What Is an RF Amplifier?

An RF amplifier, or radio frequency amplifier, is an electronic device that increases the power of an RF signal. The amplifier accepts a low-power RF signal at its input and produces a high-power version of that same signal at its output. The amount of amplification is specified as gain, measured in decibels (dB).

RF power amplifiers are commonly used to drive antennas, injection devices and other loads in EMC immunity testing, wireless communications, radar, aerospace and defense applications.

How Does an RF Amplifier Work?

At the circuit level, an RF amplifier uses energy from a power source—typically AC line power, which is internally converted to the required DC or high-voltage supply rails depending on the amplifier design—to increase the power of an incoming RF signal.

The amplifier accepts a low-power RF input signal and produces a higher-power RF output signal while maintaining the same fundamental frequency and modulation characteristics. That amplified output is then delivered to an antenna or other RF load.

How an RF amplifier increases a low-power RF input signal to a higher-power output for an antenna or RF load

Types of RF Power Amplifiers

There are many different types of amplifiers for various applications. Generally, RF amps can be categorized as pulse amplifiers, CW amplifiers, TWT amplifiers, and solid-state RF amplifiers.

Pulse Amplifier

A pulse amplifier increases the power of RF signals delivered in short pulses. Pulse amplifiers are commonly specified by peak output power, pulse width, repetition rate and duty cycle. They are used in radar, aerospace, defense and pulsed EMC immunity testing applications.

Solid-State RF Amplifier

Solid-state power amplifiers, or SSPAs, use semiconductor technologies such as LDMOS or GaN to amplify RF signals. SSPAs are available in broadband and narrowband configurations and are widely used for EMC immunity testing, wireless communications, radar and general-purpose RF applications. Compared with TWT-based systems, they typically provide fast startup, high reliability and relatively low maintenance.

CW Amplifier (Continuous Wave Amplifier)

A continuous-wave amplifier is designed to provide sustained RF output rather than short-duration pulses. CW amplifiers are commonly used for radiated and conducted immunity testing, communications testing, antenna applications and other tests requiring continuous RF power.

TWT Amplifier (Traveling Wave Tube Amplifier)

A traveling-wave tube amplifier, or TWTA, uses a specialized vacuum tube to produce high RF output power over a broad frequency range. TWT amplifiers are commonly used at microwave frequencies where high power and wide bandwidth are required, including EMC, radar, satellite and aerospace applications.

Solid-state amplifiers and TWT amplifiers each have advantages depending on frequency, output power, bandwidth, efficiency and application requirements. Compare SSPA vs. TWTA amplifiers to understand which technology is better suited for your test setup.

Amplifier Categories by Properties

Amplifiers can also be classified according to the electrical quantity they are designed to increase. Voltage amplifiers primarily increase signal voltage, current amplifiers increase available current, and power amplifiers increase the total signal power delivered to a load. RF amplifiers used to drive antennas and other RF loads are power amplifiers because their primary purpose is to increase the signal power delivered to the load.

Voltage-versus-Current-Versus-Power-Amplifier-Gain

Amplifier Categories by Signals

Amplifiers are categorized by their frequency range and purpose:

1. RF Amplifiers

Radio frequency power amplifiers are designed to convert a low-power RF signal to a higher-power signal. RF amplifiers are available across a wide range of frequencies, from kilohertz frequencies through microwave and millimeter-wave ranges depending on amplifier design.

2. Audio-Frequency Amplifiers

AF Amplifiers amplify audio signals within the audible frequency range of 20 Hz to 20 kHz. Audio-frequency amplifiers are commonly used in sound systems for applications such as concert venues, stadiums and consumer audio equipment.

3. Intermediate Frequency Amplifiers

Intermediate-frequency (IF) amplifiers amplify signals that have been converted from their original RF frequency to a fixed intermediate frequency inside a receiver or transmitter. Using a fixed IF makes it easier to provide consistent gain, filter unwanted noise and signals, and improve selectivity before further processing. IF amplifiers are commonly used in radio receivers and other communications systems.

