Capacitor Roles in High-Power Class E RF Generators

Radio frequency (RF) generators produce alternating current (AC) at specific frequencies ranging from kilohertz to gigahertz.

These essential subsystems power a wide range of applications, including: 

• Telecommunications: Used in signal transmission, modulation, and amplification for broadcasting and wireless communication.
• Medical Equipment: Powering MRI imaging, surgical ablation, and other procedures that require precise energy delivery.
• Industrial Processes: Support plasma etching, RF welding, heating, and other manufacturing operations that require high-frequency energy.
• Semiconductor Manufacturing: Generating plasma for etching and deposition.
• Scientific Research: Used in plasma experiments and particle acceleration.  

Among RF generation topologies, the Class E amplifier stands out as a unique case that bridges RF and power electronics by acting as a switching element rather than a linear amplifier.  

Class E RF generators are switching-mode power amplifiers that can achieve efficiencies of 85 to 95 percent. They leverage Zero-Voltage Switching (ZVS), where the switching transistor turns on when the drain voltage is at zero, to dramatically reduce switching losses and heat generation. 

Key Characteristics of Class E Designs: 

• Output power from a few watts to over one kilowatt 
• Uses a resonant load network for voltage and current waveform shaping 
• Leverages automatic load adaptation 
• Simple, robust designs with fewer external components (compared to linear amplifiers) 

Figure 1: Simplified Class E amplifier 

Capacitor Jobs in a Class E Amplifier 

A simplified Class E circuit, shown in Figure 1, shows a switch (Q1) in ZVS mode, creating a square wave that’s shaped into a near sinusoid through filtering. Two capacitors, C1 and C2, are central to the process. The shunt capacitor (C1) is connected in parallel with the transistor’s drain-source. It enables ZVS by shaping the drain voltage waveform and reduces peak voltage stress across the MOSFET during its off state. The series capacitor (C2) forms a series-tuned circuit with the load inductor, which couples the load to the transistor. This optimizes power transfer and maintains proper impedance relationships. 

Additional Capacitor Roles in RF Generator Circuits 

Beyond their function in Class E topologies, capacitors serve other critical roles in RF generator circuits. High-stability capacitors in the RF pulse generator circuit set the pulse width driving the switch. Additional tank capacitors in the RF output matching circuit form resonant circuits that determine operating frequency and Q factor. They also store and release energy during switching cycles to help maintain continuous power flow.  

Output matching capacitors help match the high impedance of the tank circuit to the standard 50Ω load, enabling efficient power transfer. In addition, they provide DC isolation, suppress unwanted harmonics, and optimize overall signal delivery to the external load. Within the DC power supply, bypass capacitors provide low-impedance paths for AC signals to ground. 

Specs to Look For 

Specifying capacitors for high-power RF generators, especially Class E topologies, requires careful attention to a specific set of performance characteristics: 

• High Voltage Ratings: Must handle peak voltages up to several kilovolts for high-power applications. 
• Ultra-Low ESR / High Q: Required to minimize loss and maintain efficiency. 
• Stability: Critical for consistent performance over temperature and voltage changes. 
• High Power Capability: Lager case sizes enable higher power handling. 

Knowles offers a range of capacitors designed to meet the demand of Class E and other high-efficiency RF generators. Explore stocked parts and custom assemblies.