What Is an RF Power Amplifier?

A radio frequency (RF) power amplifier is a device that increases the voltage and current of an RF signal. Usually, the RF signal being boosted is an RF carrier with sidebands or a pure carrier or just sidebands in some cases. The RF power amplifier usually has an air-cooled final stage that uses heat sinks with fins that are increased in number to come in contact more with air and cause more heat transfer for a more cooling effect.

There are audio power amplifiers that operate in the audible voice frequencies such as 400 to 4,000 hertz (Hz). High-fidelity audio amplifiers work from around 40 Hz to about 15,000 Hz. RF power amplifiers have specified operating frequencies, and the chosen operating frequency must lie within their frequency range. For an operating frequency of 150 megahertz (MHz), an RF power amplifier with a range of 145 to 155 MHz will be suitable. An RF power amplifier with a frequency range of 165 to 175 MHz will not work on the operating frequency of 150 MHz.

In radio electronics, the RF power amplifier is an electronic amplifier that may be built in a transmitter or may be a separate equipment that is connected by a coaxial cable to the output of a lower-power output transmitter. The RF power amplifier output is then connected to the external antenna. For receiver operation, the transceiver or transmitter-receiver unit may have an internal or an external transmit/receive (T/R) switch. The job of the T/R switch is to switch the antenna to the transmitter or to the receiver as needed.

The RF power amplifier circuit may use transistors or integrated circuits to produce the amplification. In RF power amplifier design, the goals are to have enough amplification to produce the desired output power and to allow a temporary and small mismatch between the transmitter and the antenna feeder and the antenna itself. The impedance of the antenna feeder and the antenna itself is usually 50 ohms. Ideally, the antenna and feeder combination will present an impedance that is purely resistive at the operating frequency.

Most antennas may be tuned so that when combined with the feeder, they present the most ideal impedance to the transmitter. This impedance matching is required for maximum power transfer from transmitter to antenna. Antennas will have slightly different characteristics over the frequency range. The important test is to make sure that the reflected energy from the antenna into the feeder and back to the transmitter is low enough. When the impedance mismatch is too high, the RF energy being sent to the antenna will be returned to the transmitter to produce a high standing wave ratio (SWR) that causes the transmit power to stay in the RF power amplifier and cause overheating and even damage to active components.