What are Class C amplifiers used for?

The main application of Class C amplifiers is in RF power amplifier stages, especially the final stage before the antenna. They are also used in some switched-mode power supplies. Class C is well-suited for applications that require high efficiency and can tolerate significant distortion.

How does a Class C amplifier work?

In a Class C amplifier, the transistor is biased so that it only conducts for less than half of the input signal cycle. This allows high efficiency but introduces a lot of distortion. A tuned load is used to filter out the distortion and present the correct impedance to the transistor.

Can Class C amplifiers be used for audio?

No, Class C amplifiers are not suitable for audio applications due to their high distortion. The tuned load also limits the bandwidth too much for audio. Class AB, D, G, and H amplifiers are commonly used for audio instead.

What are some advantages and disadvantages of Class C?

The main advantages of Class C are high efficiency, simple circuitry, and good performance at high frequencies. The main disadvantages are high distortion, narrow bandwidth, and the need for a high drive level. Class C is not suitable for linear amplification without special techniques.

Conclusion

Class C amplifiers offer a unique combination of high efficiency and simplicity that makes them valuable for many RF power amplifier applications. While their high distortion and narrow bandwidth limit their use for audio and some linear RF systems, Class C remains a key part of modern wireless infrastructure.

Advances in transistor technology, especially wide bandgap semiconductors like gallium nitride (GaN), are pushing Class C efficiency even higher. At the same time, techniques like EER and outphasing are enabling linear operation with Class C efficiency.

As wireless technology continues to evolve, it’s likely that Class C and its variants will remain important for a long time to come. Understanding the principles and trade-offs of Class C is essential for any RF engineer.