RF Amplifiers: Switching to Solid State

Radio frequency (RF) has powered electronics for decades, but solid-state technology is opening up new doors. RF amplifiers offer much more control and durability than some other technologies, like magnetrons, making them a powerful solution in everything from medicine and consumer devices to industrial and military applications. Let's take a closer look at the basics of RF amplifiers and what they can do.

What Are RF Power Amplifiers?

RF power amplifiers convert low-powered RF signals into high-powered signals. As the name suggests, they operate on radio frequency, which is 3 kilohertz to 300 gigahertz, and typically drive the antenna of a transmitter. RF power amplifiers are common in telecommunication transmissions, but newer innovations make them a great fit in other applications, like cooking appliances and medical devices.

These amplifiers work across different modes, called classes, to meet the needs of various applications. These classes — such as Class A, Class B and Class AB — often correspond to different types of RF amplifiers, which we'll discuss later. The classes outline conduction angles and other behaviors regarding how the amplifier affects the circuit.

What Is a Solid-State Power Amplifier?

Traditionally, RF power amplifiers used vacuum tubes to control electron flow, but these were rather rudimentary. They don't offer much control and are mechanically fragile. Modern RF amplifiers may use metal-oxide-semiconductor field-effect transistors (MOSFETs). Laterally-diffused MOSFETs (LDMOS) are common in wireless telecommunications. MOSFETs and similar solid-state technologies have all but replaced vacuum tubes, but you'll still find tubes in some high-powered applications.

Engineers use RF power amplifiers in a wide range of electronics, from household microwaves and industrial welding to medical analysis and diagnostics. With the improved capabilities of solid-state RF amplifiers, engineers can start using them in more precise, delicate applications.

The Uses and Benefits of an RF Power Amplifier

The three main functions of an RF power amplifier are:

• Gain: If the RF signal amplitude is too small, the amplifier can increase it. This boost helps ensure the signal-to-noise ratio stays intact as it moves through the circuit. An RF amplifier can also ensure the signal amplitude matches input ranges specified by some components.
• Buffer: RF amplifiers can maintain the shape and amplitude of a signal despite changing loads. It can also help the circuit connect to difficult loads, such as those with lower impedance. A buffer amplifier can prevent distortion or fidelity impacts. 
• Driver: In many cases, the amplifier drives a low-impedance load by sourcing and sinking appropriate current at the right operating frequency. An amplifier can also drive a load if it requires a boost in power. You may need a driver if the slew rate and sink/source capabilities exceed that of RF amplifiers. 

Most applications for RF power amplifiers include driving to a high-powered source, exciting microwave cavity resonators or, the most popular option, driving transmitting antennae.

As technology advances, RF amplifiers are offering more precision and control, so users can zero in on the right areas. For example, home microwaves often give you hot spots in food, with some cold and some hot areas. Engineers can use solid-state RF amplifiers for dielectric heating, which heats on a molecular level, affecting the whole material at once. Users also get more control over frequency. Other benefits include lower voltage requirements, low noise, small form factors, better longevity and improved reliability.

We already see RF technology in telecommunications and microwave ovens, but some exciting RF amplifier uses include:

• Medical treatments: With more control and precision, RF amplifiers can help power medical devices for ablation and sterilization to heat or destroy tissue and cells. One particularly interesting application is hyperthermia treatment, which can apply heat in precise locations to target tumors and other unwanted cells. Better heating technology may also help with warming blood and organs for more successful transfusions and transplants.
• Plasma lighting: Also called light-emitting plasma (LEP), this technology offers extremely high lumen densities, delivering a significant amount of light from a tiny space. In the future, you might see them in everything from medical scopes to large spaces like parking lots and warehouses.
• Consumer products: Even the humble home microwave might get solid-state technology. While it's mostly rolling out in industrial cooking applications, we should eventually see these amplifiers in consumer-grade products to offer more even, efficient heating.
• Car ignitions: Traditionally, car engines use spark plugs to ignite fuel in the combustion chamber, but it isn't a very precise system. Radio frequency sustained plasma ignition systems (RFSIs) can improve this process, increasing efficiency, decreasing variability and reducing carbon monoxide and unburned hydrocarbon emissions.
• Microwave-assisted chemistry: Many chemical processes rely on heating, and sophisticated RF amplifiers can help provide more precise, even and highly controlled warming. Molecular control can help improve a wide range of research applications.

What Are the Different Types of Amplifiers?

You'll find many types of RF power amplifiers that fall under one or more of the following categories:

1. Traveling wave tubes (TWT) amplifiers: TWT amplifiers are older and can provide higher output-power densities than a solid-state amplifier, but they also require higher voltages. They may be appropriate where you don't need a wide bandwidth in medium- to high-power applications.
2. Solid-state amplifiers: Solid-state amplifiers are a little newer and can support capable technologies like gallium-arsenide transistors and silicon bipolar transistors. Lower voltage requirements make them more appropriate for less-skilled technicians, while reliability and a small form factor make them good for military and industrial applications.
3. Broadband or wideband amplifiers: These amplifiers can provide moderate transmission gain across a longer bandwidth without raising the noise figure much. Engineers often use them in the receiver circuitry at the front of the antenna.
4. Linear amplifiers: A linear amplifier maintains a proportional linear relationship between inputs and outputs. They optimize linearity for high-quality performance. You'll typically see them used on transmitter and test equipment that requires high linear power.
5. Log amplifiers: Log amplifiers offer a greater gain curve when the output is the input voltage's natural log. If your installation has that shape, you may need a log amplifier.
6. Low-noise amplifiers: When signals are extremely low, such as from an antenna, a low-noise amplifier can help boost them without introducing excessive noise.
7. Waveguide amplifiers: A waveguide amplifier delivers consistent performance over one or several waveguide bands, which are larger than the typical frequency bands that amplifiers affect. They're common in test and sensing equipment.
8. Variable gain amplifier: Users can set and adjust gain within a large range in a variable gain amplifier.

The right choice for your application can vary, but solid-state RF amplifiers often overcome the limitations of other amplifier types in this list.

The Move to Solid-State RF Amplifiers

The move from TWT to solid-state amplifiers is picking up speed. It allows RF technology to replace devices like magnetrons, which generate a microwave signal through oscillation rather than amplifying an existing signal. With smaller form factors, low voltage requirements, improved control, increased precision and longer life spans, solid-state RF amplifiers have a lot to offer.

As we approach cost parity between the two technologies, solid-state RF amplifiers typically outperform magnetrons and other tube-based technologies, especially as gallium-nitride wide-bandgap transistors come into play. Solid-state technology addresses issues like the short life span of tubes and hazards related to high voltages and radiation. However, its relative newness can require outsourcing, since not as many users are familiar with it. Working with a knowledgeable provider can help you make the most of solid-state RF amplifiers.

Along with an industry push toward standardization, solid-state RF amplifiers are paving the way forward, offering better performance at comparable costs.

Add Solid-State RF Amplifiers With Astrodyne TDI

Astrodyne TDI is always on the road to innovation. For over 60 years, we've been providing reliable power supplies and filters to some of the most demanding industries, including military, industrial and medical clients. Solid-state RF amplifiers are a powerful solution to many different applications, and our knowledgeable team has the expertise to implement them effectively and affordably.

Reach out to a pro today to learn more about our power supplies, EMI filters and other products or start building a custom solution.