4. Broadband Amplifiers

Broadband RF amplifiers provide amplification across a wide frequency range, allowing a single amplifier to support multiple RF and test applications without frequent test equipment and setup changes.

Other amplifier types include video, operational, buffer and DC amplifiers. These are designed on a different scale for different electronic applications when compared to RF power amplifiers.

RF Amplifier Applications

rf-amplifier-connected-to-antenna-in-anechoic-chamber

EMC Immunity Testing

Radiated Immunity
Conducted Immunity
TEM/GTEM Cells
Bulk Current Injection
Reverberation Chamber

Aerospace, Defense and Radar

Pulse Radar Systems
Electronic Warfare (EW and E3 Testing)
Avionics
Space and Satellite Systems
Military Communications

Wireless and Communications

Wireless Device Testing
Telecom Systems
Satellite Communications
RF Component Testing
Transmitter and Receiver Development

RF Amplifier Characteristics and Specifications

Frequency Range: The range of frequencies over which the amplifier meets its specified performance, including output power, gain and other applicable specifications.
Rated Output Power: The RF power the amplifier can deliver specified as CW or Peak Pulse Power and as P1dB or Saturated Power.
Gain: The ratio between RF Output and Input Power, expressed in decibels (dB).
Gain Flatness: The variation of the amplifier's gain across the operational frequency range, typically expressed as ± dB.
Linearity and Compression: An amplifier produces a proportional output only within its linear operating range. The 1dB compression point, or P1dB, identifies where gain begins to compress noticeably.
Harmonics and Distortion: Unwanted frequency components produced by amplifier nonlinearity that may affect signal quality or EMC test field uniformity.
VSWR and Mismatch Tolerance: The amplifier's ability to operate safely when power is reflected back toward the output by an antenna, injection device, cable or improperly matched load.
Pulse Specifications: For pulse amplifiers, important specifications include peak power, pulse width, duty cycle, rise time, pulse repetition frequency (PRF).

Amplifier Classes

RF amplifier classes describe how an amplifier’s active device conducts during each signal cycle. This is referred to as the conduction angle of the amplifier. The amplifier class influences important characteristics such as linearity, efficiency, distortion and heat generation. Class A provides the highest linearity, while Class AB, B and C trade off linearity for improved efficiency.

Amplifier Class	Conduction Angle	Linearity	Efficiency	Typical RF Use
Class A	360°	Highest	Low	Highly linear RF amplification
Class AB	180°–360°	High	Moderate	Linear RF power amplifiers
Class B	180°	Moderate	Higher	Push-pull RF stages
Class C	<180°	Low	High	Tuned RF transmitters

In general, increasing conduction angle improves linearity but reduces efficiency. Classes with lower conduction angles can improve efficiency at the expense of increased distortion.

RF Amplifiers for EMC Immunity Testing

RF power amplifiers are an essential part of many EMC immunity and susceptibility test systems. The amplifier increases the RF signal from a signal generator to the power level required to drive an antenna, bulk current injection probe, coupling device or other RF load.

Depending on the product and industry, RF amplifiers may be used to support testing to standards such as:

IEC 61000-4-3 - Radiated RF immunity testing for electrical and electronic equipment.
MIL-STD-461 RS103 - Radiated susceptibility testing for military equipment and subsystems.
RTCA DO-160 Section 20 - Conducted and radiated RF susceptibility testing for airborne equipment.
ISO 11452 - Automotive component immunity testing for electrical disturbances from narrowband electromagnetic energy.

Learn more about EMC testing methods and equipment used for immunity and susceptibility testing.

How to Choose an RF Amplifier

Every RF amplifier application has a required frequency range and output power level. Selecting the right amplifier also depends on the load it must drive and how that load behaves across frequency. VSWR tolerance, reflected-power protection and amplifier foldback can be important when operating into antennas or other mismatched loads.

Amplifier gain, linearity and compression performance also matter. Some applications require operation near P1dB without excessive waveform compression, while others prioritize efficiency or maximum available power. These trade-offs can influence whether a Class A, Class AB, solid-state or TWT amplifier is better suited for the application.